Outgassing: FTIR detection of outgassing chemicals

Static headspace system detects sources of AMC

by John C. Hulteen, Ph.D.

Airborne molecular contamination (AMC) has become the primary contamination issue in both semiconductor and disk drive cleanrooms. Numerous articles related to the effects of AMC and attempts to quantify these molecular contaminants have been published (see “Fibrous filter performance: Particle control success; AMC control failure,” CleanRooms August 1999 p. 10; “Preparing for Battle,” November 1999 p. 15; and “Tips for Tackling AMC,” December 1999 p. 4). The adverse effects of AMC are well known and can be quite severe. Problems can include corrosion, hazing, adhesion and electrical shorting, which all can lead to eventual product failure. Therefore, adequate identification, control and elimination of these molecular contaminants is essential to efficient cleanroom operation.

A Fourier transform infrared (FTIR) static headspace system can simultaneously detect and quantify the molecular acids, bases, condensables and solvents of AMC. The system could be used on the manufacturing floor for routine evaluation of semi-finished or finished goods for unwanted contaminants and for control charting. It also has the potential to be used in product development and product certification activities.

The sources of AMC are varied and can include items such as tapes, adhesives, plastics, process chemicals, solvents and people.

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Photograph of the Fourier transform infrared (FTIR) static headspace (SHS) outgassing manifold.

Eliminating the source of AMC within a cleanroom is often recommended as the initial step in total AMC reduction. However, efficient identification and removal of the products that contribute to high levels of AMC require the application of a comprehensive set of analytical methods to each item under study. One of the primary difficulties with analytical testing is characterizing the type and quantity of molecular compounds present. Standard methods of detecting the molecular compounds that are emitted or “outgassed” from cleanroom materials routinely involve one of three procedures: gravametric, headspace gas chromatography (GC) or ion chromatography (IC).

Gravametric methods such as thermal gravametric analysis (TGA) or ASTM E-595 are quick procedures for determining the quantity of outgassed contaminants. However, they do not yield chemical speciation or quantification.

The second procedure, headspace GC, involves either static (IDEMA standard M8-98) or dynamic headspace (IDEMA standard M11-99, IEST standard in development WG-CC031) manifolds with a variety of detection methods, including mass spectroscopy (MS). As with gravametric procedures, unless MS is used for detection, compound identification can be impossible. In addition, it is difficult to measure the large variety of outgassing compounds accurately using GC/MS without running multiple samples with multiple instrumental parameters.

The third procedure, IC, can be used for the detection of smaller molecular acids and bases. However, this method requires the use of purge and trap systems, which can be difficult to work with. In addition, the method can be a time-consuming process, and it is not effective for other classes of molecular contaminants.

The FTIR static headspace system eliminates the need for multiple instrumentation to conduct an AMC analysis. This system combines attributes of each of the standard analytical tests previously mentioned. It is capable of rapid identification and quantification of outgassing compounds present in the sample. Outgassing compounds can be identified and quantified in as quickly as 2 minutes. The FTIR static headspace system is capable of simultaneous identification and quantification of such standard cleanroom contaminants (SEMI standard F21-95) as molecular acids (MA), molecular bases (MB), molecular condensables (MC) and residual solvents.

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FTIR spectrum of the outgas generated from a cleanroom tape along with a library standard FTIR spectrum of formic acid. The graph shows the cumulative outgassing of formic acid from this tape as a function of time.

An FTIR static headspace unit consists of a commercial FTIR connected to a static headspace unit. The headspace unit contains a drawer, which can be utilized for easy placement of samples into the sampling chamber. The operation of the FTIR static headspace instrument begins with the placement of a potential source of AMC (such as a tape sample) into the sealed sampling chamber. This chamber can be maintained at room temperature or can be heated to assist in driving off the volatile residuals present within the sample. These volatile residuals, such as acids, bases, solvents and residual monomers, will begin to outgas upon delivery into the chamber. During this outgassing process, an infrared source is directed through the chamber generating FTIR spectra of the molecular compounds present in the headspace. The FTIR absorbance spectra of the outgassing compounds are generated continuously throughout the outgassing process so that the dynamics of the outgassing can be followed. The FTIR spectra are compared to standards, and the identity and concentrations of outgassing compounds are determined instantaneously using a non-linear least squares fitting routine.

Molecular acids are typically one of the most difficult contaminants to detect and quantify, and are often cited as a major source of corrosion within the microelectronics industry. The FTIR static headspace is capable of detection and quantification of the common acids present in cleanroom tape such as hydrochloric acid, formic acid, acetic acid and acrylic acid. The figure demonstrates the ability of FTIR static headspace to detect molecular acids outgassing from a standard commercially available cleanroom tape. In this example, a 100 cm2 sample of tape was placed in the FTIR static headspace unit and heated to 85 degrees C. The FTIR spectrum of the volatile material present in the chamber after 30 minutes is presented as well as a standard FTIR spectrum of formic acid for reference. Formic acid, which is not a compound that can be detected by the standard headspace GC/MS methods, was determined to outgas from this sample at a level of 570 ng/cm2 or 31 ppm by weight. Also included in this figure is a graph of the total accumulation of formic acid in the static headspace unit as a function of time. This graph shows that formic acid was identified within two minutes after introduction of the sample into the FTIR static headspace system.

In addition to molecular acids, common molecular bases such as ammonia can easily be detected. Other compounds such as solvents (methanol, toluene, ethyl acetate) and molecular condensables, including siloxanes, acrylates and phthalates, can also be detected. All of these contaminants can be detected at low levels as required by the microelectronics industry. Typical detection limits are between 1-5 ng/cm2 or sub part per million in the gas phase.

John C. Hulteen, Ph.D., leads the electronics market analytical team located in the environmental laboratory at 3M. His work involves materials qualification and development of analytical methods for microcontamination testing of materials destined for the microelectronics industries. Dr. Hulteen received his Ph.D. from Northwestern University in physical/analytical chemistry. He is involved with the IDEMA (disk drive microcontamination) and IEST (outgassing of cleanroom materials) committees.

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