The Analytical Approach Edited by Claude A. Lucchesi Peter Cukor GTE Laboratories Waltharn, Mass. 02154
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Figure 1. Analytical approach for characterization of organic products used in the electronics industry
Analysis of Products Used in the Electronics Industry The electronics industry has used a great variety of materials in a unique manner to accomplish the communication and home entertainment revolution of our times. The design and fabrication of integrated circuits (IC) which are a t the center of this revolution consist of a series of complicated chemical processes. The purity of some of the materials used in IC manufacturing, such as silicon and boron, is extremely high. A trace amount of plasticizer extracted by i i solvent from its plastic container can get deposited on a silicon wafer during its cleaning with solvent and may ruin several integrated circuits. consequently, processing takes place under meticulously clean conditions in rooms with filtered laminar flow air and high-purity deionized water. The integrated circuits are only one of many items produced by the electronics industry. General Telephone and Electronics, Inc., is a good example of a broad base technology company with business activities extending
into areas of telephone service and equipment, lighting and home entertainment products, and chemicals and parts manufacturing. This diversified activity means that the analytical chemists a t GTE’s central research laboratory in Waltham work on a wide variety of analytical problems requiring different analytical approaches. Although most of the analytical facility is devoted to the study of inorganic materials, there does exist within the Materials Evaluation Facility, a group whose primary responsibility is the analysis of organic materials. Members of this group provide the support required by scientists involved in research, development, and production and also spend a fair amount of time on the analysis of commercial products. The major reasons for the latter activity are summarized in Table I. The analytical approach used for the characterization of organic products is shown in Figure 1. The examples that follow serve as specific illus-
trations of this general approach. The first one illustrates the determination of the chemical composition and probable polymerization mechanism of an experimental product. The second example shows how analytical work originally intended for troubleshooting led to the development of a raw material specification. Finally, an example is given from an area that is a new challenge to analytical chemists-namely, the analysis of insoluble polymers. Analysis of Experimental Electron Beam Resist
Electron beam resists generally are soluble polymers which cross-link and are rendered insoluble by exposure to an electron beam. It is possible to print an image on a substrate by coating it with a polymer and exposing the polymer coating to a beam of electrons which trace the desired pattern. Those portions of the coating which are not bombarded with electrons are readily washed away, and the exposed substrate is available for chemical treat-
Table I. Reasons for Analyzing Commercial Products Problems with vendors Batch-to-batch variations in products supplied Inability of vendor to supply sufficient quantity of product requiring either alternate vendors or in-house preparation of material Products may be used for purposes other than intended by vendor, and their use may lead t o production problems
Materials for evafuation Evaluation for a proposed use by manufacturer or by own technical personnel Evaluation in connection with management decision, i.e., acquisition Analysis of competitive products t o confirm patent claims
ANALYTICAL CHEMISTRY, VOL. 48, NO. 1, JANUARY 1976
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ment such as etching. The crosslinked polymer pattern, on the other hand, will be retained on the substrate, protecting it from the etchant. The advantage of an electron beam resist over the more commonly used photoresist is the ability to produce much finer lines in the image. An experimental electron beam resist was evaluated for a potential supplier who also requested a detailed chemical analysis. General examination of this sample (odor, heating a small portion on a hot plate) indicated that the resist was dissolved in a solvent. Infrared spectra identified the volatile portion as 2propanol and the residue as a silicone. With this preliminary information, it was possible to map a strategy for more detailed characterization as shown in Figure 2. The volatile fraction amounted to 80% of the sample, and its evaporation in the thermogravimetric analyzer under nitrogen was completed around 200OC. The residue was thermally stable up to 65OoC,beyond which it exhibited a gradual weight loss. By GC-IR it was shown that the 2-propanol was of high chemical purity. The nonvolatile portion of the beam resist was studied by IR and by proton NMR in CHC14 solution. In addition, the elemental composition of the nonvolatile was determined by several techniques: X-ray fluorescence, optical emission spectrography, neutron activation, and C, H, and N organic microanalysis. These data are summarized in Table 11. The 1R band in the vicinity of 1100 cm-' definitely suggested the Si0-Si type structure observed in 5 2 0 2 , silicones, and other siloxanes. The exact location of this band (1080 cm-') further indicated a cyclic-siloxane, and its position and shape suggested a tetramer (Si-O)4:
I
I
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I
-Si+-Si-
This structure was further inferred by comparison of the intensity of the Si0-Si stretching band with the C-H band intensities. The relatively strong Si-0-Si band indicated a minimum number of groups attached to the silicon atoms. The NMR spectrum indicated that the -CH3 and -OH groups present were on the same carbon atom. The spectral data are consistent with the structure shown a t the top of the next column. 52A
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2-Propanol
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High-Purity 2-Propanol
TGA and Distillation
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IR Spectrum
Cyclic Silicone Containing Methyl Vinyl and Hydroxyl Side Groups
Further Confirms Structure
N M R Spectrum
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Figure 2. Analysis scheme of electron beam resist
Elemental composition calculated on the basis of this structure is in good agreement with elemental analysis data as shown in Table 11. The low value shown for carbon probably is due to Sic formation. Gel permeation chromatography was used to measure weight average molecular weight of the polymer. A calibration curve constructed from n paraffins yielded an average molecular weight of 360 for the polymer; a calibration curve based on polyglycols gave 660 as the average molecular weight. The curve was skewed toward the high molecular weight side, and resolution of the curve into two Gaussian peaks showed that the molecular size of the two species was roughly in a ratio of 2:1, indicating the possible presence of a dimer. The calculated molecular weight of the proposed structure is 633 which is in good agreement with the molecular weight obtained by GPC (660) when
ANALYTICAL CHEMISTRY, VOL. 48, NO. 1, JANUARY 1976
the polyglycols calibration curve was used. Besides determining the structure of the resist material, the analytical investigation also yielded information about how the resist functioned. Upon exposure of a film of dry resist to a stream of electrons, IR showed a noticeable decrease in the intensities of the C-H and OH bands in relation to the Si-0-Si band intensity. The appearance of a new band a t 1700 cm-' suggested the formation of C=O, and this correlated with a pronounced decrease in the vinyl group absorption a t -960 cm-', indicating that the cross-linking was accomplished through the vinyl groups. This was further supported by the decrease of the vinyl overtone a t 1950 cm-I. The spectra also showed a shift in the Si-0-Si band to lower frequencies which suggested an opening of the tetramer ring and the formation of an open chain polymer.
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look a t the manufacturing process revealed that the problem occurred not dqring, but prior to, firing. That is to say, the decomposition temperatures of all three waxes were satisfactory, but their wetting characteristics were vastly different. Infrared analysis of the waxes was carried out, and the one with the highest CH&Hp absorption ratio displayed the best wetting characteristics. These findings explained why only one of the three waxes proved to be a usable binder. The investigation resulted in the establishment of specification for paraffin wax binders in terms of TGA and IR data. Analysis of Plastic Wire Coating
Analysis of Cemented Carbide Binder
This example illustrates the problems that may occur when a change in raw material suppliers takes place. In the preparation of cemented carbides, an organic binder is used to hold the powdered material together prior to firing. During firing, the binder is decomposed and is given off as low molecular weight volatile matter while the powdered carbide is cemented together. One of the most popular binders has been a certain paraffin wax. Due to a shortage of petroleum products, the supplier was unable to continue providing this material. No obvious alternative supplier could readily be found, and an assortment of possible binders was evaluated. A screening procedure based on the use of the thermogravimetric analyzer (TGA) and firing conditions as close as possible to those 54A
used in the manufacturing process was set up, and the thermograms of the candidate binders were compared with that of the original paraffin wax. This rapid screening procedure eliminated all but three paraffin waxes as possible substitute binders. Next, small batches of carbide cements were prepared with each of the three waxes. In these tests only one of the three performed satisfactorily. The question then arose as to what other property besides the profile of the thermal degradation influences the behavior of the binder. Accordingly, the three waxes which passed the TGA screening test were further characterized. Molecular weight distributions were obtained by GPC analysis, and the distribution curves corresponded very closely to the TGA curves, suggesting that the decomposition temperatures were a direct function of the molecular weights of the components. Another, more careful
ANALYTICAL CHEMISTRY, VOL. 48, NO. 1, JANUARY 1976
One of the more recent challenges t o analytical chemists has been the analysis of insoluble or partially soluble polymers. In analyzing these materials, the most efficient tools of characterization, transmission infrared spectrophotometry and various forms of chromatography, usually are not applicable without modification. Hence, the increasing interest in reflectance and pyrolysis infrared spectrophotometry and in pyrolysis gas chromatography. Also differential thermal analysis may be used to generate an identifying fingerprint thermogram of many insoluble plastics. Neutron activation and X-ray fluorescence have proved to be effective ways of analyzing plastics for metals, halogens, and oxygen without the need for cumbersome wet ashing procedures. A cable containing a plastic wire coating was analyzed. A flow diagram of the analysis is shown in Figure 3. With an ATR spectrum and DSC curve, the principal constituent of the coating was identified as polyvinyl chloride. The presence of an estertype plasticizer was also detected. The sample was extracted with methanol to remove the plasticizer. The methanol extract, which was equal to 20% of the total sample, was subjected to liquid chromatographic separation, and the material of the major peak was collected and identified by its IR spectrum as diethylhexyl phthalate. Analysis for C, H, N, 0, and C1 confirmed the finding that the coating was plasticized PVA, but it also indicated that not all of the material was accounted for, since the total added up to only about 92%. TGA yielded a 10% residue. Emission spectrographic analysis of the residue showed antimony as the principal constituent. Neutron activation analysis of the original plastic confirmed this finding. The composition of the wire coating may be summarized as follows: 70% polyvinyl chloride, 20% diethylhexyl phthalate plasticizer, and 10% antimony trioxide fire retardant. The examples cited illustrate the
complexity of the analyses required for the characterization of commercial products. I t should be obvious that a variety of techniques have to be utilized for rapid and effective analysis of these samples. I t is also important to point out that in most cases only the major and the minor components are found and identified. Additives which may be present in trace quantities are only recognized if their presence is suspected and specific effort is made to detect them.
Acknowledgment The author acknowledges the contributions of many people to the work discussed here, particularly those of Carmine Persiani and Edward Lanning, also Amy Fermin, Michael Rubner, Arthur Russell, Betty Orofino, Frank Mason, James Kranick, and Jorge Flores. He is further grateful for the contributions of all other members of the Materials Analysis Department of GTE Laboratories.
Figure 3. Analysis of wire coating
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