Pyrolytic Polyimide Is Semiconductor Material may have promising electronic applications at high temperatures Vacuum pyrolysis of poly[N,N'-(p,pf'oxydiphenylene) pyromellitimide], Du Pont's H-film, at 850° C. results in a product stable at high temperatures. Dr. Stephen D. Bruck of the Applied Physics Laboratory of The Johns Hopkins University (Silver Spring, Md.) finds [Polymer (Lond.), 6, 319 (1965)] that the material has intrinsic resistivities of about 5 X 10 - ohm-cm. (measured at 25° C ) . This high-temperature stability and low resistivity might make Dr. Bruck's polyimide suitable for high-temperature semiconductor devices. So far, the highest operating temperature (about 200° C.) has been achieved with inorganic semiconductors. Inorganic semiconductors attain their (extrinsic) resistivity, which suits them to electronic applications, upon addition of impurities to the crystal lattice. For a particular device function, these impurities (making the solid p or n type) must be localized. Diffusion at high temperatures, however, upsets this electronic structure, thereby preventing the device from functioning properly. By contrast, the pyrolytic polyimides developed by Dr. Bruck have an intrinsic resistivity potentially useful for electronic application. The APL chemist is quick to point out, however, that the conduction mechanism in his polymer is not yet known. Organic polymers have been investi-
gated by many scientists during the past decade. In 1955, Dr. Field H. Win slow and his co-workers at Bell Telephone Laboratories (Murray Hill, N.J.) studied the pyrolytic degradation of polyvinylidene chloride and preoxidized polyvinylbenzene. They found that the nonvolatile degradation products made at high temperatures (about 1000° C.) have semiconducting properties at room temperature. The product, however, did not have dimensional stability. The late Aleksandr V. Topchiev of the U.S.S.R. Academy of Sciences (Moscow) found semiconduction with the pyrolytic condensation product of polyacrylonitrile. But this product's resistivity ranges from 10 5 to 10 7 ohmcm., approaching the upper limit of the semiconduction range. Recent studies on this material by J. Manassen and J. Wallach of the Weizmann Institute of Science (Rehovot, Israel) show that it is a good catalyst, as many semiconductors are. Some biopolymers (such as plasma albumin, fibrinogen, and edestin) have been found by others to possess semiconductivity. Nobel prize winner Dr. Albert Szent-Gyorgyi of Muscle Research Institute, Marine Biological Laboratories (Woods Hole, Mass.) suggested in 1941—while at the University of Szeged, Hungary—that mobile electrons in proteins could lead to semiconduction in living systems.
Pyrolysis. Dr. Bruck used samples of H-film in his studies. He heated the samples in vacuum at 5 X 10~6 torr, using a sealed quartz tube. After pyrolysis, the sealed tube was cooled in liquid nitrogen. In one to three hours, electron paramagnetic resonance (EPR) absorption measurements were made at 9140 megacycles per second. Resistivity was measured on separate samples by radio-frequency induction at 50 megacycles per second. Finally, density measurements were made. The APL chemist finds that the relative EPR absorption first increases with pyrolysis time. After going through an inflection, the curve climaxes, and finally declines. During the early part of pyrolysis—corresponding to the inflection in the c u r v e about 40% weight loss occurs. Then the weight of the residue remains roughly constant. An exception is the samples, pyrolyzed at low-temperature (550° C ) , which attain their 40% weight loss slowly. The maximum and the shape of the curve are a function of temperature. The inflection in the curve becomes more pronounced with decreasing temperature in the range of 620° to 550° C. At the same time, the EPR absorption maxima increase with temperature up to 620° C. At 700° C. and above, however, no microwave absorption was measurable. Therefore, Dr. Bruck believes that the curve has a drastic drop. This drop in EPR absorption coincides with a sharp decrease in resistivity and an increase in density (after an initial reversal at about 600° C ) .
Aromatic Polypyromellitimide Gradually Converts into a Semiconducting Compound
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At 600° C., semi-isolated regions of condensed rings form. Above 700° C , increasing reorganization and condensation occur and semiconduction sets in. Above 800° C , the network of fused rings further grows and semiconduction increases JULY
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Physical Properties Change During Pyrolysis
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EPR absorption maxima increase with temperature between 550° and 620° C. But at 700° C , no microwave absorption is measurable. In the range of 620° to 850° C , density sharply increases and resistivity decreases with leveling off above 850° C.
During weight loss, CO and CO., are given off, and limited poly condensation of the aromatic rings is possible. The CO conies from the cleavage of carbonyl groups in the polyimide, whereas COL» probably originates from the carboxyls of polyamide impurities (present in the sample in small amounts). Between 700° and 850° C , increased carbonization takes place. Higher temperatures would probably lead to graphitization, Dr. Bruck comments. Elemental microanalyses indicate that up to 800° C , considerable amounts of nitrogen, oxygen, and hydrogen are retained by the pyrolyzates. The fact that these pyrolyzates have resistivities comparable to those of semiconductors suggests that graphitization is not a requirement for semiconduction in pyrolytic polymers. Structure. Dr. Bruck attempts to correlate resistivity with structural changes in the polyimide. He thinks that in the first stage of pyrolysis— when most of the weight loss takes place—semi-isolated regions of condensed rings form. These semi-isolated regions are, however, not sufficient to bring about pi-orbital overlap 38
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to cause semiconduction. (Conjugation itself is not sufficient for semiconduction, as evidenced by the fact that many conjugated molecules are typical insulators.) During the second stage of pyrolysis, increasing reorganization of the pyrolyzate takes place. The semi-isolated condensed ring systems gradually merge into a continuous network of fused aromatic and heterocyclic rings in which an increasingly effective piorbital overlap develops. At this stage semiconduction sets in (resistivity < 10" ohm-cm.), he adds. In the third stage, the network of fused polynuclear regions further grows and unsaturation increases. Semiconductivity accordingly increases in this stage, Dr. Bruck explains. He thinks that it is the increased mobility rather than the greater number of current carriers that is responsible for the onset of semiconduction. This increase in the mobility of current carriers is facilitated by the enhanced pi-orbital overlap, which in turn is brought about by a molecular reorganization and fusion process at high enough temperatures. Besides the low resistivity, there are other features that may make these polyimide films useful for electronic applications, comments Dr. Bruck. These are: • Dimensional stability at high temperature. • Reproducibility of resistivity with temperature cycling (between 900° and 25° C ) . • 5 X 10~- ohm-cm. resistivity is measured at 25° C. • Material can be molded into special shapes before pyrolysis. In addition, poly[N,N / -(p,p / -oxydiphenylene) pyromellitimide] may serve as a model for studying electrical conductance in pyrolytic polymers, Dr. Bruck concludes.
New Phase Discovered In Tantalum Films A new phase in tantalum films has been discovered by Mildred H. Read of Bell Telephone Laboratories (Murray Hill, N.J.) and Dr. Carl Altman of Western Electric Engineering Research Center (Princeton, N.J.). The new phase, called beta phase, has electrical properties significantly different from those of normal tantalum.
The pair finds by S-ray diffraction techniques that when tantalum films are formed by cathode sputtering in a system containing 1 X 10~2 to 3 X 10" 2 ton* of argon, the films become beta phase if the total pressure of all other gases in the system is less than 1 X 10" 5 torr. Beta tantalum can be made, however, by evaporation and by vapor deposition as well, the scientists say. Beta tantalum converts to normal tantalum when heated to about 750° C. in vacuum. Its resistivity ranges from 180 to 220 microhm-cm.; its temperature coefficient of resistivity ranges from —100 to 100 p.p.m. per ° C ; and its superconducting transition temperature ranges from 0.57 to 0.49° K. By contrast, these values for normal tantalum films are 24 to 50 microhm-cm., 500 to 1800 p.p.m. per ° C , and 3.3° K. So far, beta tantalum has been produced and studied only in film form, Mrs. Read and Dr. Altman comment. Studies are being continued to determine if beta tantalum can be made in bulk form. Additional studies are being conducted to verify whether beta tantalum is an allotrope of normal tantalum or is an impurity-stabilized phase. Beta tantalum has a tetragonal crystal structure, whereas normal tantalum films are body-centered cubic. Nevertheless, it oxidizes by anodic treatment as readily as normal tantalum. It can be used for the familiar electronic applications of normal tantalum (such as capacitors for thin-film circuits). But in addition, it can be used as a thin-film resistor because of its high resistivity and its low temperature coefficient of resistivity.
BRIEFS The nuclear magnetic resonance signal in a bulk nickel sample is extremely intense, a study by Dr. Lawrence H. Bennett of the National Bureau of Standards (Washington, D.C.) shows. The shape and strength of the NMR signal are comparable to those of signals obtained in fine nickel powders. This is in contrast to findings on nonmagnetic bulk metal samples, where weak signals are obtained. The work indicates that bulk specimens may be used for NMR studies of magnetic materials, eliminating the time and expense necessary to prepare powdered materials.
