TECHNOLOGY
Improved Photoresists for Integrated Circuit Chips Devised Two-layer resins that increase resolution and organometallic resins that protect during ion etching are among new developments Chemists at many companies and universities have made great progress in developing photoresists to meet demands of the electronics industry for ever-decreasing sizes of features to be doped, etched, or plated onto integrated circuit chips. Photoresists are light- or other energy-sensitive resins that can be applied as films and patterned to protect some, and expose other, areas of silicon wafers for doping, etching, or plating. Though the electronics industry uses many chemicals, photoresists give chemists their widest latitude to be creative. Many of the products of that creativity became apparent at the recent American Chemical Society national meeting in Chicago, where the society's Division of Polymer Chemistry held four sessions on imaging science. Topics included multilayer photoresist systems to increase resolution, organometallic resins to give protection during ion beam etching, and polymers that amplify the effects of imaging light w i t h cascades of chemical reactions. Multilayer photoresists potentially can increase the resolution of smaller and smaller features, as the electronics industry increases the number of devices on surfaces of chips whose size remains about the same. In multilayer systems, a top layer receives the image to be transmitted through the other layers to the substrate surface. A bottom layer,
called a planarizing layer, covers the often-irregular substrate surface to provide a flat surface. Sometimes a middle layer is used to protect the bottom layer during development of the image in the top layer. With the single-layer photoresists used now, film thicknesses vary with the topography the films cover. Films are thicker in "valleys" than over the tops of "mountain ranges." The light that adequately exposes the valleys may overexpose the higher regions. Also, the depth of focus of projection printers may limit resolution in areas of greater thickness. Resolution may suffer further as chip makers start using deep ultraviolet light because shorter wavelengths need greater apertures in lenses or mirrors, which, in turn,
reduce depth of focus. And light that is reflected back through the film from the substrate can broaden exposed areas, further eroding resolution. But multilayer systems have disadvantages too. A p p l y i n g three layers and using three image developing steps add costly complexity. And multiplying the number of layers multiplies the chances for defects that cause chips to be rejected. Elsa Reichmanis of AT&T Bell Laboratories described a simpler two-layer system that eliminates the middle layer by making the top layer not only photosensitive but also protective against oxygen ion etching. Working with Gerald Smolinsky and Cletus W. Wilkins Jr., she applied a copolymer of trimeth-
Photoresists may be positive or negative Light
-i-i-
-Mask -Resist -Silicon dioxide -Silicon
\ Develop Negative image
Positive image
Etch
JZ1
Strip
October 7, 1985 C&EN
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Technology
Uneven photoresist thicknesses...
s
-Resist -Substrate feature
. . . are eliminated by multilayer systems Imaging layer Planarizing layer
ylsilylmethyl and 3-oximino-2-butanone methacrylates over a planarizing layer of phenolic resin. In that system, deep UV depolymerizes the top layer of resin in the areas exposed. A developing solvent dissolves the depolymerized resin, uncovering the bottom layer in those areas. Oxygen ions etch away the uncovered phenolic resin down to the substrate surface. Those ions convert areas of intact top-layer resin to a thin film of silicon dioxide, which resists further etching. Thus, the two-layer system both simplifies processing and extends two-layer technology into the deep UV. A two-layer system is also the ap-
proach of physical organic chemist Paul R. West at General Electric Co. Working with organic chemist Gary C. Davis and physicist Bruce F. Griffing, he covered a film of conventional phenolic novolac photoresist with a second film of polystyrene doped with a nitrone [R — CH = N(O) — R] contrast enhancing agent. The problem that the GE workers try to overcome results from optics. The ideal exposure on a photoresist surface is all or none. In patterning a line, for example, the line should get full exposure and the surrounding area none. The ideal result would be a "canyon" with perpendicular sides extending to the substrate surface. In practice, light intensity tapers off at the edges, producing a sloping valley. The GE solution is the nitrone, which is opaque to light wavelengths from 315 nm well into the visible, but which is photolyzed to a transparent compound. The a-dimethylaminophenyl-Nphenylnitrone West used is photolyzed to 3-dimethylaminophenyl-2phenyloxaziridine by the intense light in the center of the image but not by less intense light at the edges. Light passes through this transparent compound to photolyze a 2-diazonaphthoquinone solubility inhibitor in the novolac to a 1-indanecarboxylic acid. The GE workers
Phosphors printed on color TV screens Although most chemists in the electronics industry think of photolithography as a technology to image integrated circuit chips or circuit boards, researchers at Hitachi have used it to apply phosphors to inner surfaces of color television picture tubes. According to Saburo Nonogaki, the idea is to produce patterned sticky spots on the glass. Phosphor powders dusted over the glass stick only to those spots. The process is continued until red, blue, and green phosphors have been applied in the desired, high-resolution pattern. Working with Hajime Morishita at Hitachi's Tokyo research laboratory and with Yoshifumi Tomita and Masahiro Nishizawa at the Mobara City plant, Nonogaki coated glass to a dried thick-
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October 7, 1985 C&EN
ness of 0.3 M m with a water solution of 2 to 3 % p-dimethylaminobenzenediazonium zinc chloride, 0.1 to 0.2% polypropylene glycol alginate, and 0.8 to 1 % ethylene glycol. Photolysis of the coating through a mask with a high-pressure mercury lamp produced p-chloro-N,N-dimethylaniline, which volatilized, and zinc chloride, which absorbed atmospheric moisture and made exposed spots on the coating sticky. The Hitachi investigators obtained good resolution of a test pattern of 15-/xm lines and spaces. The amount of phosphor that stuck to a spot depended on the amount of light energy used. That amount leveled off at 2 mg per sq cm with 700 millijoules of light energy applied per sq cm.
Opacity photolyzes to transparency (CH3)2,N - ^
\-CH=K
(CH3LN Opaque contrast enhancement material at top becomes transparent compound when photolyzed with UV
then strip off the contrast-enhancing top film with toluene-anisole solvent and develop the image in the novolac with aqueous alkali. Alkali leaches out the carboxylic acid, rendering the novolac in that area soluble in alkali also. The more perpendicular sides of the trenches produced by the GE method show up not only in photomicrographs but in the computerg u i d e d calculations of Dietrich Meyerhofer of RCA Laboratories. Meyerhofer uses a program developed at the University of California, Berkeley, and improved at RCA. The program calculates light distribution in images, the exposure effect in the resist, including bleaching, and the resulting profiles of images. In the area of organometallic resists, Scott A. MacDonald of IBM described two new organotin resins that are sensitive to electron beam imaging. Later oxygen or fluorocarbon plasma etching converts the organotin to stannic oxide or fluoride, which resist further etching. Working with Jeff W. Labadie and C. Grant Willson, MacDonald copolymerized such c o m p o u n d s as dibutylbis(dimethylamino)tin with p-diethynylbenzene to a resin with dibutylstannylene and p-phenylenediethynyl groups. Selective electronbeam irradiation crosslinks the resin, rendering exposed areas insoluble in developing solvents. Alternatively, resins like poly(2-dimethylstannyl-2-propyl methacrylate) are depolymerized by electron beam ra-
diation, making exposed areas more soluble. Thomas R. Pampalone of RCA government systems division told of a new electron-beam resist containing silicon. He copolymerized vinyltrimethyltin and sulfur dioxide at - 7 8 °C to a polysulfone. Next Pampalone depolymerized this resin with an electron beam and dissolved it away from an underlying planarization layer in exposed areas with 2-methoxyethanol. He etched through the planarizing layer with oxygen ions, which converted the silicon resin in unexposed areas to protective silicon dioxide. A self-developing silicon-containing photoresist was described by organic chemist John M. Zeigler of Sandia National Laboratories. Working with physical chemist Larry A. Harrah and physicist A. Wayne J o h n s o n , Zeigler copolymerized methyl-tt-propyl- and methylisopropyldichlorosilanes with sodium to a relatively noncrystalline polysilane. Exposure to a krypton fluoride excimer laser (248 nm) broke the silicon-silicon bonds. Reaction of photoproducts in exposed areas with air oxygen formed six- to 10-membered cyclodialkylsiloxanes, which volatilized. Zeigler suggests that photolysis produces silyl free radicals, which further decompose to dialkylsilylenes. These, in turn, react
Gas-phase photolysis allows direct imaging Light
-Organogold vapor -Deposited gold * Substrate
with oxygen to form the cyclosiloxanes. In a technique that avoids use of resists altogether, Carol R. Jones of IBM used patterned UV light to photolyze vapors of a gold complex, depositing the gold directly onto the substrate. Working with organic chemist Thomas H. Baum, Jones selected acetylacetonatodimethylgold, which melts at 81 °C and has a vapor pressure of 9 torr at room temperature. In their best results to date, the IBM chemists used irradiation at 248 nm to get gold films 82 to 117 nm thick, containing 11% carbon and 13% oxygen. Jones noted that carrying out the process in an oven or using the heating effect of the laser lowers the nongold content still more, indicating that more work along this line may improve the quality of the films. •
Polymer networks meet demanding applications Among the new types of polymeric materials to appear recently have been those made with interpenetrating polymer networks (IPNs). Though known in the laboratory for many years, IPNs blossomed only in the early 1980s and have provided valuable, although rather expensive, materials for a number of demanding applications. Speaking at the recent national meeting of the American Chemical Society in Chicago, Leslie H. Sperling of the Materials Research Center at Lehigh University noted that IPNs were defined originally as a combination of two polymers in network form. At least one of them was synthesized and/or crosslinked in the immediate presence of the
other. The original definition implied that only chemical crosslinks were present. Today this definition of IPNs has been extended considerably. It includes, for example, sequential materials formed when a polymer network is swollen with a second monomer plus its initiator and then polymerized in situ. A second type of IPN is the material formed when both polymerizations are conducted simultaneously. A third type is the semi-IPN, which has one polymer crosslinked and the other linear. In addition, many other types are included, some of them involving more than two polymers and several types of compounding and manufacture.
With the recent extension of the definition, IPNs now can include materials with physical bonding between polymer chains. The physical bonds may be from the hard blocks as in a block copolymer, the ionic linkages in an ionomer, or even the bonds in semicrystalline materials. Blends of all these types yield a great variety of hybrids. One newer material is that containing cocontinuous phases, wherein both polymers exhibit continuous phases throughout. However, to qualify as an IPN, the material must have crosslinks, either chemical or physical. Sperling notes that at present, at least a dozen commercial materials incorporate IPNs. They usually are hard, wear-resistant plastics and have been used in applications such as artificial teeth, leathery products, and materials with great tensile strength. A typical example, cited by Sperling, is Shell's Kraton, an IPN that is leathery with high elon-
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October 7, 1985 C&EN
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Technology gation and tensile strength. It retains its mechanical properties right up to the melting point (above 200 °C) and at temperatures as low as -50 °C One of the more active patent areas at present concerns IPNs made with thermosetting and thermoplastic polymers. This area is the special interest of Jane M. Crosby, a research chemist with LNP Corp., Malvern, Pa. The "entanglement" of polymers in the physical crosslinking of these IPNs restricts the polymer mobility. Elevated temperatures reduce the strength of the physical crosslinks. Also interesting, Crosby says, are the hybrids made from thermosets and thermoplastic polymers. Both types of polymers contribute recognizable properties, but the composite itself has its own unique properties. In several n y l o n 66 IPNs formed with silicone networks, the mechanical strength appears to peak at low silicone concentrations. The friction coefficients and wear resis-
tance of the composites are improved sharply over those of nylon 66 and the heat distortion temperatures increase significantly. The greatest improvements in these properties are observed with IPNs having branched-chain silicone networks. One nylon 66-silicone IPN was found to be particularly useful in applications such as high-speed journal bearings for paper handling equipment. The silicone in the IPN appears to function as a nonmigratory lubricant. Normally lubricants function by migrating to the wear surface, but local debris often interferes. In paper-handling equipment, the debris is paper dust, which is abrasive and seems to be trapped effectively at the wear surface by conventional lubricants. The intrinsic lubricant in the IPN displayed no such tendencies. The same materials also have been used in gears for business machines, which require stability, high mechanical strength, and very precise molding. •
Detector locates leaks in plastic pond liners A device that detects and locates leaks in plastic pond liners has been invented by scientists at Foote Mineral Co. and its parent, Newmont Mining Corp. They developed the detector for use in the 220 acres of polyvinyl chloride-lined solar evaporation ponds at Foote's Salar de Atacama lithium brine deposits in Chile, but the technique could be employed to detect leaks in the ponds of a wide variety of industrial plants. To make the detector, chemist Dan Boryta at Foote and geologist Misac Nabighian at Newmont apply a 12to 30-volt potential across two electrodes, one buried 10 feet below ground outside the pond and the other immersed in the pond. Burial at 10 feet ensures that the outside electrode reaches the moisture level in the soil outside. Normally, moisture in the soil under the pond
24th Annual Eastern Analytical Symposium Technical Sessions/Technical Films/Short Courses/Mixer
EXPOSITION
New York City, New York November 19 - 2 2 , 1985 New York Penta Hotel FOR ADDITIONAL INFORMATION: G. M. NAKANE E. R. Squibb & Sons P.O. Box 4000 Princeton, NJ. 08540 (201) 846 1582
Plan To Be There! 30
October 7, 1985 C&EN
$ travels upward by a wicking effect, providing conductivity from the outside electrode to the underside of the pond. If a leak occurs, current travels between the two electrodes in the inner circuit through the brine, the leak, and ground moisture and registers on a very sensitive galvanometer in the outer circuit. To locate leaks, Boryta and Nabighian designed the probe immersed in the pond in a way that takes advantage of the greater conductivity of copper wire compared with that of brine. For the probe, they attach two lengths of stiff, insulated copper wire to the galvanometer. One length with an uncovered end extends forward under the liquid surface. The other length extends backward, also under the liquid, to a point where it joins a cable that goes outside the pond to the buried electrode. The operator swings the whole assembly around as a direction finder. The galvanometer deflection is greatest when the colinear array of copper wire points toward the leak. Walking along that line, the operator sees the deflection increase as he or she approaches the leak and change polarity when the probe is moved past it. The reason for the directionality of the galvanometer deflection is the high conductivity of copper compared with that of brine. When the copper wire points toward the leak, the current in the inner circuit meets less resistance because it has to travel through less brine, and the galvanometer deflection is greater. When the length of copper points away from the leak, the conductivity of copper offers no advantage over that of the brine, and the deflection is less. Boryta and Nabighian caution that for the detector to work there must be no other electrical connection from the pond to the ground. Thus, the top of the pond liner must be entirely above ground. Also, there must be no pipes entering the pond that have contact with the ground. At the Chilean site, for example, the scientists had to insulate pumps immersed in the pond by supporting them on rubber tires. •
A Hands-On Course Offered by the American Chemical Society
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October 7, 1985 C&EN
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