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Sean Kennedy

Many aspects of our lives—transportation, communication, computers, home entertainment, medical equipment, and instrumentation—have been profoundly affected by the microelectronics revolution. This dynamic industry is propelled by constant technological changes, which have brought improved, innovative products to the end user. Polymers play a critical role in the advancement of the microelectronics industry. They serve as photoresists in microlithography and as insulating dielectric materials in chips, displays, interconnects, and photonic devices (1–5). The continuous evolution toward portable, high-frequency microelectronic systems places high demands on material properties, notably low dielectric constants, low dissipation factors, low moisture uptake, and good thermal stability.

The Dow Chemical Co. (Midland, MI) has recently developed a variety of low-dielectric-constant (low-k) polymers to replace silica as dielectrics for on-chip interconnects (6–9). One commercially important family of polymers, developed at Dow in the late 1980s, is based on benzocyclobutene (BCB, also known as biscyclo[4.2.0]octa-1,3,5-triene or 1,2-dihydrobenzocyclobutene). BCB polymers are used for microelectronic packaging and interconnect applications. During the 1990s, they gained commercial status in applications including the fabrication of gallium arsenide integrated circuits, bumping and redistributing GaAs chips, and for planarization and isolation in flat-panel displays (10 –12). Recently, BCB resin-coated copper foil has been commercialized in Japan for use in the next generation of high-frequency printed wiring boards for telecommunication (13).

We will review polymers based on divinylsiloxane bis(benzocyclobutene) (DVS-bis-BCB) as a dielectric material in microelectronics and describe the synthesis, processing, properties (see box, “BCB properties”), and applications of this material. Two of the key advantages of BCB-based polymers are that the curing process does not emit any volatiles and that the products from the BCB ring-opening reaction are nonpolar hydrocarbon moieties. We illustrate how workers at Dow took advantage of the BCB ring-opening reaction, chose a monomer to focus on, and made the polymer an important dielectric material in microelectronics. The chemistry and properties of polymers from other BCB monomers are reviewed elsewhere (15–18).

Monomer synthesis and B-staging
The BCB hydrocarbon can be made by pyrolyzing -chloro-o-xylene (Figure 1) (19–21). Treatment of the hydrocarbon with bromine provides 4-bromo-BCB in excellent yield (22). Palladium-catalyzed coupling of 4-bromo-BCB with divinyltetramethylsiloxane produces the monomer DVS-bis-BCB (23). Distilling the monomer can reduce ionic impurities to the ppb level if this purity is required.

The monomer can be B-staged; that is, the curing reaction is started, then deliberately stopped before all of the monomer molecules are cross-linked. The resulting oligomer can be completely cured at a later time. The BCB four-membered ring opens thermally to produce o-quinodimethane (Figure 2). This very reactive intermediate readily undergoes Diels–Alder reactions with available dienophiles. In the absence of a dienophile, it reacts with itself to give the dimer, l,2,5,6-dibenzocyclooctadiene or undergoes a polymerization similar to that of a 1,3-diene to give poly(o-xylene). The Diels–Alder reaction predominates in the B-staging of DVS-bis-BCB and in the subsequent step to generate the cured product.

B-Staged BCB solution is formulated and was commercialized by Dow Chemical as the CYCLOTENE 3022 product series in 1992. (Information on CYCLOTENE advanced electronic materials can be found at www.cyclotene.com.) The CYCLOTENE 4000 series of photosensitive polymers (launched in 1994) contains a diazo cross-linker (24, 25).

Thin-film patterning processes
A thin film generally has to be patterned to serve its function in microelectronics. The patterning of a nonphotosensitive polymer requires the use of a photoresist and more processing steps than for photo-BCB. The processes for transforming photosensitive and nonphotosensitive CYCLOTENE solutions into patterned thin films are compared below and illustrated in Figure 3.

Dry-etch BCB resins. An adhesion promoter is applied to a substrate, and dry-etch (nonphotosensitive) BCB resins are spin-cast onto the substrate immediately afterward to form a thin film. The polymer can be cured using a variety of tools, including hotplates, convection ovens, vacuum ovens, tube furnaces, or reflow belt furnaces (26). The curing kinetics for films made of BCB resins have been studied in detail (27). Cure times range from a few minutes at 285 °C to a few hours at 205 °C. Films made of BCB resins must be cured in atmospheres containing <100 ppm oxygen.

Patterns are produced on films made with dry-etch BCB resins by combining a soft- or hard-mask process with a plasma process that uses oxygen and a fluorine-containing gas. The soft mask process begins with depositing a sacrificial photoresist layer on top of the dry-etch BCB film, patterning that resist layer using standard photoresist processing procedures, then plasma etching to transfer the pattern in the photoresist mask through the film made of dry-etch BCB (28). Hard-mask processes are often used for higher resolution work. In the soft- and hard-mask processes, the mask must be removed after the plasma etching is complete.

The plasma conditions used to etch films of the dry BCB resins require a fluorine component to cleanly etch the silicon moiety in the backbone of the dry-etch BCB materials. Mixtures of SF₆–O₂, CF₄–O₂, or CHF₃–O₂ pro duce controlled etch rates >1 µm/min using parallel-plate or reactive ion plasma etchers.

Photosensitive BCB resins. Photo-BCB films are spun onto the substrate directly after the adhesion promoter application and spin dried. After spin coating, the films are heated for a short time to drive out residual solvent.

Table 1. Typical processing conditions for photo-BCB polymer films CYCLOTENE resin type Thickness after soft bake, µm Exposure dosea Final film thickness, µm 4022 5.2 20 3.8 4024 7.4 25 5.2 4026 13.3 60 10.2 amJ/cm2 per µm of pre-exposure film thickness

Photosensitive CYCLOTENE resins are negative acting; that is, the exposed regions are cross-linked and remain behind after development. After the soft bake, the substrates are allowed to cool to room temperature before photolithography. The photo-BCB films should be given an exposure dose appropriate for the thickness of the film. Typical exposure doses and film thicknesses at a spin speed of 2500 rpm for the three CYCLOTENE 4000 products are listed in Table 1.

For example, a film of CYCLOTENE 4024 spun at 2500 rpm will have a thickness after soft bake of 7.4 µm; thus the recommended dose will be 25 mJ/(cm²•µm) × 7.4 µm = 185 mJ/cm². These doses were measured at I-line (i.e., the 365-nm emission from a UV lamp) and were determined on a proximity–contact aligner (a device that aligns the mask with the polymer-coated wafer) with broadband exposure (the intensity of the exposure was determined by measuring the output of the light source at all wavelengths).

After exposure, the pattern is developed by puddle, immersion, or spray techniques. In puddle development, the exposed wafer is placed on the chuck of the spin coater. A puddle of developer is dispensed onto the surface to completely cover the wafer. The wafer sits in the developer for a predetermined time to allow dissolution of the unexposed areas. When the puddle time is complete, the wafer is rinsed at slow speed and then spun at high speed to remove the developer solvent and dry the wafer. The wafer is baked on a hotplate immediately after developing to further dry the film and stabilize the sidewalls of the etched openings (called “vias”).

After photolithographic processing is complete, the film is cured. Two cure profiles are commonly used: “soft” cure (~75% conversion) and “hard” cure (>95% conversion). Soft cure is used for the lower BCB layers when there are multiple BCB layers in a structure; it provides improved adhesion between the polymer layers. Hard cure is used for only one layer or for the last layer in a multilayer build to give the film maximum chemical resistance and stable mechanical and electrical properties. In a box oven, soft curing is performed at 210 °C for 40 min, and hard curing at 250 °C for 60 min.

Following cure, a thin film, or scum, of polymer residue left behind in the development process is removed by brief exposure to a plasma. This residue is typically <0.1 µm thick; hence a de-scum process that removes 0.1–0.2 µm of polymer is generally sufficient. A parallel plate etcher operating in either plasma etch or reactive ion etch mode is recommended for this step. A typical etch gas is an 80:20 mixture of oxygen (to etch the organics) and CF₄ (to etch the silicon).

Applications
CYCLOTENE resin is the dielectric material of choice for many applications in the electronic industry because of its low dielectric constant, a low electrical current loss factor at high frequencies, low moisture absorption, low cure temperature, high degree of planarization, low level of ionic contaminants, high optical clarity, good thermal stability, excellent chemical resistance, and good compatibility with various metallization systems. Dry-etchable BCB is used to planarize thin-film transistor display plates, which allows subsequent indium tin oxide deposition on a completely flat surface. This produces displays that are brighter and require less electrical power (“high-aperture” displays). Figure 4 compares the old and new technologies of high-aperture active-matrix liquid crystal displays. The new technology requires the high levels of planarization that BCB offers. LG Philips (Seoul, South Korea) makes use of this feature in its ToPixel displays (29).

A low curing temperature (210 °C), low water absorption, rapid curing, and low viscosity make dry-etchable BCB the material of choice for a variety of applications in gallium arsenide integrated circuits. Companies including TriQuint Semiconductors (Hillsboro, OR) (30) and Nortel Networks (Brampton, ON) (31) use BCB to form multilayer interconnects on their GaAs chips (Figure 5). Other companies, including telecommunications components manufacturer Anadigics (Warren, NJ), use BCB to encapsulate and protect air bridges for their plastic packaging (32). Photosensitive BCB polymer layers may be used to reroute or reconfigure metallized structures on the substrate known as pads (Figure 6) (33–35). BCB is useful for producing waveguide structures because of its low power loss (<0.2 dB/cm at 1.3 µm) (36). It is also the material of choice for high-freqency RF (radio frequency) components. Its low dielectric constant, loss, and moisture uptake, and its high compatibility with copper make BCB highly suitable for integrating passive components such as filters and op amps (operational amplifiers).

A versatile group of dielectric polymers
DVS-bis-BCB prepolymers formed by B-staging the monomer in mesitylene have very good film-forming properties. Curing the prepolymer film at >200 °C completes the polymerization reaction to produce a thermoset polymer with excellent chemical resistance and a high T_g. The polymer does not emit any volatiles, such as water, during the curing process—a highly desirable feature in microelectronic fabrication. Because the polymer is a hydrocarbon, it has a low dielectric constant, low dissipation factor, and low water uptake. The polymer film shows a very high degree of planarization, which is an important property for building up multilevel microelectronic structures. Polymers in the CYCLOTENE 4000 product series are photosensitive. The combination of these properties makes DVS-bis-BCB–based polymers the choice dielectric material for a variety of microelectronic applications.

Acknowledgments
We thank Britton Romain for the preparation of the manuscript and our colleagues at Dow for their contributions to the BCB program.

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Ying-Hung So is a technical leader in the advanced electronic materials laboratory at the Dow Chemical Co. (1712 Building, Midland, MI 48674; 989-636-9279; yinghso@dow.com). He received his B.S. in chemistry from Chung Chi College, The Chinese University of Hong Kong. He began his graduate studies in organic chemistry at Colorado State University, Fort Collins, with Larry Miller. His research interests include photosensitive polymers, low-k polymers, polymerization mechanisms, polymer photodegradation and stabilization, and monomer synthesis. He is an honorary associate professor at the University of Hong Kong, where he teaches industrial polymer chemistry and industrial organic chemistry.

Philip Garrou is commercial director of thin-film materials in Dow Chemical’s advanced electronic materials business (919-248-9261; pegarrou@dow.com). He received his B.S. in chemistry from North Carolina State University, Raleigh, and his Ph.D. in chemistry from Indiana University, Bloomington. He is an IEEE Fellow and is currently technical vice president of the IEEE Components, Packaging, and Manufacturing Technology Society and associate editor of IEEE Transactions on Components and Packaging. He is a fellow of the International Microelectronics and Packaging Society, where he has served as president and technical vice president. In 2000, he won the William Ashman award for technical achievement in microelectronics packaging.

Jang-Hi Im is a research scientist at Dow Chemical, where he has been employed for 25 years. He currently leads the materials science and adhesion efforts for Dow’s microelectronic materials. He obtained his B.S. in mechanical engineering from Seoul National University, South Korea; his M.S. in mechanical engineering from the Massachusetts Institute of Technology; and his Ph.D. degree in materials science and engineering from MIT. He holds nine U.S. patents and has published more than 50 papers in fracture mechanics of metals and reinforced composites, microlayer coextrusion, liquid crystal fibers, and electronic polymers.

Daniel M. Scheck is responsible for technical customer support, leadership of global field engineering, and process development on new and existing products at Dow Chemical. He received his B.S. in chemistry from the City University of New York and his Ph.D. in chemistry from the University of Wisconsin, Madison. He joined Dow Chemical’s Central Research Department in 1981 and began working on BCB electronic materials in 1993.