Polymeric Materials for Electronics Packaging and Interconnection

Chapter 16

Polymer Insulating Layers for Multilayer Hybrid Circuits 1

L. M . Baker , J. L. Markham, and R. D. Small

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Downloaded by TUFTS UNIV on December 9, 2016 | http://pubs.acs.org Publication Date: September 5, 1989 | doi: 10.1021/bk-1989-0407.ch016

AT&T Bell Laboratories, Engineering Research Center, Princeton, NJ 08540 A new technology for the fabrication of hybrid circuits was developed which greatly enhances the circuit density. Multiple levels of circuitry are built up by alternating layers of an insulating polymer with the metallization. The polymer is a negative tone, photosensitive formulation developed within AT&T. Itsfinalcure temperature is low enough to prevent degradation of devices, such as resistors, capacitors, and conductors, previously prepared on the substrate. The cured polymer is resistant to commonly used cleaning chemicals and to soldering conditions. It has good self-adhesion, as well as adhesion to ceramic and metal. The polymer performs satisfactorily in accelerated environmental tests. The polymer and its properties will be discussed in this paper. Hybrid integrated circuits (HICs) have been used widely for a number of years in switching and transmission equipment and in consumer products. A HIC consists of a ceramic substrate on which thin film resistors, capacitors, and conductors have been formed. Discrete active and passive devices are then bonded to the circuit by one of several techniques, such as soldering, thermal compression bonding, or die and wire bonding. This combination of circuitry types gave rise to the use of the term "hybrid." Circuitry can be placed on one or both sides of the substrate. In addition, the density of the circuitry can be increased by using a fired glaze dielectric to cross one conductor path over another. However, it is difficult to control the impedance of the line accurately or to achieve very high interconnection densities with cross-overs and cross-unders due to difficulties with line width control. In this respect, an organic dielectric has some advantages over an inorganic glaze. (1) An organic dielectric applied in a uniform thin film and appropriately patterned allows increased routing capability,flexibility,and density and, consequently, a reduction in circuit size. Distributed power and ground planes can be used, and the lower dielectric constant of the polymer results in reduced capacitance and conductance between lines. There are several characteristics that would be desirable or essential in a polymeric dielectric for hybrid applications. A major consideration is compatibility with standard hybrid processing. This processing usually includes exposure of the substrates to various chemicals and to elevated temperature. Common processing chemicals to which the polymer should be resistant include trichloroethane, concentrated HC1, methylene chloride, methanol, and dilute hydrofluoric and sulphuric acids. The polymer must be thermally stable to soldering conditions, and, it is helpful if the T of the polymer is high enough to maintain dimensional stability. This is particularly important for reliability considerations, such as thermal cycling. On the other hand, the cure temperature of the polymer must not be so high as to damage any components present in previous g

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Current address: AT&T Bell Laboratories, 1600 Osgood Street, North Andover, MA 01845 Current address: Shipley Company, Inc., 2300 Washington Street, Newton, MA 02162

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0097-6156/89/0407-0192$06.00/0 © 1989 American Chemical Society

Lupinski and Moore; Polymeric Materials for Electronics Packaging and Interconnection ACS Symposium Series; American Chemical Society: Washington, DC, 1989.

16. BAKER ET AL.

Insulating Layers for Multilayer Hybrid Circuits

193

layers of metallization. A major improvement in processing would be achieved if a suitable dielectric were photodefinable, and, in order to build multiple layers of circuitry, the polymer must be metallizable. Material Formulation A survey of materials available at the time this development was undertaken revealed none that had the necessary thermal and mechanical properties and photosensitivity. The combination of chemical resistance and thermal stability with ease of processing suggested that a thermosetting polymer would be the likeliest candidate. Previous experience with triazine resins had demonstrated their superior electrical properties, chemical resistance, adhesion to a variety of substrates, and thermal stability. The T of unmodified triazine is generally around 250 C, depending on cure, and thermal decomposition starts above 450 C. These resins cure through reaction of pendant cyanate ester groups to form s-triazine rings (Figure 1). (2-4) Triazine can be processed much like bisphenol A epoxies, but has a higher crosslink density when thermally or catalytically cured. Because of the high crosslink density, cured triazine is quite brittle. It is not photosensitive. For this application, the triazine resin is modified with an acrylated acrylonitrile-butadiene rubber to improve resistance to cracking and thermal shock and to impart photosensitivity. Workers at Interez, Inc. used thermoplastic resins to modify triazines for improvedfracturetoughness in structural composite matrix applications. For their purposes, the butadiene-acrylonitrile rubbers were not satisfactory. (5) Here, however,fracturetoughness and loss modulus are not as important as maintaining electrical performance and adding photosensitivity. Structural demands on the polymer are slight. The rubber does not react with the triazine resin, so an acrylated epoxy resin is added as a link between the rubber and the triazine. It is known that the cyanate ester reacts with epoxies to form oxazolinerings,and that the reaction is quite efficient (Figure 2). (6) Several other minor constituents are added to optimize properties. In addition to the photoinitiator (2,2dimethoxy-2-phenylacetophenone), these include pigment, hardener (N-vinyl pyrrolidone), a photo-crosslinking agent (trimethylolpropane triacrylate), surfactant, and an adhesion promoter (glycidoxypropyl trimethoxysilane). The entire formulation is blended in a good solvent for all components. A generalized formulation is listed in Table I. (7) The structures of the components are shown in the Figure 3. e

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Table I. Polymer Formulation Level Ingredient 40-65% Triazine Resin 0-30% Rubber 0-50% Acrylated Epoxy Resin 0-12% N-vinyl pyrrolidone 0-8% Trimethylolpropane triacrylate 0-5% Glycidoxypropyl trimethoxysilane 2,2-Dimethoxy-2-phenylacetophenone 0.5-3% 0-1% Pigment 0-1% Surfactant

The rubber and the triazine resin are immiscible, and are kept homogeneous prior to application through dissolution in a mutual solvent. As the solvent evaporates, the triazine and rubber separate into distinct domains, in a manner analogous to rubber modified epoxy resins. (8, 9) This domain structure is substantially frozen during photo-cure when the components containing acrylate and vinyl functionality undergo free radical polymerization. During subsequent thermal cure, the cyanate ester and epoxy groups polymerize to give a hard film. The cured polymer is resistant to organic solvents, but it is attacked by strong base and loses adhesion to the substrate on exposure to hot hydrogen peroxide.


194

POLYMERS FOR ELECTRONICS PACKAGING AND INTERCONNECTION

BPA


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0

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M Figure 1. Polymerization of cyanate ester groups to form s-triazine rings.

Ar-OCN

ArO

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Ν OR

Figure 2. Reaction of cyanate ester with an epoxy to form an oxazoline ring.


16.

BAKER ET AL.

195


(CH =CHCO CH ) CC H t

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Figure 3. Chemical components used i n the polymer formulation.


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Figure 4. Schematic cross-section of POLYHIC fabrication process.


16. BAKER ET AL.


197

Due to the incorporation of the rubber, the thermal properties of the cured polymer are somewhat lower than those of the triazine alone. The thermal properties, along with the electrical properties, are given in Table II.

Table II. Properties of Cured Polymer Value Property Τ 150 C Thermal Stability 180 C long term 210 C short term 300*C spikes Thermal Conductivity 0.196 W/mK 90 C 0.209 W/mK 125 C 3.61 Dielectric Constant (ASTM D150) 353 KV/cm Dielectric Strength (ASTM D149) at 25 C ambient humidity >3.7 χ 10 ohm-cm Volume Resistivity (ASTM D257) > 4.25 χ 10 ohm-cm Surface Resistivity (ASTM D257) e

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e

e

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12

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Results of accelerated testing of circuits prepared with this polymer as the dielectric layer are satisfactory. Circuits comprising two metallization and one polymer layer withstand a minimum of 500 cycles from -40 to +130C with no loss of adhesion, cracking of the polymer, or open circuits. No mechanical or electrical failures occur during 1000 hours at 150 C in air, and the insulation resistance remains above 200 megohm during 500 hours at 85°C/85% RH/60 V (THB). Tantalum nitride resistors under the polymer change < 0.5% under these THB conditions. e

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Application The polymer formulation is applied to the ceramic substrate, and the solvent is removed by a gen tle bake to give a uniform film 2 mils thick. The polymer is patterned by exposure through a mask in a standard uv tool (off contact), and the unexposed regions are removed by solvent development. Polymer residues are cleared away in a plasma process. The polymer is then ther mally cured at approximately 200 C. The polymer is treated with another plasma process to enhance the adhesion of the next level of metallization, which is applied by sputter deposition. The conductor layer is patterned using standard techniques and plated to the necessary thickness. (10) This process is represented schematically in Figure 4. Alternating layers of polymer and metal can be applied by repeating this process. Typical circuits constructed with this procedure have 6 mil vias and 6 mil lines and spaces. e

Conclusions A novel, photodefinable, polymeric material was formulated to meet the needs of a particular cir cuit technology. Rubber modification of a thermoset resin with good thermal, chemical, and electrical properties generated a formulation that met stringent processing and reliability require ments. Literature Cited 1. Shiflett, C. C.; et. al., Proc. International Symposium on Microelectronics, 1986. 2. Grigat, E.; Putter, R., Angew. Chem. 1967, 79, 219.


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3. Ayano, S., Chem. Economy and Engineering Review 1978, 10, 1. 4. Graver, R.B., ACS Polymer Preprints 1986, 27, 491. 5. Shimp, D.A.; Hudock, F.A.; Bobo, W.S., Proc. InternationalSAMPE Technical Conference, 1986. 6. Shimp, D.A.; Hudock, F.A.; Ising, S.J., Proc. InternationalSAMPE Symposium, 1988. 7. Small, R.D., U . S. Patent 4,554,229. 8. Sultan, J.N.; Laible, R.C.; McGarry, F.J., Proc. Applied Polymer Symposium, 1971, 127. 9. Brown, P.J.; Markham, J.L., Proc. SPE ANTEC, 1988, 900. 10. DeForest, W.S., Photoresist Materials and Processes; McGraw-Hill: New York, 1975.


RECEIVED June 2, 1989


Polymeric Materials for Electronics Packaging and Interconnection

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