Polymers for Microelectronics and Nanoelectronics - American

Chapter 21

Downloaded by UCSF LIB CKM RSCS MGMT on November 20, 2014 | http://pubs.acs.org Publication Date: February 7, 2004 | doi: 10.1021/bk-2004-0874.ch021

Benzocyclobutene-Based Polymers for Microelectronic Applications Ying-Hung So, Phil E . Garrou, Jang-Hi Im, and Karou Ohba Advanced Electronic Materials Department, The Dow Chemical Company, Midland, MI 48640

Benzocyclobutene polymerization chemistry is unique and at tractive in microelectronics because the thermally activated BCB ring-opening reaction does not produce any volatiles such as water, and products from BCB reactions are nonpolar hydrocarbon moieties. The excellent film-forming properties of divinyltetramethylsiloxane bisbenzocyclobutene (DVSbisBCB) oligomer solutions and the outstanding electrical, thermal, and planarization properties of the cured final product make these polymers the dielectric materials of choice in a va riety of microelectronic applications. The ductility of DVSbisBCB-based polymer is enhanced with the incorporation of an elastomer in the resin matrix. This paper discusses the chemistry of polymers based on DVS-bisBCB and recent ad vances in product development.

Introduction The continuous evolution of microelectronic systems towards higher fre quency, better performance, lower cost, and robust reliability requires dielectric materials with demanding properties. Some of the notable requirements are low dielectric constant, low dissipation factor, high glass-transition temperature, minimum moisture uptake, good mechanical properties, and good thermal stabil-

© 2004 American Chemical Society

In Polymers for Microelectronics and Nanoelectronics; Lin, Q., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2004.

279

280 ity. No evolution of volatiles during fabrication is also a highly desirable feature


Two key advantages of polymers based on benzocyclobutene (BCB) are (a) the curing process does not emit any volatiles and (b) the products from the BCB ring-opening reaction during polymerization are nonpolar hydrocarbon moieties. This paper focuses on the chemistry of polymers based on divinyltetramethylsiloxane bisbenzocyclobutene (DVS-bisBCB) and how recent ad vances continue to make these polymers important dielectric materials in micro electronic applications.

Benzocyclobutene Ring-Opening The benzocyclobutene ring opens thermally to produce o-quinodimethane. This extremely reactive species readily reacts with dienophiles via a Diels-Alder reaction. In the absence of a dienophile, o-quinodimethane reacts with itself to give a dimer, 1,2,5,6-dibenzocyclooctadiene, or undergoes a polymerization reaction similar to that of a 1,3-diene to give poly(o-xylene) (d, 7).

Polymerization based on the BCB ring-opening reaction has become a popular topic since the mid-1980s. Reaction of the BCB ring-opening intermedi ate, o-quinodimethane, has been used to build macromolecules (8-11). Cross-linked Polymer

Linear Polymer


281 Researchers have also made polymers with pendant BCB moieties and used them as crosslinking agents to transform a linear polymer into a thermoset poly mer {12-14),


Crosslinked Polyamide

DVS-bisBCB Monomer Synthesis and B-Staging The scheme shown below describes the synthesis of divinyltetramethylsiloxane bisbenzocyclobutene (DVS-bisBCB) monomer. The BCB hydrocarbon (15) may be madefromthe pyrolysis of a-chloro-o-xylene (16). Treatment of the hydrocarbon with bromine provides 4-bromo-BCB with excellent yield. Palla dium-catalyzed coupling of 4-bromo-BCB with divinyltetramethylsiloxane pro duces the monomer DVS-bisBCB. Distillation of the monomer can lower ionic impurities to the ppb level if required (17). The viscosity of DVS-bisBCB monomer is 100 cP at room temperature as measured with a Brookfield viscome ter at 20 rpm.

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2

Brominatfon

The Diels-Alder reaction is the predominant reaction in B-staging of DVSbisBCB and in the subsequent curing step to generate the cured product. B-staged DVS-bisBCB solution is formulated in mesitylene and was commer cialized by The Dow Chemical Company as CYCLOTENE 3022 electronic


282 resin. (CYCLOTENE is a trademark of The Dow Chemical Company.) These products are dry-etchable. Products from the CYCLOTENE 4000 electronic resin series contain a diazo crosslinker, 2,6-bis(4-azidobenzylidene)-4-alkyl cyclohexanone. These products are photosensitive.


Curing of DVS-bisBCB Polymer Thin Film The BCB ring starts to open at approximately 160°C and the reaction inten sifies at or above 200°C. The curing kinetics of DVS-bisBCB polymer has been studied in detail (18). The infrared (IR) absorption peak at 147S cm" is assigned to the BCB four-member ring (Figure 1). The peak progressively decreases dur ing cure and virtually disappears at higher than 95% conversion. The absorption peak at 1500 cm" is attributed to the tetrahydronaphthalene group being formed in the product This peak is absent in the monomer and progressively develops into a strong absorbance. The peak at 1251 cm" is the rocking mode of the methyl groups attached to die silicon atoms. The intensity of this peak is unaf fected by the polymerization reaction. The degree of BCB conversion can be determined by the ratio of peaks at 1475 cm" and 1251 cm" . Curing can be completed in minutes at 285°C or in a few hours at 205°C. DVS-bisBCB polymer films are cured in nitrogen. If more than 100 ppm of oxy gen is present in the DVS-bisBCB polymer curing process, IR spectroscopy of the DVS-bisBCB polymer film shows absorption peaks in the carbonyl region, which suggests possible formation of ketones, carboxylic acids, and carboxylic acid anhydrides. The resulting film is a darker color. 1

1

1

1

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DVS-bisBCB Properties The polymer madefromDVS-bisBCB has a low dielectric constant (Figure 2) and dissipation factor, a high T , very low moisture uptake, and a high degree of planarization (Figure 3). The electrical, thermal, and mechanical properties of thin films from photosensitive products are summarized in Table I (19). Poly mersfromthe dry-etchable products have similar properties. Dry-etchable DVS-bisBCB polymer film has >90% transmittance across the visible spectrum (Figure 4). Figure 5 shows the coefficient of thermal expansion for photosensitive DVS-bisBCB polymer as a function of temperature. g

Thin Film Patterning Processes A thin film generally has to be patterned to serve its function in microelec tronics. The patterning of a non-photosensitive polymer typically requires the



Figure L IR spectra of cured and uncured dry-etchable DVS-bisBCB resin containing 20% elastomer.


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284

Figure 2. Dielectric constant of DVS-bisBCB resin as a function oftemperature andfrequency.

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285

Table I. Properties of Thin Films from the Photosensitive Products


Property

Dielectric constant (1 kHz-20 GHz) Dissipation factor (lkHz-lMHz) Breakdown voltage (V/cm) Volume resistivity (Ω-cm) Thermal conductivity (W/m°K) at 25°C CTE (ppm/°C) 50-150°C Tg (°C) Water uptake at 84% relative humidity and 23°C (%) Tensile modulus (GPa) Tensile strength (MPa) Elongation at break (%) Poisson's ratio Residual stress on Si at 25°C (MPa)

Value

2.65 0.0008 3 χ 10 1 χ 10 0.29 52 >350 0.14 2.9 87 8 0.34 28

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Figure 4. UV-Vis spectra of dry-etchable DVS-bisBCB resin and other materials.


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100 125 150 175 200

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Figure 5. Coefficient of thermal expansion for photosensitive DVS-bisBCB res as a function oftemperature. use of a photoresist and more processing steps than for a photosensitive material. Photosensitive DVS-bisBCB formulation is negative acting and is sensitive to Gand I-line. The patterning of dry-etchable DVS-bisBCB resins and photosensi tive DVS-bisBCB resins has been described previously (17).

Elastomer-Modified DVS-bisBCB A limitation of DVS-bisBCB polymer is its restriction in building thick (>50 μχη) stacks because of its moderate fracture toughness. The ductility of DVSbisBCB polymer is significantly enhanced by the addition of an elastomer. Elas tomer-toughened thermosets and thermoplastics are widely used in many diverse industries. Recent advances in this area can be found in three American Chemi cal Society Symposium Series (20-22). The following illustrates recent developments in elastomer-modified DVSbisBCB formulations. A given amount of an elastomer was added to B-staged DVS-bisBCB. Thin films were spin-coated and cured in a nitrogen-purged oven. Transmission electron micrographs (TEM) of DVS-bisBCB polymer films with 10% and 15% elastomer did not show discrete domains of the additive. In a sample with 20 wt % elastomer, some domains with approximate diameter of 0.1 (tm were observed. The areafractionof the observed elastomer was considerably less than 20%. As the amount of the elastomer increased, the amount and size of the domains increased. At 30 wt % the size of the domains was approximately 0.5 μιη, and at 40%, the size was approximately 1 μχη (23). At lower concentra tions, the elastomer is soluble in DVS-bisBCB resin, and as the solubility limit is reached, domains of the elastomer are formed.


287 Chemistry of the Elastomer During DVS-bisBCB Curing Possible chemical reactions of double bonds in the elastomer, which is a poor nucleophile, with o-quinodimethane from DVS-bisBCB were investigated. IR spectra of DVS-bisBCB resin with elastomer before and after cure are shown in Figure 1. Peaks at 987 cm" and 966 cm" are due to the double bonds in DVSbisBCB resin and the elastomer, respectively. The after-cure spectrum shows the obvious intensity reduction of the DVS-bisBCB double bond. The peak for the elastomer double bond did not change noticeably. The result suggests that the elastomer did not react extensively with DVS-bisBCB during cure. Chemical bonding between the elastomer and DVS-bisBCB resin was, how ever, demonstrated by gel permeation chromatography (GPC) coupled with ul traviolet (UV) and refractive index detectors. A mixture of the elastomer and DVS-bisBCB monomer was heated for a short period of time. Only DVSbisBCB absorbs in the U V range. As an aliphatic compound, the elastomer does not absorb in UV. The reacted mixture was injected into a GPC column, and the eluded components were monitored by UV and refractive index detectors. The eluded elastomer was UV-sensitive, which suggests grafting of DVS-bisBCB on the aliphatic chain.


1

1

Electrical Properties Incorporation of the elastomer did not increase the dielectric constant and dissipation factor of DVS-bisBCB resin. At 1 MHz, dielectric constant and dis sipation factor were 2.65 and 0.0008, respectively.

Thermal Analysis DVS-bisBCB resin with 10,15, and 18% elastomer was studied by dynamic mechanical spectroscopy (DMS), and the result for DVS-bisBCB with 18% elas tomer is shown in Figure 6. Only a slight decrease of storage modulus was ob served. Glass transition was not detected in the temperature range of -100°C to 300°C. Thermal stability of DVS-bisBCB resin as free-standing films of about 11 μιη thick and with 0, 5, 10, 15, and 20 wt % elastomer was studied. Samples were held isothermally at 350°C for 2.5 h. Weight loss for all samples was less then 2% per hour. Table II lists coefficient of thermal expansion (CTE) values of DVSbisBCB resin formulation with different amounts of elastomer. The incorpora tion of an elastomer increases the CTE of DVS-bisBCB resin.


288

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Figure 6. DMS of DVS-bisBCB resin with 18 wt % elastomer.

Table Π. CTE Values of DVS-bisBCB Resin with Different Amounts of Elastomer Elastomer %

CTE(ppm/°C)

0 5 10 15

52

57 65 80

Table m. Tensile Properties of DVS-bisBCB Resin with Elastomer Additive Content (%)

0 5 10 18

Tensile Strength (MPa)

87 94 90 95

Strain (%)

8 13.5 18 35

Modulus (GPa) 2.9

2.7 2.5 2.4


289


Mechanical Properties Thin film ductility, as measured by strain at break, was used to determine the toughness of elastomer-modified DVS-bisBCB resin. Tensile properties of DVS-bisBCB resin with different amounts of elastomer are listed in Table III. Strain at break of DVS-bisBCB resin was significantly enhanced with the elas tomer, and tensile modulus was slightly reduced. Figure 7 is the stress versus strain curve of DVS-bisBCB resin with 18 wt % elastomer. There have been reports in the past that formation of discrete domains is important in polymer toughness enhancement (19-22, 24). However, formation of discrete elastomer domains does not seem to be necessary for DVS-bisBCB resin toughness en hancement as TEM of the polymer film with 15% elastomer did not show dis crete domains of the additive. The mechanism for improved mechanical properties of the matrix is being investigated.

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Moisture Absorption The low moisture absorption of DVS-bisBCB resin is maintained in modi fied DVS-bisBCB resin. At 85% relative humidity, for DVS-bisBCB resin sam ples with 5 to 20 wt % elastomer, moisture uptake was less than 0.2 wt %.

Adhesion Properties A DVS-bisBCB resin formulation with 5 and 10 wt % elastomer was ap plied on wafers with Si0 , Si N , aluminum, and copper coatings. AP3000 was used as an adhesion promoter. The wafers were placed in a pressure vessel at 125°C and 2 atm for 96 h. The samples were evaluated by the tape peel test, and 2

3

4


290

Table IV. Adhesion Performance of DVS-bisBCB Resin with Elastomer Wafer Type

Si N Si0 Al Cu 3


2

4

Tape Peel Test Results S% Elastomer 10% Elastomer 0% Elastomer 5B 5B SB SB 5B 5B SB 5B SB SB SB 5B

no detachment of DVS-bisBCB resin from any of the surfaces was observed (Table IV).

Applications DVS-bisBCB resins are notable for their low dielectric constant, low loss factor at high frequency, low moisture absorption, low cure temperature, high degree of planarization, low level of ionics, high optical clarity, good thermal stability, excellent chemical resistance, and good compatibility with various met allization systems. These properties make DVS-bisBCB resins the dielectric ma terials of choice for many applications in the electronics industry. Dry-etchable DVS-bisBCB resin is commercially used by L G Phillips in ToPixel technology in which indium tin oxide is deposited on a completely flat DVS-bisBCB polymer surface. A high degree of planarization of DVS-bisBCB polymer is essential to tins "high-aperture" display technology. A low curing temperature (210°C), low water absorption, rapid curing, and low viscosity make dry-etchable DVS-bisBCB resin an excellent choice for a variety of applications in gallium arsenide (GaAs) integrated circuits. Triquint Semiconductor, Nortel Networks Corporation, and others use DVS-bisBCB to form multilayer intercon nects on their GaAs chips. Others, such as Anadigics, use DVS-bisBCB to en capsulate and thus protect air bridges during plastic packaging. In bumping/redistribution/wafer level chip scale packaging, polymer layers are used for rerouting of perimeter bond pads to an internal array or for recon figuring pads for solder bumping. Photosensitive DVS-bisBCB resin is used extensively for this application in the U.S., Taiwan, Korea, and Europe. Extremely low dielectric constant and low loss at GHzfrequencies,low moisture uptake, and copper compatibility make DVS-bisBCB resin an excellent dielectric material for integration of passive RF components. Waveguide struc tures, which use photosensitive DVS-bisBCB resin as the core material and dryetchable DVS-bisBCB resin for cladding, have been demonstrated by Ericsson and others. The major advantage of DVS-bisBCB resin is its low loss of

Polymers for Microelectronics and Nanoelectronics - American

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