WO2002068525A1

WO2002068525A1 - Positive photodefinable composition of polycarboxylic acid, phenolic and thermocurable resins

Info

Publication number: WO2002068525A1
Application number: PCT/US2002/005909
Authority: WO
Inventors: Shenliang Sun; Yin-Nian Lin
Original assignee: Enthone, Inc.
Priority date: 2001-02-26
Filing date: 2002-02-26
Publication date: 2002-09-06
Also published as: CN1500119A

Abstract

A positive-working photodefinable dielectric composition with minimal thickness loss in the dielectric layer during posTexposure development and/or lessened exposure to ultraviolet radiation during curing comprises (a) a base-developable polycarboxylic acid resin selected from (1) an esterified or hydrolyzed maleic anhydride-styrene copolymer, (2) an acrylic acid (co)polymer, or (3) a (meth)acrylic acid (co)polymer, (b) a base-developable phenolic resin, (C) a thermally curable resin such as an epoxY resin, and (d) a photo-active compound.

Description

POSITIVE-WORKING PHOTODEFINABLE DIELECTRIC COMPOSITION AND

METHOD

BACKGROUND Printed wiring boards (PWBs) are used throughout the electronics industry. These boards serve to mount and secure electronic components while providing electrical connection among them.

Future growth in the electronics industry will be shaped by miniaturization of components and features. The consumer demand for smaller and lighter electronic products has caused product manufacturers to demand small semiconductor packages with increased levels of functional integration. These smaller, more-dense, highly-integrated packages require printed wiring boards having a large number of interconnection paths per unit board area. The high density of interconnection (HDI) required to support these highly-integrated packages can be achieved using multilayer printed wiring boards, wherein several layers of circuitry are laminated within a multilayer board. This type of board offers more potential interconnections per unit area for packaging electronic components than do conventional double-sided boards.

Increased interconnection density in a multilayer board requires vias through the board's dielectric polymer layer, wherein the vias join conductive patterns printed in separate layers on the board. These vias may need to be smaller than can be economically achieved by conventional mechanical through-hole drilling. Vias smaller than what can be achieved by conventional mechanical drilling are referred to as "microvias." The following four technologies have been used by board manufacturer to produce microvias: resin coated foil, laser ablation, plasma machining and photodefinable dielectric materials. Photodefinable dielectric chemistries are classified as either positive or negative working. Positive-working chemistry is rendered soluble to subsequent developer attack following exposure to ultraviolet (UV) radiation. In contrast, negative-working chemistry is rendered insoluble by exposure to UV radiation. Negative-working photodefinable dielectric materials are most common in manufacturing printed wiring boards in the current market. An advantage of positive-working photoresist is that it can be used to naturally generate a "V'-type (shape) via because the top of the film receives a level of

UV exposure that is greater than that which penetrates to the bottom of the film; therefore, the top of the film is rendered slightly more soluble. The V-type of via is desirable in order to achieve effective metallization. Positive-working polymer materials provide higher resolution because, unlike negative-working polymer materials, positive-acting polymers do not swell during development. If used in printed wiring board applications, board yields can be increased by switching from negative- to positive-working photodefinable dielectric materials. Therefore, in theory, positive-working photodefinable dielectric materials would be preferred by printed wiring board manufacturers. However, attempts to produce positive- working photodefinable products have typically been plagued by excessive thickness loss in the dielectric layer during post-exposure development and/or by a necessity for very high exposure energy during the UV exposure step.

SUMMARY A positive-working photodefinable dielectric composition of this disclosure includes a polycarboxylic acid resin and a phenolic resin, both of which are base- developable. The polycarboxylic acid resin includes at least one of the following components: a substantially-esterified or substantially-hydrolyzed copolymer of maleic anhydride and styrene; an acrylic acid polymer or its copolymer; and a methacrylic acid polymer or its copolymer. The positive- working photodefinable dielectric composition further includes a base-developable phenolic resin, a thermally-curable resin, and a photoactive compound. A printed wiring board can be fabricated by laminating a substrate with an electrically-conductive layer (e.g., a copper layer). The positive- working photodefinable dielectric composition is then coated on the electrically-conductive layer. Further, the photodefinable dielectric composition can be exposed to radiation, such as ultraviolet radiation, to enhance the solubility of the dielectric composition in a basic solution. Consequently, unexposed portions of the photodefinable dielectric composition will remain substantially insoluble, while exposed portions of the composition will be rapidly dissolved when exposed to the developer solution. A conductive material (e.g., in the form of a metallic plating) can then be deposited on the dielectric layer by electroless and electrolytical plating processes. The positive-working photodefinable dielectric compositions of this disclosure offer several advantages. First, the combination of the polycarboxylic acid resin and the phenolic resin provides a workable positive-working photodefinable dielectric composition in which particular properties of the final coating can be modified by adjusting the ratio of the two resins. For example, an increased proportion of polycarboxylic acid resin can enhance the developability of the composition, while an increase in the proportion of phenolic resin will improve control over composition surface loss. Second, the compositions can provide higher resolution because the compositions do not swell during development. Third, the glass transition temperature of a film formed with positive-working photodefinable dielectric compositions of this disclosure can have a glass transition temperature around 160°C (320°F), which enables the film to withstand the routine processing and working conditions to which a PWB is subjected. Fourth, positive-working photodefinable dielectric compositions of this disclosure can be plated with copper to produce a coating with high peel-strength adhesion.

BRIEF DESCRIPTION OF THE DRAWINGS The FIGURE is a sectional drawing of a multi-layer printed wiring board with vias passing through a positive-working photodefinable dielectric layer. The drawing is not necessarily to scale.

DETAILED DESCRIPTION

A positive-working photodefinable dielectric composition of this disclosure includes the following four components: a base-developable polycarboxylic acid resin, a base-developable phenolic resin, a thermally-curable resin, and a photo-active compound. The composition is particularly useful for forming a dielectric layer, particular regions of which can be exposed and removed in a photolithography and development process.

The base-developable polycarboxylic acid resin includes a substantially-esterified or substantially-hydrolyzed copolymer of maleic anhydride and styrene; an acrylic acid polymer or its copolymer; and/or a methacrylic acid polymer or its copolymer. As these terms are used herein, "substantially-esterified" or "substantially-hydrolyzed" mean that a substantial majority of anhydride functional groups in the copolymer have converted to acid or acid plus ester groups. Accordingly, the residue of the anhydride groups in the copolymer is less than 10%. The presence of a significant amount of anhydride in the compositions of this disclosure can interfere with the development process due to reaction of the anhydride with hydroxyl groups of novolac resin during the tack dry process. Specific examples of base-developable polycarboxylic acid resins that can be used in accordance with this disclosure include low-molecular- weight styrene acrylic copolymers, such as CARBOSET GA-1160, GA-1161, GA-1162 and GA-2299 polymers from BF Goodrich Specialty Chemicals of Cleveland, Ohio.

A phenolic resin, such as novolac resin (e.g., novolac resin P-4 from Diversitec Corp.), is also incorporated, in combination with the polycarboxylic acid resin, as a base- developable resin. After being deposited as a dielectric layer and upon exposure to UV radiation, the photo-active compound is converted from being insoluble to being soluble in the developer, thereby increasing the solubility of exposed regions of the dielectric layer and consequently creating a substantial differential in the respective solubilities of exposed and unexposed regions of the dielectric layer. Photo-active compounds that can be used in this composition include any positive- working photosensitizer including an ortho-quinone diazide group. Such compounds include l,2-naphthoquinone-2-diazido-5-sulfonyl chloride and esters with phenol and its derivatives, phenol polymers and the like. In particular, an ester of dihydroxybenzophenone, such as 2-diazo-l-naphthol-5-sulfo ester with 2,4-dihydroxy benzophenone from Diversitec Corp. (Fort Collins, Colorado, USA), can be used. It is believed that the photo-active compound (e.g., a diazonaphthoquinone (DNQ) compound) functions by increasing the solubility of the coating in the exposed areas and by decreasing the solubility of the unexposed areas. However, the dissolution rate in the unexposed areas typically will be non-zero, so the thickness of unexposed regions of the dielectric layer will be thinned somewhat during development, though it will be thinned at a rate much slower than that of the exposed regions. It is desired to reduce thickness thinning of unexposed regions as much as possible. Compositions of this disclosure can exhibit thickness thinning of less than 10% in unexposed regions.

The thermally-curable resin can be epoxy; exemplary epoxy resins include aromatic glycidyl ethers, aliphatic glycidyl ethers, other resins having at least diglycidyl ether functionality, and their mixtures. Epoxy resins that are particularly suitable include aromatic and aliphatic glycidyl resins having two or more functional groups per molecule, such as novolak epoxy. The thermally-curable resin can react with phenolic and polycarboxylic resin during thermal (post) curing to form an extended network structure. In addition, a curing catalyst, such as an amino resin (e.g., CYMEL 300, 301, etc. resins from Cytec Industries Inc. of West Paterson, New Jersey, USA), and a catalyst, such as CYCAT 4040 (also from Cytec) can be included to facilitate curing. The positive-working photodefinable dielectric composition can additionally include other components such as a filler composition, such as SYLOID 74 silica filler from W.R. Grace & Co., Davison Chemical Division (Columbia, Maryland, USA) and/or HYDRAL 710 aluminum oxide filler from ALCOA World Chemicals (Bauxite, Arkansas, USA); a defoamer, such as TSA750 silicone from Toshiba Silicone Co. (Tokyo, Japan); a non-aqueous solvent, such as methyl ethyl ketone, 2-ethylbutyl acetate, dipropylene glycol methyl ether (DPM), propylene glycol methyl ether acetate (PMA) and/or dipropylene glycol methyl ether acetate (DPMA).

Suitable concentration ranges for the components, listed above, in the positive- working photodefinable dielectric composition are as follows: 5% to 50% polycarboxylic acid resin, 5% to 30% phenolic resin, 20% to 70% thermally-curable resin, 5% to 20% photo-active compound, 0% to 15% filler, 1% to 3% defoamer, and 30% to 70% non- aqueous solvent. In particular examples, the concentration ranges for these components are as follows: 5% to 30% polycarboxylic acid resin, 5% to 20% phenolic resin, 30% to 50% thermally-curable resin, 10% to 15% photo-active compound, 3% to 6% filler, 1.5% to 2% defoamer, and 40% to 50% non-aqueous solvent.

Additional components that can also be included in the composition include a coupling agent, such as 3-glycidoxypropyltrimethoxysilane and/or trimethoxy[2-(7- oxabicyclo[4.1.0]hept-3-yl)ethyl]silane; a photo-acid generator, such as MP-triazine or TME-triazine; and/or an onium salt group acid generator such as di-(p-tert-butylbenzyl) iodinium trifluoromethanesulfonate.

The positive-working photodefinable dielectric composition 12 can be applied, in liquid form, on a copper layer 14 cladding an epoxy-glass laminate substrate 16. A PWB product 10 incorporating these three layers is illustrated in the FIGURE. The dielectric composition can be applied via a conventional coating process, such as screen printing, curtain coating, spraying, etc. Solvent is removed by heating the dielectric layer at around 160°F (71°C) in a ventilated convection oven or similar drying equipment to achieve a tack-free condition. The board is then exposed imagewise through a phototool in which 500 to 700 mJ/cm² UV radiation passes through a mask and onto the board. The mask defines apertures corresponding to the regions of the positive-working photodefinable dielectric layer that are to be removed.

The exposed dielectric layer is then sprayed with or immersed in a basic developer. The basic developer, which has a pH in the range of 12 to 14, dissolves and thereby removes the exposed regions of the dielectric layer. The developer can be, for example, sodium carbonate, sodium hydroxide, or a mixture of the two.

The developed board is then heated in an oven at 150-200°C for 1-2 hours to further crosslink the dielectric layer so as to achieve the physical and mechanical properties needed for effective functioning of the finished PWB. Next, the board can be plated with an electrically-conductive material, such as copper, to form vias 18 (where the dielectric layer was removed) extending to the underlying electrically-conductive layer. The vias 18 provide interconnect passages between copper layers 14, 20.

EXEMPLIFICATION

EXAMPLE 1

A positive-working photodefinable dielectric composition of the following composition was prepared:

The polycarboxylic acid resin was made by the following procedure. First, 303 g of SMA 1000 (copolymer of styrene and maleic anhydride from ATOFINA Chemicals (formerly, Elf Atochem) of Philadelphia, Pennsylvania) was mixed in 305 g of 2-ethylbutyl acetate. This mixture was heated at 100°C and stirred for two hours. 0.66g of lithium acetate in 24 g of water was then added into the SMA 1000 solution. Stirring was continued at 100°C for 24 hours. The temperature of the mixture was then set to 70°C, and 20 g of methanol was added into the mixture. Stirring continued overnight and a Fourier transform infrared (FTIR) measurement of a sample made in accordance with this procedure showed that most of the anhydride groups were hydrolyzed or esterified.

The above-listed ingredients were added to a mixing vessel and stirred in the vessel with a sharp blade stirrer at 300 revolutions per minute (rpm). In an effort to obtain optimum dispersion, the mixture was passed through a three-roll mill. The milled dielectric composition was then screen printed onto a copper-clad glass substrate to form a first coat having a thickness of 1.1 mils (0.028 mm). The first coat was tack dried at 160°F (71°C) for 20 minutes. A second coat of the dielectric composition was then screen printed on top of the first coat and tack dried at 160°F (71°C) for 40 minutes. The second coat was 1.2 mils (0.030 mm) thick, thereby providing a combined thickness of 2.3 mils (0.058 mm) for the two coats.

After the dielectric composition was coated onto the copper-clad substrate and dried, the board was placed under a phototool, where the dielectric composition was exposed to 700 mJ/cm² of UV radiation. Following exposure, the board was developed in an aqueous solution including 1.5 weight-percent NaOH for two minutes, then rinsed with de-ionized water for one minute. The unexposed region of the dielectric layer was found to have lost less than 5% of its thickness during development, in which time, the exposed region was completely removed, exposing the underlying copper cladding.

The board was then subjected to a pre-cure procedure in which the board was heated at 180°F (82°C) for one hour, then at 200°F (93°C) for one hour, and then at 310°F (154°C) for two hours. The board was then subjected to a desmear (or "surface- treatment") process in which the surface was cleaned and roughened, followed by electroless and electrolytic copper plating, as described below.

In the surface treatment process, the board was immersed in the following compositions in the following sequence for two cycles:

(1) ENVISION MLB-497 permanganate-based oxidizer at 175°F (79°C) for 5 minutes followed by a 5-minute rinse in de-ionized water; and

(2) ENVISION MLB-791 neutralizer for manganate residue at 150°F (66°C) for 2 minutes followed by a 5-minute rinse in de-ionized water. Following surface treatment, metallization of the board commenced, wherein the board was immersed in the following compositions in the following sequence: (1) ENVISION conditioner PPD-9201 at 120°F (49°C) for 10 minutes followed by a 5-minute rinse in de-ionized water;

(2) ENPLATE AD-485 mild etch solution for microroughening at 92°F (33°C) for 1 minute followed by a 1 -minute rinse in de-ionized water; (3) 10% vol. sulfuric acid solution for 1 minute followed by a 1-minute rinse in de-ionized water;

(4) ENPLATE PC-236 acidic activator for 1 minute;

(5) ENPLATE Activator 444 palladium catalyst at 90°F (32°C) for 5 minutes followed by a 5-minute rinse in de-ionized water; (6) ENPLATE PA 493 post-activator at room temperature for 3 minutes, again followed by de-ionized water rinsing for 2 minutes;

(7) ENPLATE electroless CU-406 copper at 90°F (32°C) for 30 minutes;

After the metallization process, the plated board was post-cured at 310°F (154°C) for 20 minutes. The successfully-plated board exhibited a peel-strength adhesion of 4.0 lb/in² (28 kPa) between the plated copper and the dielectric layer.

Each of the above-listed ENVISION and ENPLATE products is commercially available from Enthone-OMI, Inc. (New Haven, Connecticut, USA).

EXAMPLE 2 A positive- orking photodefinable dielectric composition of the following composition was prepared:

This dielectric composition was mixed, screen printed and exposed as described in Example 1, above. The dielectric composition was then developed in a 1.5 weight-percent NaOH solution for two minutes and then rinsed with de-ionized water for one minute. The unexposed region of the dielectric layer was found to have lost less than 5% of its thickness during development, in which time, the exposed region was completely removed, exposing the underlying copper cladding.

After the dielectric layer was developed, the board was pre-cured at 180°F (82°C) for 1.5 hours, then at 200°F (95°C) for two hours, and then at 310°F (154°C) for two hours. The board was then subjected to the desmear and copper plating processes described in Example 1. After copper plating, the board was further post cured at 310°F (154°C) for 20 minutes. The sucessfully-plated copper had a peel strength adhesion of 4.9-5.0 lb/in² (34-35 kPa) between the copper layer and the dielectric layer.

While this invention has been particularly shown and described with references to embodiments thereof, those skilled in the art will understand that various changes in form and details may be made therein without departing from the scope of the invention, as encompassed by the appended claims.

Claims

What is claimed is:

1. A positive-working photodefinable dielectric composition comprising:

(a) a base-developable polycarboxylic acid resin including at least one component selected from the following group: (1) a substantially-esterified or substantially-hydrolyzed copolymer of maleic anhydride and styrene;

(2) an acrylic acid polymer or its copolymer; and

(3) a methacrylic acid polymer or its copolymer;

(b) a base-developable phenolic resin; (c) a thermally-curable resin; and

(c) a photo- active compound.

2. The positive-working photodefinable dielectric composition of claim 1, further comprising filler.

3. The positive- working photodefinable dielectric composition of claim 1, further comprising defoamer.

4. The positive- working photodefinable dielectric composition of claim 1, further comprising non-aqueous solvent.

5. The positive-working photodefinable dielectric composition of claim 1, wherein the thermally-curable resin is epoxy.

6. The positive-working photodefinable dielectric composition of claim 5, wherein the thermally-curable resin includes at least one component selected from the group consisting of: aromatic glycidyl ether; aliphatic glycidyl ether; and other components having at least diglycidyl ether functionality.

7. The positive- working photodefinable dielectric composition of claim 1, wherein the photo-active compound is a positive-working photosensitizer including an ortho-quinone diazide group.

8. The positive- working photodefinable dielectric composition of claim 7, wherein the photosensitizer includes a diazonaphthoquinone compound.

9. The positive-working photodefinable dielectric composition of claim 7, wherein the photo-active compound includes an ester of di-hydroxybenzophenone.

10. A multi-layer printed wiring board, comprising: (A) a substrate; (B) an electrically-conductive layer coated on the substrate; and

(C) a positive- working photodefinable dielectric composition coated on the electrically-conductive layer, the dielectric composition including:

(1) a base-developable polycarboxylic acid resin including at least one component selected from the following group: (a) a substantially-esterified or substantially-hydrolyzed copolymer of maleic anhydride and styrene;

(b) an acrylic acid polymer or its copolymer; and

(c) a methacrylic acid polymer or its copolymer;

(2) a base-developable phenolic resin; (3) a thermally-curable resin; and

(4) a photo-active compound.

11. The multi-layer printed wiring board of claim 10, wherein the electrically- conductive layer includes copper.

12. The multi-layer printed wiring board of Claim 10, wherein the photo-active compound is a positive-working photosensitizer including an ortho-quinone diazide group.

13. A method for fabricating a printed wiring board comprising the steps of: coating a substrate with an electrically-conductive layer; and coating the electrically-conductive layer with a positive-working photodefinable dielectric composition including a base-developable polycarboxylic acid resin including at least one component selected from the following group: a substantially-esterified or substantially-hydrolyzed copolymer of maleic anhydride and styrene, an acrylic acid polymer or its copolymer, and a methacrylic acid polymer or its copolymer; a base-developable phenolic resin; a thermally-curable resin; and a photo-active compound.

14. The method of claim 13, further comprising the step of exposing the photodefinable dielectric composition to radiation.

15. The method of claim 14, wherein the radiation is ultraviolet radiation.

16. The method of claim 13, further comprising the step of applying a developer composition to the photodefinable dielectric composition, the developer composition selectively removing regions of the photodefinable dielectric composition that were exposed to the radiation.

17. The method of claim 16, further comprising the step of applying an electrically- conductive composition to regions where the exposed dielectric composition was removed.