2

Electrodepositable resists

D. Merricks

2.1 INTRODUCTION

Electrodeposition, also called electro coating, is a process by which organic materials are coated from aqueous suspension, or solution, onto a conductive substrate under the influence of electricity. The process utilizes direct current for depositing resins, so that predominantly electrophoretic processes operate. Electrophoresis in this context is understood to mean the migration of colloidal or suspended particles in an electric field. The particles migrate, according to their charge, to the anode (anaphoresis) or to the cathode (cataphoresis).

Although the principle of electrophoresis has been known since 1809, from the work of Reuss, it has remained confined to a very few areas of application in medical, analytical and other technological fields. The process of electrophoretically depositing paints and lacquers could only be applied industrially when new ionizable paints and resins were developed that could be diluted with water and deposited from an aqueous medium under the influence of an electric current, similarly to the electrodeposition of metals (although the electrodeposition of organic material is much more complex). It was not possible to electrodeposit conventional organic-based paints, since these did not form ions, and known water-soluble paints that could be applied by conventional immersion or spraying techniques were too expensive.

The new technique gained industrial significance when the Ford Motor Co. [1] elaborated a method for prime-coating metal automobile bodies. Following several years research to produce inexpensive, safe, water­soluble electrodepositable (ED) paints, the first production facility opened in 1963. The superior coating performance, uniformity on complex surfaces, freedom from pinholes, efficient use of paint solids, reduced solvent emission and reduced overall costs led to rapid worldwide market penetration in the automobile and other sheet-metal

J. A. Chilton et al. (eds.), Special Polymers for Electronics and Optoelectronics© Chapman & Hall 1995

38 Electrodepositable resists

coating industries. Offset against these advantages are the relative difficulty in formulating paints and the greater control of bath stability and operation required, although the advantages far outweigh the disadvantages.

Early electropaints were anaphoretic (anionic), and consisted of negatively charged particles depositing onto a positively charged substrate. Cataphoretic (cationic) paints were introduced in the early 1970s and rapidly became dominant because of the reduced metal erosion and staining achievable by negatively charging the substrate.

ED paints may be thermally or photochemically cured for improved performance. None the less, it was some time before serious attempts were made to use ED photocurable films as resists for metal patterning. It had been foreseen that dry-film photoresists, which have been the mainstay of the printed circuit board inner-layer fabrication process for the last two decades, would soon reach their resolution limit and that a process that coated much thinner layers of resist would take over. ED resists that were capable of coating layers up to five times thinner than dry film seemed the natural successors. In 1986 the Rohm and Haas Co. [2] issued a patent describing a photoresist composition for cataphoretic deposition onto copper during the process of forming a printed circuit board. Many other patents in this field, describing both cataphoretic and anaphoretic deposition of a wide variety of resins, have been issued since then.

ED photoresists are stable aqueous micro-emulsions from which a film of organic photoresist may be deposited onto a circuit board or other substrate. They are used primarily for high-resolution selective etching and pattern plating in the manufacture of printed circuit boards (both inner and outer layers), 3D moulded interconnect devices, multichip modules and other interconnect-level electronic devices. Other applications include use as permanent dielectrics, in primary imaging for chemical milling (i.e. lead frames and printing rollers), as decorative coatings, and replacements for metallic etch resists such as tin-lead solder. Some of these processes can utilize non-photoimageable ED resists.

With their attractive properties, these resists have the potential to displace both solvent-based liquid resists and dry film photoresists in most present-day applications involving conductive substrates.

This chapter concentrates on the chemistry and application of ED resists for the printed circuit board and chemical milling industries. Details of other applications of ED resists have been extensively covered elsewhere; see e.g. [3].

Following this introduction, the chapter continues with a description of the principles of electrodeposition (section 2.2). The various types of polymers employed in ED resist formulations are then described (section

Principles of electrodepusition 39

2.3), as are the other components necessary for the preparation of practical resist emulsions (section 2.4). Finally, in section 2.5 the different applications of ED resists are described in more detail.

2.2 PRINCIPLES OF ELECTRODEPOSITION

2.2.1 Introduction

Electrodeposition, which also encompasses metal electroplating and the coating of various dispersed solids, is used here specifically with reference to the deposition of emulsified organic material onto a con­ductive substrate. The two methods of electrodeposition are cataphoretic coating, in which the part to be coated is made the cathode, and ana­phoretic coating, in which the part to be coated is made the anode.

In both cataphoretic and anaphoretic resists an ionized polymer acts as a surfactant to emulsify itself and the other resist ingredients in water. The resulting micelles are typically of the order of 50-200 nm in diameter and bear a surface charge. A typical micelle in a cataphoretic emulsion, surrounded by a diffuse layer of counter-ions, is shown in Fig. 2.1.

Coulombic repulsion of like charges keeps the particles sufficiently separated to avoid flocculation and settlement.

-RC02

+NHR2

RC02

-RC02

+NHR2

DYE PHOTOSYSTEM

SOLVENT

+NHR2

-RC02

Fig. 2.1 Micelle in cataphoretic resist.

-RC02

+NHR2

-RC02

40 Electrodepositable resists

The following discussion concentrates on the mechanism of cataphoretic coating for simplicity. The principles of anaphoretic coating are basically the same, the only difference being reversal of the charges on the micelles and electrodes.

In a cataphoretic emulsion, when an electric field is applied (approximately 1 kV m -1), micelles migrate by electrophoresis toward the cathode at the rate of micrometres per second. In addition, all the water-soluble components also migrate with the micelle. The conductivity of the solution permits controlled electrolysis, and water decomposes to raise the pH at the cathode and lower the pH at the anode. The anode is usually made of an inert material, such as stainless steel, since, being the oxidizing electrode, dissolution is possible.

When the micelles reach the cathode, their positive charges are neutralized by hydroxide ions produced by the electrolysis of water. The micelles then become destabilized, and coalesce on the surface of the cathode to form a self-limiting, insulating film that emerges nearly dry from the coating bath. As the resistance of the film increases, so does the potential across the film, causing water and occluded ions to be forced out of the coating by electro-osmosis. The small size of individual micelles results in good packing densities and even coatings. Thus the com­bination of electrophoresis and electrolysis produces a highly uniform and defect-free coating. The self-limiting nature of ED coatings is mainly dependant on voltage, coating time and bath temperature (section 2.2.3), and conditions can be used that build up quite thick (greater than 50 ,um) films. Under optimum conditions for these coatings, film growth continues until its resistance is so high that the electric field across the emulsion is too low to induce delivery of the micelles, or the current flow becomes low enough that the cathodic pH is too low to induce coalescence. As a result, films should be uniform, even if they deposit at different rates across a part.

Meanwhile, on reaching the anode, carboxylate anions are neutralized by hydrogen ions from the electrolysis of water. As resist solids are removed from the bath at the cathode, there is a gradual build up of ionizer in the bath. Therefore, to maintain bath chemistry, free acid must be removed by ultrafiltration, or drag-out, or the use of semipermeable membranes (anolyte boxes) [3].

Although ultrafiltration can easily control resist-bath conductivity, any small water-soluble molecule is allowed to pass through the membrane, including solvent, which constitutes an environmentaVwaste treatment problem. Many ED resists have been formulated to include water-soluble solvents, and maintaining bath solvent level therefore requires periodic analysis and addition of fresh solvents. (This is not the case with resists that have been ultrafiltered during manufacture to remove water-soluble solvent (section 2.4.10).)

Principles of electrodeposition 41

2.2.2 Cataphoretic and anaphoretic emulsions

Manufacturers of cataphoretic and anaphoretic resists have previously reported the benefits of their own particular processes, and in doing so some of the advantages of their product and disadvantages of competitors products seem to have been rather exaggerated [4]. Many of these advantages and disadvantages are not relevant [5].

For example, stated advantages of anaphoretic systems are their self-cleaning nature, due to copper surfaces being micro-etched in the ED bath during coating and their ability to deposit more than 25 Jim, unlike cataphoretic systems, which only coat up to 12 Jim. The first advantage can also prove to be a disadvantage in some resists, since copper ions are incorporated into the resist and can lead to developing problems and a limited bath life. The second advantage is not relevant, since cataphoretic systems have been formulated that can easily coat over 50 Jim. Similarly, stated advantages of cataphoretic systems are that their faster coating rates enable greater throughput and make them easier to control, while their ability to deposit much thinner coatings than anaphoretic systems leads to greater product yields and resolution. Again, anaphoretic systems have been formulated that deposit coatings as fast and as thin as cataphoretic resists.

However, the major potential problems associated with each type of resist do depend on whether it is coating the reducing (cathode) or oxidizing (anode) electrode.

In cataphoretic deposition no metal ions are incorporated into the coating from electrode dissolution, but the volume of hydrogen produced can cause problems through the formation of resist defects (pinholes) if it is not removed.

In anaphoretic deposition much less gas is evolved from the anode, but metal ions dissolved from the anode are incorporated into the film, very few of them passing through into the resist bath. It has been recognized in the electrodeposition of paints [3] that anaphoretic coatings are more susceptible to corrosion than cataphoretic coatings, and they result in poorer performance.

In fact, both of these potential problems have solutions described in section 2.4.9.

Figure 2.2 shows cell diagrams for typical cataphoretic and anaphoretic resists.

2.2.3 Factors affecting electrodeposition

There are several important system variables that control the electro­deposition of a coating. These include the chemistry of both the aqueous and organic phases, the applied voltage and electrode geometry, coating time and bath temperature.

+

~ ~

°z

H2

H20

-2e

-+

2W

+!

02

H20

+

e

-__

_ -

OH

~ H

2

NR

3+

6 -R

C0

2-

W +

RC

02-

ONR 3

-+-O

H

RC

02H

f Q

-N

R2

+ H

20

(a)

Cat

apho

retic

Fig

. 2.2

Cel

l d

iag

ram

s.

- II

1'-

+ f

~

~ II

02 HZ

_iii

H20

-2e

-+

2H

+ +~ 0

2 H20

+

e----

-O

H ~ H

z

CO

:!"

-0

C

O2-

W+

G

t 6JZH

R3

HW

-

R3H

N"

+ -

OH

R3N

+ H

;p

(b)

Ana

phor

etic

Principles of electradepasitian 43

The most important aqueous-phase parameters appear to be conduct­ivity and acidity. Likely effects of variation of these parameters are an alteration of micelle migration rate (due to changes in surface charge) and control of the electrolysis of water, which may have significant implications for the interfacial microstructure of the final coating and therefore film adhesion. Typical conductivities are in the (2-8) X 10- 2 S -1 m range.

Since the electrophoretic mobility and charge neutralization rate both depend on the amount of ionized groups on the micelle, the exact chemical nature of its contents is important. For a closely related group of formulations in which the concentration of ionizable groups does not vary widely, the most important variable may be the extent to which the groups can populate the micellar surface for ionization. This in turn may depend on the arrangement of groups on the polymer backbone, polymer glass transition temperature Tg, molecular weight distribution, dielectric constant and hydrophilicity of the organic droplets. Percentage solids present in the emulsion have very little effect on coating thickness and quality.

For a given emulsion and cell geometry, the rates of electrophoretic transport and electrolysis should both be approximately linear in applied voltage. Increasing the applied voltage (and hence the field strength) usually leads to the deposition of thicker coatings, especially at higher bath temperatures.

Both constant-voltage and constant-current coating methods have been employed, the latter eliminating large current 'spikes' that can occur on switching on the voltage and can cause problems with some resists at the beginning of electrodeposition.

The effect of changing voltage, however, is closely linked with other variables such as coating time and bath temperature. Generally, at constant voltage, increasing coating time leads to thicker deposits, but usually only at relatively high temperatures, where the coatings are not self-limiting (Fig. 2.3). Typical ranges for coating time and applied voltage are 10 s-3 min and 10-250 V respectively.

Figure 2.3 shows a plot of resist thickness versus bath temperature at constant coating time and voltage for a typical acrylic-based resist emulsion.

The temperature that gives minimum thickness (T min) can be obtained from such a graph. Coating tends to be self-limiting in nature at or close to T min' At low temperatures (more than, for example, 10 °C below T min)

complete coalescence is inhibited, a hard porous resist builds up without limit and the adhesion is very poor. At temperatures around 5-10 °C below T min film growth appears to be limited by coalescence of micelles arriving at the substrate. As a continuous film begins to form, the electric field driving electrophoresis gradually diminishes, since more of the cell

44 Electrodepositable resists

20

15

E ::1.

U) U) Ql c ~ 0 10 E l-v; ·iii Ql a:

5

20 25 30

Temperature(°C) Fig. 2.3 Resist thickness versus bath temperature.

35 40

voltage drops across the growing film than across the bath emulsion. Reproducibility of coating thickness can be quite good in this region of the curve. The form of the low-temperature branch of the curve probably reflects the rapid change in organic-phase viscosity near the glass transition temperature Tg . At high temperatures (above T min) film growth continues more rapidly, and very thick coatings can be obtained. The micelle viscosity, which is lower, and the increased film conductivity contribute to the increase in thickness. The increase in conductivity through the film could arise from increased ionic transport, helped by continuous rupture and regrowth of the low-dielectric-strength film.

Other important variables are the chemical and physical natures of the surface to be coated. Changes in the origin and pre-clean treatment of the conductive substrate can have profound effects on both the thickness and performance of electrodeposited coatings.

2.3 POLYMERS FOR ELECTRODEPOSIT ABLE RESISTS

2.3.1 Introduction

A variety of electrodepositable film forming resins are known, the most often utilized to date being acrylates, epoxies and novolacs, although the

Polymers for electrodepositable resists 45

use of polymers containing more diverse functionalities is gradually increasing.

Structural examples of the three most commonly used resins are shown in Fig. 2.4.

The synthesis of these polymers is carried out using standard methods, and is described in detail elsewhere [6]. In order for a polymer to be electrodepositable, it must contain a distribution of ionizable groups along its molecular chain. Polymers may be cationic, containing basic sites such as amino groups, or they may be anionic, containing acidic sites such as carboxylic, sulphonic or phosphoric acid groups.

In some instances amphoteric polymers are known with both acidic and basic groups in the same chain.

In the case of resins for photoresists the polymer may also contain photosensitive functionalities, such as sites of unsaturation in the case of negative-working photoresists, or have photo-active groups such as diazo naphthoquinone (DNQ) attached, as in positive-working photo­resists. Although some polymers are formulated with such functionalities present, in the most commonly used polymers such groups are absent and the main function of the polymer is as a carrier for the other resist additives (section 2.4.1).

Another requirement of the polymer is that it must be able to form a stable emulsion with suitable ionizers such as inorganic or organic acids (which protonate the ionizable groups in cationic resins), or bases (which deprotonate the ionizable groups in anionic resins) in the presence of other resist additives, including photoinitiators, diazonaphthoquinone photoactive compounds (DNQ-PACs), cross-linking agents and so on. Some polymers used in commercial electrodeposition processes contain sufficient ionizable groups to be water-soluble, so that both emulsified and dissolved material is simultaneously deposited along with sus­pended pigment and other components.

ED polymers exhibit a wide range of glass transition temperatures Tg•

Resists made from polymers with high Tg usually require the presence of hydrophobic solvents that act as plasticisers. This ensures that the deposited coating is not brittle and therefore exhibits good adhesion to the conductive substrate.

2.3.2 Anionic acrylic polymers

By far the most commonly used polymers for electrodeposition are those based on copolymers of acrylic or methacrylic acid with their esters and amides. The ionizable group is provided by the parent acid, while other monomers are incorporated in varying amounts to control resin properties such as Tg, flexibility and strengh (methacrylate monomers usually promote rigidity, while acrylate monomers tend to impart flexibility to the final polymer).

Acr

ylic

Epo

xy

Nov

olac

r CH'-~

l

1 c:-

r R

H

A

cryl

ic a

cid

R =

CH3

Met

hyl m

etha

cryl

ate

/0

I

OH

1

0 C

HZ

.:cH

-CH

2l O

-@

t@-O

CH

z-tH

-CH

zl0-

@t@-

oCH2

-~-'

tHz

Bis

phen

ol A

-epi

chlo

rohy

drin

epo

xy

OH

O

H

~CHZ'CcCHZ

VV

"\.

:7 I

? I

~

CH

3 C

H3

m-C

reso

l o-

link

ed n

ovol

ac

Fig

. 2.

4 R

esin

s u

sed

in E

D r

esis

ts.

Polymers for electrodepositable resists 47

Examples of widely used (meth)acrylate monomers are given in Table 2.1.

Anionic acrylic resins have been formulated in both negative- and positive-working photoresists. A number of negative-working resists are described in patents filed by the Kansai Paint Co. Ltd, Japan [7]. Some examples of polymers used in these resists are shown in Table 2.2. These copolymers are not solely carriers for other resist additives when formulated into emulsions, since they also contain sites of un saturation on the backbone. Therefore, they do not require the addition of separate crosslinking agents. (In negative-working photoresists, after irradiation with ultraviolet (UV) light, the crosslinked areas become insoluble in suitable developers; section 2.5.1). The amount of acrylic acid is controlled to give the desired acid value in the polymer (the number of ionizable groups and the degree of neutralization by a basic counter-ion control the micelle size in the final resist emulsion).

The polymers for negative-working photoresists (Table 2.2) typically have epoxide groups as the site of reactivity. Vinyl groups are also incor­porated into acrylic polymers as the crosslinking moiety [7]. An example

Table 2.1 Acrylic monomers

H I

CH2=C I C~H

Methacrylic acid

Acrylic acid

Butyl acrylate

Hydroxyethyl methacrylate

Methyl acrylate

Methyl methacrylate

Glycidyl methacrylate

Tab

le 2

.2 A

nion

ic a

cryl

ic r

esis

ts

No.

1 2 3

Pol

ymer

a

r: CH

3 ~E

H

jE

H

=rE

C

==

rH3

C 3H

L

H2

-?C

H2

-? "C

H2-

?

CH2-

?

CH2-

? .

C

0 2CH

3 CO

z C 4

H,

CO

C0 2

H

CO2

~H

I ,0,

CH3 +

CH

3 CH

2CHC

H2

CH2C

OCH

3

(2)

(1)

(1)

(1)

(1.2

)

E H

H

H

H

CH

3

CH

c?

~EH2-?

jEH2-?~H2-?~2-?~

Ph

COzC

H 3

CO ~ L

cOzH~L

COz ~

~H

I t,

CH3+

CH

3 CH

2CHC

H2

CH2C

OCH

3

(2)

(1)

(4)

(3)

(3.5

)

-E CH

3 jE

H

---=

rr;

H--=

rr: C~

CH

2-t

2C

H3

CH

2-t

~ L

H2-t

-.JL2

-t Oz ~

COz"

C 4H

, CO

zH

1,0

, CH

2CHC

H2

(2)

(2)

(1)

0.2

)

Solv

ent

Neg

ativ

e-w

orki

ng a

nion

ic

acry

lic

resi

ns

2-M

etho

xypr

opan

ol

But

yl c

ello

solv

e

2-M

eth

ox

yp

rop

ano

l

Pos

itiv

e-w

orki

ng a

nion

ic

acry

lic

resi

ns

Ref

.

[7a]

[7a]

[7b]

-E C~

CH

] ~E

~ ~E ~~

Iso

pro

pan

ol

[7b]

4

CH2-

~ CH2-~CH2-y n

CH

2-Y

CO

2 C

0 2C

H]

CO

2 C

4!t,

C

02H

I

. yH

2

CH

3 yH

2

CH3~N--S02

CH,

©9 N

2

° (5

.8)

(5)

(4.3

) (1

)

-E H~

CH

] jE

C~

Dig

lym

e [8

] 5

CH2-

~ CH2-~ 'C

H2-t

m,H

oo

,"C

,', 9

CH

] C

H]

N=C

=O

(1)

(8.4

) (3

.8)

-E2 -{:

-3EH2

-{

jE2-

{~

2-M

eth

ox

yp

rop

ano

l [9

] 6

Ph

CO

lC4!

t,

C~H

(1.8

5)

(6.4

) (1

)

Tab

le 2

.2 C

ontin

ued

No.

7 8

Poly

mer

"

r= C

H3 II

: C

H3 I"

cH

)1

r::

CH

) I

L2-t-=:JLH2-t---=rcCH2-t~H2-t--=r

I I

I I

C0

2CH

J C

02H

C

02C

H2C

HzO

H

C02CH2IH~H5

(CH

2))

I .

CH

)

(3.9

5)

(1)

(4.2

6)

(3.9

5)

r= cH3

1i:

H

j~

CH

J II

: H

I

LH2-t-

=rLH

2-~C

H2-~

=-==n==.H2-~=r

I °

C02nCi~

CO

:!CH

3 C

O

II I

CO

:!(C

Hz}

30C

+N

H ~OH

CH

3 C

HJ

IQJ

CH

2SO

)H

(5.7

8)

(3.5

4)

(4.1

3)

(1)

Solv

ent

Ref

·

2-M

eth

oxyp

rop

anol

[1

0]

2-M

eth

oxyp

rop

anol

[1

0]

9a

9b

c C~

H

JE c~

LH2-

~ rLrl

-L .-

J~H2

-~ n

CH2

-t

.-J

CO:z

CH3

CO

2 C4~

I

° +

11 C

OiC

H:z

)P

C I CH

2 °

C3H7-tH-~CO:zH

(1.0

8)

lQJ

(1)

(1)

[~:(o,

H OR

OR

Ho

,c~

OC

H2-

tH-C

H2+

OC

H2C

H2 +

oC

HiH

-CH

zO

n

a N

um

ber

s in

par

enth

eses

indi

cate

par

ts b

y w

eigh

t.

But

yl c

ello

solv

e [1

1]

2-M

etho

xypr

opan

ol

[11]

52 Electrodepositable resists

is a 2:1 adduct of hydroxyethyl methacrylate with toluene diisocyanate, which has two acrylate groups, one of which remains free after incorpor­ation into the polymer.

Acrylic polymers that do not contain any reactive site of unsaturation have also been utilized [12]. These polymers, which contain acrylic or methacrylic acid, are simply mixed together with a hydrophobic monomer containing two or more photopolymerizable unsaturated groups.

Some examples of acrylic polymers used in positive working photo­resists are also shown in Table 2.2. The photoactive group can be incorporated into the polymer backbone or added to the resist as a separate component (section 2.4.5; in positive-working photoresists the areas irradiated with ultraviolet light become soluble in certain developers). Compounds 7 and 8 are examples of polymers that do not contain photoactive groups.

Polymers such as compound 4 in Table 2.2 may have all the t-butyl­aminoethyl methacrylate moieties converted to DNQ-sulphonamides by preforming the sulphonamide monomer and then effecting polymer­ization with the other monomers to give the desired product. Alter­natively, polymers such as compound 5 can be made with varying percentages of the isocyanate sections containing the photoactive compound (PAC), by preforming the polymer containing the reactive isocyanate group and then reacting this with the desired amount of a PAC as shown in Fig. 2.5 [8].

Polymers 9a and 9b are mixed together in the formulated resin. The R groups in 9b are the photoactive DNQ-sulphonyl groups.

An alternative acrylic copolymer for positive-working photoresists is one containing t-amylacrylate or methacrylate with acrylic acid and n-butylacrylate [13]. A photo-acid generator is also present in the resist; after irradiation with UV light, the t-amyl groups are degraded by the acid produced to leave an increased concentration of carboxylic acid groups in the exposed regions. The exposed region thus becomes soluble in a basic developer. .

R-N=C=O + ~N2 Qy

S02NCH2CH20H I CH3

Fig. 2.5 Preparation of a photo-active compound.

o II

N~ y9

R-NHCOCH2 CH2- N-S02 I CH3

Polymers for electrodepositable resists

CH3 I

CH2=C I C02CH2CH2N( CH3)2

Fig. 2.6 Dimethylaminoethyl methacrylate (DMAEMA).

2.3.3 Cationic acrylic polymers

53

The acrylic monomers that make up cationic polymers are usually the same as those that make up anionic polymers (Table 2.1). Cationic acrylic polymers contain a basic ionizable group, which is usually introduced in one of two ways. The most common method is copolymerization of monomers with an amine group containing monomer, such as dimethyl­aminoethyl methacrylate (DMAEMA, Fig. 2.6).

The alternative method is to preform a copolymer that includes a reactive site capable of being attacked by a nucleophile, such as an epoxide group. In this case, the nucelophile is an amine, such as N-methylethanolamine. Table 2.3 gives typical examples of cationic acrylic polymers along with the type of resist in which they are used.

The first polymer in Table 2.3 is a typical cataphoretic acrylic resin, to which a monomer is added to provide sites of unsaturation. Polymer 2 is reacted with N-methylethanolamine to give the ionizable t-amine group. Polymer 3, which is incorporated into a positive-working photoresist, is preformed containing the PAC, whereas the PAC is mixed with polymer 4 during resist formulation. The final example in the table is not formu­lated with any photosensitive group. The resist here is used as an etch­resist, replacing tin-lead solder, in the manufacture of printed circuit boards.

2.3.4 Amphoteric acrylic polymers

Acrylic copolymers that contain both basic and acidic functionalities are also known. These resins can form either cataphoretic or anaphoretic emulsions by neutralization with acids or bases respectively [10]. An example is the polymer shown in Fig. 2.7, which contains methacrylic acid and DMAEMA; the latter is neutralized with lactic acid, producing a cataphoretic resist emulsion.

2.3.5 Epoxy polymers

Useful properties of epoxy resins include good adhesion, high chemical resistance and flexibility. However, when used in ED resists, they appear to be more difficult to remove than acrylic coatings. Epoxy resins used for

Tab

le 2

.3 C

atio

nic

acry

lic

po

lym

ers

No.

1 2

Pol

ymer

"

-E C

H3 ~E

H ~E C~H3

H2

-{

CH

Z -{

cH

z-b

CO

zCH

3 C

OlC

4Hg

I C°z

CH

2CH

2N(C

H3)

2

(8.5

) (3

.4)

(1)

CH

2-{

CH

2 -{

CH

2-b

-E

CH

3 jE

H ~E C~HJ

C~CH

3 C~nC4Hg

I (1

) (1

.2)

/0

,

C~CH2 -

CH

-CH

2

(1.2

)

3 r=

CH

3

LH

z -b

] ~

I E

HZ

-C

I r-::

H

o

t::-::

:JL"

'-t

=j~

CH

C

,0

,'

CH

"

I ~ ¢

r"

CH

C

o,"

c"

-,-

-c '"

--

, C

H

' '

.'.

,=-=

r C

H

" rn

, co

,rn,

J

CH

-<

IP

I CH

3~1

2 N

(CH

) ~N

32

·--S

02

CH

3

(2.9

) (1

) (2

.1)

(2.1

)

Res

ist t

ype

Ref

·

Neg

ativ

e [2

,14]

Neg

ativ

e [8

]

Pos

itiv

e [7

b]

4 -E

H ~E H~

CH

3

C'"-t

~H'i

~ L"1~

CO

lC4f!

, C

QzC

H2C

H2N

(CH

3)2

(4.1

) (1

.3)

(1)

5 -E

CH

':3

E

c~

H--=

U=

H

I

C"1

-m,t

-=:J

LH'-r-

;:JCC

H'1-=

r C

0 2C

H2C

H2N

(CH

3h

C~CH3

C~ C

4f!,

II

CH

3

o

(1)

(8.5

) (2

.9)

(0.1

3)

a N

um

ber

s in

par

enth

eses

indi

cate

rea

ctio

n p

arts

by

wei

ght.

Pos

itiv

e

No

n-p

ho

to­

imag

eab

le

[9]

[15]

56 Electrodepositable resists

rh CH3 CH3 CH3

+lli -rl C"'-f --t f"'- cH 1~ C--+ COlCH3 COlCH1CHlOH COlH C01CHlCHlN(CH3)l

(9) (8) (2) (1)

Fig. 2.7 Amphoteric acrylic polymer.

ED resists are usually manufactured from a phenol, such as bisphenol A, by reaction with epichlorohydrin.

Ciba-Geigy have described the use of epoxy resins in organic ED coatings that replace tin-lead solder as etch-resists [16]. A typical resin, which is formulated into a cataphoretic resist, is shown in Fig. 2.8.

Both negative and positive working photoresists have been formulated with epoxy resins [11, 17]. The example in Fig. 2.9(a) is mixed with a polyamine resin to give a cataphoretic, negative-working resist [17], while that in Fig. 2.9(b) is reacted with a DNQ-sulphonyl chloride to give an anaphoretic, positive-working resist [11].

2.3.6 Novolac polymers

Novolacs, which are a class of phenol-formaldehyde resins, are polymers formed by the interaction of a phenol, or a mixture of phenols, and formaldehyde. A molar excess of a phenol or cresol is reacted with formaldehyde under acidic conditions, the exact ratio and reaction conditions being controlled to give the desired average molecular weight and degree of ortholpara substitution. The final product therefore consists of a complex mixture of polynuclear phenols linked by 0- and p-methylene groups. The degree of linearity can be controlled somewhat by using alkyl-substituted phenols such as cresols or t-butylphenols. The novolac structures shown here are representatives of a wide range of structural isomers (ortholpara-substitution ratio, cresol isomer ratio, distribution of ionizable and photosensitive groups etc.). Normal novolacs need to be modified so that they are electrodepositable. This is carried out either by using phenols containing an ionizable functionality or by preforming the novolac and carrying out a Mannich reaction with formaldehyde and a suitable compound with basic or acidic groups.

Though novolacs have been used in formulating both positive and negative-working photoresists, the vast majority are used in the former. A few examples are shown in Table 2.4. In the case of positive-working resists the desired amount of a photo-active group can be added to the novolac resin as a preformed compound (polymers 1 and 2 in Table 2.4) or

OH

O

H

OH

I~I ~I

(HO

CH

2CH

2) 2

N-CH2CHCH20~OCHZ-CH-

CH

r O~ O

CH

2CH

CH

2N( CH2CH20H~

n

Fig

. 2.

8 C

atap

hore

tic

epox

y po

lym

er.

o II

/0

\

OC

CH

= C

H2

0

----

'~I~

I

\~I~_

/\

CH2-CH-0~OCH2-C-CH20~OCH2CH-CH2

(a)

CH

2CH

2CH

2N ( C

H3)

2

I O~O

mix

ed w

ith ~_

m

OH

O

H

OH

,!Jlr

0C

H,b

HC

H2

0-(

@f@

-OC

H2

-bH

-C

H2-ff

lt@-

OCH2

bHCH

20~

C02

H

(b)

CO

zH

Fig

. 2.

9 E

D p

oly

mer

s fo

r ph

otor

esis

ts.

Tab

le 2

.4 P

osit

ive-

and

neg

ativ

e-w

orki

ng n

ovol

acs

No.

P

olym

er

OH

O

H

1 ~H,II

II

CH

2N(C

H2C

H20

Hh

CH2~

CH

3

OH

O

H

2 ~3

t-©-

cH,~

CH2-N-CH2C~H

CH

3

Res

ist t

ype

Cat

apho

reti

c!

posi

tive

Ana

phor

etic

! po

siti

ve

% H

ydro

xyl

este

rifie

d R

ef·

[10]

[10]

0

¢",

3 ~~~}

An

aph

oret

id

12

[10]

p

osit

ive

CH

3

0

¢f

tb-~*}

An

aph

oret

id

10

[18]

4 p

osit

ive

CH

3

Tab

le 2

.5 L

ess

com

mo

n e

lect

rode

posi

tabl

e po

lym

ers

No.

1 2 3

Pol

ymer

~~ /C

~-±

-C

H-C

H

/ ,

n N

(CH

3h

OC

OC

H=

CH

, m

CH

2CH

2CH

2N(C

H:0

2 I N

.0

I ~,

n

-r-+CH

,-o-C

HP nJ

, CH'

~HPE

~o)~

t lQ

J R

R=

H,

25%

R

=C

02H

,75

%

Type

Cat

apho

reti

c ne

gati

ve

Cat

apho

reti

c ne

gati

ve

Ana

phor

etic

po

siti

ve

Ref

·

[17]

[17]

[18]

Electrodepositable resist formulation 61

added by reacting the novolac itself with DNQ-sulphonyl chloride (polymers 3 and 4).

Polymer 3 in Table 2.4 contains no ionizable group, and can be added to polymer 2 as the PAC-containing component.

2.3.7 Miscellaneous polymers

This section describes a number of less common electrodepositable polymers that do not fit into the other classes. Table 2.5 shows examples of some of these compounds.

The first example in Table 2.5 was made from a partly epoxidized butadiene resin by reaction with dimethylamine followed by acrylic acid. The second example was again a reaction product of polybutadiene with maleic acid, followed by treatment with N,N-dimethylaminopropyl­amine. This polyamine was mixed with a novolac resin containing sites of unsaturation, an example of which is shown in Fig. 2.10.

The final example is a polyester containing an alternative photo-active o-nitrophenyl acetal group. These resins, however, appear to be difficult to process, often requiring brushing during alkaline developing and stripping.

2.4 ELECTRODEPOSITABLE RESIST FORMULATION

2.4.1 ED polymers

The ED polymers described in section 2.3 are not effective on their own as resists for primary imaging, etch-resists or any other application. The addition of other components to the resist is necessary for the formulation of workable products. Some typical resist additives for both negative- and positive-working photoresists are described in the following sections. Most of these additives are also incorporated into ED resists for other applications.

Polymers for ED resists have been covered extensively in section 2.3. The polymer contains ionizable groups for both electrodeposition

o 0 / \ II

~:~:+ CH3 CH3

Fig. 2.10 Novolac polymer with reactive sites.

62 Electrodepositable resists

and emulsification, and acts as a carrier for other resist components. Sometimes photopolymerizable/photo-active groups are bonded to the polymer backbone.

2.4.2 Hydrophilic solvents

Usually the polymerization solvent is formulated into the resist emulsion, in cases where the polymer is not isolated from solution. In some cases the solvent is partially or wholly removed by ultrafiltering the resist emulsion after formulation. Since ultrafiltration tends to increase manufacturing costs as well as removing small amounts of other resist components, ideally this step is left out. The removal of the polymeriz­ation solvent can increase the Tg of the deposited coating, and can therefore have a direct effect on coating conditions (such as voltage, bath temperature and coating time). Typical solvents include 2-methoxy­propanol, 2-methoxyethyl ether, 2-butoxyethanol, 2-propanol and 1,4-dioxane.

2.4.3 Unsaturated monomers

These compounds are usually added to negative-working photoresists, and provide sites of unsaturation, which take part in photopolymer­ization, in cases where the polymer itself does not contain pendant unsaturation [2]. The number of reactive double bonds per unit, the functionality, has a definite effect on cure speed. However, increasing amounts of monomer usually result in an increase in tack of the final coating. Therefore a balance must be achieved between minimizing tack and decreasing photospeed. The choice of monomer also affects the

CH202 C CH =CH2 I

CH2 = CH C02 CH2 . C - CH2 CH3 Trimethylol propane triacrylate I CH2 02 C CH = CH2

CH2 02 C CH = CH2 I

CH2 = CH C02CH2 - C - CH202 C CH =CH2 I

Pentaerythritol tetraacrylate

CH202 C CH = CH2

CH2 = CH C02 CH2 CH2 0 CH2 CH2 02 C CH = CH2 Diethylene glycol diacrylate

Fig. 2.11 Common unsaturated monomers.

Electrodepositable resist formulation 63

hardness and flexibility. Certain types of unsaturated monomers are also used as plasticizers in non-photo-imagable ED resists [15].

A few examples of the more common unsaturated monomers utilized are shown in Fig. 2.11. Many other suitable compounds are given in the Rohm and Haas patent [2]; these compounds usually have carbon-carbon double bonds in conjugation with the carbonyl group of an ester or amide.

2.4.4 Photoinitiators and photo sensitizers

(a) Introduction

All of the negative-working ED photoresists contain a photoinitiator and, in some cases, a photosensitizer as well. The photoinitiator is a compound that absorbs incident UV radiation of an appropriate wave­length from the exposure lamp during the imaging step and splits into free radicals, which initiate polymerization of the unsaturated monomer (or unsaturated polymer) (see e.g. Fig. 2.12).

Given an appropriate light source, efficient photoinitiation depends upon several factors, including suitable absorption coefficients and wavelength sensitivities for the initiator molecule, initiation quantum yields in the range 0.1-1.0, and the requirement that neither the initiator molecule nor any of its photofragments should function as chain­terminating agents.

Sometimes light of the appropriate wavelength for initiating the above reaction with a particular photoinitiator is unavailable in sufficient intensity from the standard exposure units used. In these cases another photo-active compound is added with the photoinitiator; this is usually referred to as a photosensitizer. The photosensitizer is chosen so that it absorbs the appropriate wavelength of light and also is matched to the photoinitiator in a way that it can transfer the absorbed energy to the latter, causing it to split into active radicals. Section 2.4.4(b) describes the mechanism in detail.

The choice of photosystem varies greatly from one resist to another; however, it must be compatible with the other resist components and not separate from them on standing or during electrodeposition. Sometimes a compound that acts as a photoinitiator in one resist can function as a photosensitizer in another. Examples of the more common photo­initiators are a-hydroxyisobutylphenone, benzoin ethyl ether, 2-t-butyl-

o " hv • •

R-C-R ~ RCO + R

Rl nCH2=CHRI I ---~~ R-fCH-CH2±-

n

Fig. 2.12 Photo-initiation of polymerization.

64 Electrodepositable resists

R"N~~::© R{ 0 0

Fig. 2.13 Photosensitizer for use with 488 nm radiation.

anthraquinone, diethoxyacetophenone, Irgacure 651 and Irgacure 907 (the latter two are both available from Ciba-Geigy [18]). Many others are described in the various references cited in this chapter.

Virtually all of the photo systems used in ED photoresists are sensitive to UV light. However, a composition has been reported that is sensitive to the visible radiation of an argon ion laser at 488 nm [19]. This employs the compound shown in Fig. 2.13 as a photosensitizer.

(b) Photochemistry of negative-working ED resists

When a negative-working photoresist coating is exposed to light of the appropriate wavelength through a patterned mask, the exposed areas crosslink and then become less soluble than the unexposed regions in a developing solution. In this way a negative tone image of the artwork is formed on the substrate.

The exact mechanism involves, initially, absorption of the incident UV radiation from a suitable source (usually a 3,5 or 7kW medium-pressure mercury vapour lamp for applications in which ED resists are used) by the photoinitiator, which then splits into active radicals and initiates cross­linking of the unsaturated monomer. The initial radicals produced, depending on their stability, may initiate polymerization themselves or split further, giving other radicals. Some initiators can also abstract hydrogen from the polymer, leading to favourable grafting effects. For example, using acetophenone derivatives as photo initiators, the initial excited state on light absorption is the singlet state (51), which then undergoes intersystem crossing to the excited triplet state (II). The triplet state then dissociates to form the radicals by a Norrish Type 1 cleavage [20] (Fig. 2.14).

The photoinitiator is chosen so that its peak absorption wavelengths are matched with the major wavelengths emitted from the mercury vapour lamp to give maximum photocrosslinking. If the photoinitiator wavelength is insufficiently close to a major mercury emission wave­length, there are two ways to improve efficiency. First, a doped mercury vapour lamp can be used that has a more appropriate spectral output relative to the absorption of the photoinitiator. Metal halides are usually used to accomplish this. However, medium-pressure mercury lamps

Electrodepositable resist formulation 65

o R' II I 2

Ph-C-C-R 13 R

Ph - ECH2 - CHt CH2 - CH - CH -CHt I I I R R R n

Fig. 2.14 Dissociation of acetophenone derivatives.

o II

Ph-O +

R' I 2

·C- R I 3 R

emit most of their UV energy at wavelengths longer than 330 nm where many of the common photoinitiators do not absorb well. Also, the vacuum frames (comprising a glass base and a polyester cover sheet) on most printers used in the printed circuit board industry, as well as the artwork, absorb radiation of wavelengths less than 330 nm.

Therefore the second method of increasing photopolymerization is to use a photosensitizer, which absorbs strongly across the spectrum and transfers energy efficiently to the photoinitiator by means of spin exchange [20]. Thioxanthones are very efficient photosensitizers (Fig. 2.15).

The photospeed of a negative-working photoresist can be defined as the minimum exposure energy (J m -2) needed for complete development

o

~R hv> 330 run ~

Radicals 3[ J* o R' II I 2

Ph - C-k3- R

Fig. 2.15 Thioxanthones as photosensitizers.

II I 2 Ph-C-C-R

I 3 l OR'

R

o

+~R

66 Electrodepositable resists

of the unexposed resist and retention of the exposed region, gtvmg acceptable resist sidewall definition. The thickness of the resist and the wavelength of radiation is normally specified, along with developing conditions. In practice the photospeed can vary somewhat owing to differences in the exposure and development conditions and techniques. Contrast curves can be drawn up of normalized resist thickness after development versus exposure dose [21].

An example is shown in Fig. 2.16, for a typical negative-working ED photoresist.

The contrast curve is measured by exposing resist films to a range of energies, and measuring the thickness of resist remaining after development. At low exposure doses the crosslink density is low enough for the developing solvent to still dissolve and therefore remove the resist. At some threshold dose, known as the gel dose D~, the solvent can penetrate the resist but not solubilize it. Exposure doses above D~ lead

1.0

III III Q) C

.><

.~

.s:: .... ti 0.5 ·iii ~

-0 Q)

~ Cll

E 0 z

Exposure dose

Fig. 2.16 Contrast curve for a negative-working photoresist.

Electrodepositable resist formulation 67

to an insoluble residue, which remains after development if the adhesion to the surface is good.

The gel dose only approximates to a threshold, and the degree to which this approaches a sharp threshold is known as the contrast y. It is usually defined as the gradient of the linear portion of the contrast curve, which has the form

1 (2.1)

y = log (D~'o - D~)

where D~'o is the exposure dose required for 100% resist retention, extrapolated from the linear portion. Contrast depends on the dist­ribution of molecular weights in a polymer: the wider the distribution, the lower the contrast of the resist [21]. Also, the higher the contrast, the better the resolution.

2.4.5 Photoactive compounds for positive-working ED photoresists

The photoactive compounds (PACs) that find use in positive-working photoresists are those that rearrange on absorbing incident UV light of the appropriate wavelength. They produce functional groups that react with a developing solution and are therefore selectively removed from a coated substrate, leaving behind the unexposed resist (and PAC). By far the most commonly used P ACs are those containing o-diazonaphtho­quinone (DNQ) groups (Fig. 2.17).

As mentioned in section 2.3, the DNQ group can be attached to the polymer backbone (polymer 4 in Table 2.2) by reaction of the DNQ­sulphonyl chloride with an amine group on the polymer. Alternatively, the DNQ-sulphonyl chloride can be reacted first with a 'ballast group' before mixing with the polymer. The most common class of ballast groups comprises hydroxybenzophenone analogues, an example of which is shown in Fig. 2.18.

The DNQ-PAC shown in Fig. 2.18 was incorporated into a cataphoretic, positive-working resist with the novolac polymer 1 shown in Table 2.4. The number of hydroxyl groups or the degree of esterification can vary widely, depending on the resist.

As in negative-working ED resists, the PAC must be compatible with

R = H,

R R'

2,1,4-PAC

2,1,5-PAC

Fig. 2.17 Photoactive compounds containing the DNQ group.

68

o

2 rpN' +

S02C1

Electrodepositable resists

~OH ~'f

OH

Fig. 2.18 Synthesis of a DNQ-PAC.

Base

-2 HC1

the other resist components and co-deposit with them during electro­deposition.

When a positive-working resist coating is exposed to light of the appropriate wavelength through a patterned mask, the PAC in the exposed areas undergoes rearrangement to give a compound that is more soluble in the developer solution than the unexposed area. In this way a positive-tone image of the artwork is formed on the substrate. The initial

0

[~J rpN' hv o h ~ ~

-N2

R R

DNQ Carbene

0 II C C02H

¢ H2O >- C?6 R R

Ketene Acid

Fig. 2.19 Photorearrangement of a DNQ-PAC.

Electrodepositable resist formulation 69

reaction involves absorption of light by the DNQ group in the resist, which loses nitrogen to give a transient carbene intermediate. This transforms via a Wolff rearrangement to a ketene, which reacts with water present in the film to give a carboxylic acid (Fig. 2.19).

It is therefore important that the resist coating contain enough water for the photoreaction. Carrying out the imaging step after allowing the coating to stand for a short period in an atmosphere of around 50% relative humidity usually suffices. If the ketene is formed and there is insufficient water to form the acid, crosslinking can occur with other resist components, leading to image reversal.

Since most positive-working ED resists are anionic, carboxylic acid groups are already present on the polymer and are widespread through­out the unexposed areas. Therefore a potential problem is loss of un­exposed resist during developing with a basic solution. However, if the resist has been formulated carefully, only a very small loss of unexposed resist is observed. As in resists for the semiconductor industry, DNQ­PACs can act as dissolution accelerators. For example, complete photo­decomposition of the DNQ results in a dissolution rate that is equal to, or greater than, the intrinsic dissolution rate of the polymer alone, whereas the unexposed resist, containing unreacted DNQ, dissolves some orders of magnitude more slowly in aqueous base solution than films of the polymer alone. It is this photochemically generated difference in dissolution rate in aqueous base that is exploited in the generation of images.

The photospeed of a positive-working resist can be defined as the minimum exposure energy (J m -2) needed for complete development of the exposed resist and retention of the unexposed region, giving acceptable sidewall definition.

Again, the photospeed is dependent on resist thickness, the wavelength of light used and developing conditions. DNQ-P ACs that are sensitive to light in the 350-420 nm region of the spectrum are usually used.

The sensitivity of positive-working photoresists is similarly determined (as for negative-working resists; section 2.4.4b) by measurement of resist thickness with varying exposure energy [21].

The contrast curve is shown in Fig. 2.20. The extrapolated dose at which 100% of the resist remains after development is D~, and D~ is the exposure energy at which the resist is just removed without affecting the thickness of the unexposed resist. The contrast is given by

1 Y = 0 i

10g(Dp - Dp) (2.2)

A method of reversing a positive-tone image to a negative-tone image has been reported [22]. The ED resist used is a cataphoretic, novolac-

70 Electrodepositable resists

1.0-"T-----------..-:::-

Exposure dose

Fig. 2.20 Contrast curve for a positive-working photoresist.

based resin containing DNQ-P AC groups. After exposure through a mask, the substrate is simply heated at 90°C for 10 min, subjected to flood exposure and finally developed in dilute base, giving a negative-tone image of the artwork.

2.4.6 Hydrophobic solvents

Hydrophobic solvents (or plasticizers) are often added to electro­depositable resists to lower the Tg of the polymer, enabling electro­deposition to take place at low temperatures and allowing easier control of the film thickness. The coated resist can also flow better (coalesce) during baking, when a plasticizer is present, to give a more compact, defect-free surface.

In some cases solvents, which increase flexibility by causing the resist to flow during baking, remain in the resist after the bake step and lead to a

Electrodepositable resist formulation 71

certain degree of tackiness. Other more volatile solvents can be used that are completely removed during baking, giving no tack in the coating, but sometimes leading to a brittle deposit. Plasticizers therefore need to be chosen to maximize film coalescence and flexibility and to minimize tack. Plasticizers that are too volatile may need constant replenishment in the resist bath. Solvents that have a moderate degree of water solubility may also lead to problems, causing insufficient amounts necessary for improving film quality to be co-deposited with the other resist components.

Classes of hydrophobic solvents that have been used include phthalate, aliphatic acid ester, ethylene glycol ethers, ketones and many others.

2.4.7 Dyes

The main reason for adding a dye to a photoresist is to give visual contrast with the substrate after imaging and developing. The dye should be chosen so that it is transparent in the region of the spectrum in which the photoinitiator or photosensitizer absorb. It is also advantageous if a latent image of the artwork is seen after imaging. This enables the coated substrate to be examined before developing.

The two main requirements of a dye for ED resists are as follows.

1. It must be completely soluble in the organic phase (polymer, photosystem etc) and show no separation on electrodeposition.

2. It should not interfere with the photoreaction, e.g. as a triplet-state quencher of a photoinitiator or photosensitizer, or as a free-radical polymerization inhibitor.

Typical examples of suitable dyes include the Orasol® (Ciba-Geigy) and Zapon® (BASF) ranges, methyl violet and rhodamine B, and further details are given in the references cited in this chapter.

2.4.8 Ionizers

In the manufacture of cataphoretic ED resists an acid is added to a polymer containing basic groups to ionize, or partially ionize, them (20-100% neutralized), forming salt groups on the polymer. The micelles thus formed are spherical in shape and have cationic surface charges surrounded by acid anions, which help to stabilize the micelle. Alterna­tively, micelles in anaphoretic ED resists have anionic surface charges surrounded by base cations. Here a base has been added to a polymer with ionizable acid groups. The micelles are therefore surrounded by an electrical double layer, one charge fixed on the micelle surface and the other freely mobile on the surrounding counter-ions.

The amount of ionizer present in an ED resist is the main factor

72 Electrodepositable resists

determining conductivity, low conductivity giving rise to low electro­phoretic mobility, and high conductivity accelerating the electrolysis of water (section 2.2). The micelle size also depends on the amount of ionizer, greater amounts of ionizer added during manufacture usually leading to smaller micelles.

Acids suitable for adding to cationic resins include both organic acids such as acetic, lactic, maleic, glycolic and p-toluenesulphonic acid, or inorganic acids such as hydrochloric, sulphuric and phosphoric acid.

Bases suitable for adding to anionic resins include both organic bases such as tertiary amines, ethanolamines and morpholine, and inorganic bases such as sodium or potassium hydroxide or carbonate.

2.4.9 Miscellaneous additives

The previous sections have described the main components of typical ED resist formulations, although they often also contain additional compounds to control certain resist properties more finely. Some selected examples are given here.

Some negative-working resists contain an inhibitor compound, usually based on hydroquinone, that prevents spurious thermal crosslinking of the monomer on standing, thereby increase shelf life.

One series of negative working anaphoretic resists [12] that has been described contain benzotriazole derivatives having one or more carbonyl groups. These additives are reported to chelate copper (II) ions, which are dissolved from copper anodes during electrodeposition and which would normally chelate with the carboxyl groups of the polymer, causing undesirable 'pseudo-crosslinking' and leading to imperfect development after irradiation.

A potential problem with cataphoretic ED resists is the evolution of hydrogen generated from the electrolysis of water during electro­deposition. If this is not effectively removed from the surface of the resist (e.g. by cathode vibration), pinholing can result, leading to areas of exposed substrate. An effective chemical means of removing hydrogen has been reported [23], utilizing hydrogen scavengers in the form of organic nitro compounds. These drastically reduce the amount of evolved hydrogen in the bath by reacting with it to form amines. The more effective compounds are those that produce partially water-soluble amines, which can enter the aqueous phase and be re-oxidized back to the active nitro compound at the anode.

2.4.10 ED resist manufacture

The preparation of ED resists basically involves two stages. The first is preforming of the polymer, followed by addition of the photo system,

Applications of electrodepositable resists 73

solvents and dye. The presence of polymerization solvent aids the solubility of the components and homogeneity on mixing.

The second stage involves addition of the ionizer, followed by deionized water. The water is added very slowly at first, in portions, since the viscosity of the mix initially increases gradually, as a water-in-oil emulsion forms. The viscosity increases with increasing water content, until inversion occurs and an oil-in-water emulsion results. This stage needs very efficient mixing and temperature control, since local exotherms caused by various solvation reactions and mixing can lead to a range of micelle sizes. Large micelles may be unstable and eventually settle out, although particle size seems to have very little effect on resist functional performance.

The micelle size produced depends on the temperature and ionizer content at inversion, since emulsion formation here is irreversible. After inversion has taken place, the remaining water can be added faster, since no further change in micelle size occurs.

Thus, with efficient control, emulsions with the desired micelle size, conductivity and solids content can be obtained. Solids content is usually in the 10-25% range.

At this stage some resists can be ultrafiltered to remove the water­soluble polymerization solvent. Ultrafiltration removes small molecules, but large aggregations of molecules (i.e. micelles) cannot pass through the membrane. Small amounts of other, water-soluble resist components may also be removed, but this can be compensated for by adding slightly more of them at the beginning.

Ultrafiltration tends to increase the manufacturing costs somewhat, but lowers the VOC (volatile organic content) and simplifies bath control during processing (section 2.2.1).

2.5 APPLICATIONS OF ELECTRODEPOSITABLE RESISTS

2.5.1 Photo resists

(a) Introduction

With the growing use of surface mount technology and related techniques such as chip-on-board, printed circuit board manufacturers are continually seeking new processes to enable production of boards with increased circuit densification and decreased circuit feature sizes. Consequently, line widths are now approaching those of early semi­conductor devices, and this has in turn placed new demands on the resolution capabilities of the inner-layer and outer-layer photoresists used in the industry.

Initially, liquid solvent-based photoresists were used as the primary imaging medium in printed circuit board manufacturing, and application

74 Electrodepositable resists

was by one of two methods: screen printing or roller coating. Liquid resists had superb resolution capabilities, but both methods were troubled by non-uniformity, solvent vapour containment and other issues.

In the early 1970s dry-film photoresists were introduced, and these soon dominated the industry. The coating of printed circuit board inner and outer layers was simplified dramatically, yet at a lithographic price that is only now being realized. There is a practical resolution limit to the use of dry film, the general consensus being that the manufacturing capability diminishes rapidly below 100 flm.

Apart from high resolution, an effective photoresist must have good etch resistance, along with good adhesion, no tack, fast photospeed, excellent contrast giving straight sidewalls, and good processability.

(b) Primary imaging

Both negative- and positive-working ED photoresists can be used in primary imaging applications, the former being used mainly as an etch resist in inner-layer manufacture and the latter mainly for outer-layer imaging, where protection of plated through holes is necessary.

Figure 2.21 shows a schematic representation of the lithographic process using both resist types. In negative-working resists the exposed areas are rendered less soluble by crosslinking, while in positive-working resists the exposed areas are rendered more soluble by a photo-induced molecular rearrangement. After development, the base metal is etched, or further metal deposition can take place, before stripping of the resist by wet methods.

Rather than being replacements for dry-film photoresists, ED resists were designed as a technology extension for the next generation of devices possessing higher densities of fine features on printed circuit boards, and there are a number of installations, especially in the Far East, employing this new technology.

ED photoresists can offer better performance than dry film provides, but they are more sensitive to the lithographic technology of imaging, resulting in higher resolution [14]. As already mentioned, the resolution of the conventional dry-film photoresist process reaches its limit around 75 flm, whereas the ED resist process is able to resolve features down to around 10 flm. Electrodeposition allows coatings that are much thinner and better conforming than those made by dry-film lamination. Also, the absence of a cover sheet on the ED resist means an order-of-magnitude difference in thickness between the two processes.

The thin resist therefore offers superior resolution and no line growth, giving excellent sidewall definition, even when conventional non­collimated light sources are used. However, because of the superior

Applications of electrodepositable resists

1I!!!!!!f!!!~"'" Resist ~ - Base metal

L-______________ -J "

Laminate

75

Electrodeposit resist

Exposure

Develop

Etch base Metal

Resist Strip

Fig. 2.21 Lithographic process for negative- and positive-working photoresists.

resolution, a greater standard of cleanliness in the pre-clean, coating and imaging areas during processing is necessary, since extremely small contaminants in or on the surface of the resist will also be resolved, resulting in printed defects.

The thin ED resist coating results in resist patterns with very low aspect ratio, improving the flow of etchant between spaces, compared with the relatively high aspect ratio features seen with dry-film resist. A faster etching rate (typically 30% faster), better centre-to-edge and top­to-bottom etch uniformity and improved uniformity between dense and isolated circuit areas make these resists well suited to the production of controlled impedance circuitry and ultrafine line work.

Conventional liquid resists provided complete coverage over defects in the laminate such as handling scratches and undulations in the surface,

76 Electrodepositable resists

but suffered from uniformity constraints, since high spots on the laminate had minimal resist coverage, and low areas had excessively thick films. Dry film virtually eliminated the uniformity issue by providing a flat surface over the entire innerlayer. However, this surface did not conform to the surface in the event of scratches, but rather tented over the defects.

The ED resist, by its very nature, provides the advantages of both types of system without the corresponding disadvantages. Being liquid-based, the material provides coverage on every wetted surface, regardless of the topography seen. However, the resist coating characteristics are those of a dry, organic film, providing uniform coverage through the self-limiting coating method. This provides coverage of defects that dry film typically tents over, as well as complete uniformity in coverage for the entire inner layer.

With increasing circuit densification and the widespread adoption of technologies such as surface mount, via-holes are rapidly becoming smaller in diameter and greater in number. With a dry film resist the via itself was never actually coated with resist, merely tented over. However, an ED resist coats the via-hole, giving adequate coverage with a far higher degree of preservation than tenting can provide.

Similarly, when using an ED resist on outer layers, the plated through­holes are completely coated with resist, as opposed to being merely tented. For high-aspect-ratio holes a positive-working photoresist is desirable, since, because the unexposed regions remain through the development step, no light is required down the holes. A problem when using a negative-working resist would be getting sufficient light onto the resist-coated hole walls to initiate crosslinking and to offer adequate protection through development.

Another advantage of using a positive-working ED resist is the potential for reducing the diameter, or in fact eliminating pads around holes on printed circuit boards, enabling the packing density of the board to increase. These are a necessary feature when using dry film to anchor the resist over the hole.

Figure 2.22 shows a general process sequence for an ED photoresist (used here as an etch-resist). The bake step is necessary for resist adhesion, and some resists may require additional bake and/or exposure steps after developing. Examples of processes and typical ancillary chemistries are described in detail in the various patents cited in this chapter.

(c) 3D moulded interconnect devices

Unlike conventional printed circuit boards, where the desired circuit pattern is formed using screen-printed polymer plate and/or etch­resistant inks, or dry-film photopolymer laminated films, a moulded

Applications of electrodepositable resists 77

Fig. 2.22 ED photoresist process sequence.

interconnect having irregular topographies, varying planes, complex shapes and surfaces requires imaging techniques compatible with three­dimensional non-planar geometries.

Prior to 1989, known techniques for 3D imaging and catalysation for subsequent plating were based on the use of either photoreducible non­noble metal salts or 'in-mould' selective catalysation, using two-shot injection moulding [24]. Both of these approaches are based on the use of fully electroless plating for metalizing circuitry and through-hole

78 Electrodepositable resists

features, and a variety of unique moulded interconnect devices have been constructed in this way.

Each of these approaches offer capabilities in 3D circuitization. However, growing requirements for fine line trace and space widths, along with electrolytically deposited metals, such as copper, nickel, tin, tin-lead and gold, as well as panel plating processes, required the use of alternative technology. ED photoresists are well suited, owing to their high resolution and their ability to conform to three-dimensional features. ED resists have been successfully applied to low-volume production of moulded interconnects [24], although, to date, these devices have not become a totally viable and accepted technology within the electronics market place.

(d) Photochemical machining

ED photoresists are increasingly being successfully applied to photo­chemical machining processes in the manufacture of products such as colour TV receiver tube aperture masks (shadow masks), integrated circuit lead frames and printing rollers. These resists are used as etch­resists for a variety of metals and alloys, and can withstand the harsh etching conditions used in these processes.

Currently, liquid resists applied by conventional methods, such as dip coating are used, but problems with drops and streaks and non-uniform coating mean that a more effective alternative process is required.

ED resists, by their very nature, give excellent uniformity, defect-free, chemically resistant coatings on most metal and alloy surfaces, and their high resolution means that much finer features can be successfully etched.

(e) Plating resists

Negative-working cataphoretic ED resists, such as Shipley's Eagle™ resist, which are normally stripped using a solution of an organic acid under mild conditions, are able to function as electrolytic plating resists, withstanding highly acidic solutions over long periods. A variety of metals have been successfully plated, including copper, tin-lead, rhodium, gold and silver. It is necessary for the resist coating to be thicker than the deposited metal, and this can easily be achieved with some ED resists (section 2.2.3).

In addition to printed circuit boards, many other objects have been plated in this way using ED resists, including lead frames, pen nibs, silicon wafers, car parts, spectacle frames and other decorative items.

References 79

2.5.2 Solder replacement

ED resists have been used for the protection of circuit pattern tracks and plated through-holes during subtractive etching of copper in the manu­facture of printed circuit boards [15, 25]. This involves the deposition of a polymeric etch-resist in place of the traditional metallic tin or tin-lead solder, providing a faster, simpler and more environmentally acceptable process that eliminates the need for disposal of toxic wastes from the plating and stripping chemistries used.

The coating is applied to printed circuit board outerlayers to which aqueous-developable dry film has been previously laminated, imaged and developed. The ED film readily withstands the subsequent dry-film stripping and copper-etching operations before being itself removed. An aqueous-developable dry film is necessary so that the ED resist is not removed at the same time as the dry film.

These coatings are usually non-photoimagable. However, a photo­system can be incorporated to improve performance if required [15].

2.6 SUMMARY AND CONCLUSIONS

Organic resists applied by electrodeposition are capable of producing high-resolution images useful for the production of a variety of products in the electronics and photochemical machining industries. Con­sideration of the chemical and manufacturing aspects of ED photoresists versus current dry-film technology shows that the former process provides significant advantages for technology-driven printed circuit boards that require ever finer and denser circuitry.

Thin films that are difficult to apply by the dry-film process are readily produced by electrodeposition. Three-dimensional substrates can also be coated.

ED resists are relative newcomers to the printed circuit board and photochemical machining industries, and as yet have not been accepted as widely as the well-established techniques. However, as these tech­niques are increasingly pushed to their limits and ED resist technology continues to improve, the utilization of ED resists is expected to grow dramatically over the next few years.

REFERENCES

1. Ford Motor Co. (1960) UK Patent 933 175. 2. Rohm and Haas Co. (1986) US Patent 4 592 816. 3. Machu, W. (1978) Handbook of Electroplating Technology, Electrochemical

Publications, Weinheim. 4. Murray, J. (1991) Printed Circuit Fabrication, 14, 44.

80 Electrodepositable resists

5. Collins, J.M. (1991) Printed Circuit Fabrication, 14, 62. 6. Saunders, K.J. (1983) Organic Polymer Chemistry. Chapman & Hall, New York. 7. (a) Kansai Paint Co. Ltd. (1991) US Patent 5 070 000; (b) Kansai Paint Co. Ltd.

(1990) US Patent 4965 073. 8. Kansai Paint Co. Ltd. (1990) US Patent 4898 656. 9. Kansai Paint Co. Ltd. (1990) European Patent 0383 223.

10. Ciba-Geigy Co. (1992) US Patent 5 080 998. 11. Nippon Paint Co. Ltd. (1991) US Patent 5055374. 12. Hitachi Chemical Co. Ltd. (1991) European Patent 0481 709. 13. Hitachi Chemical Co. Ltd. (1991) European Patent 0489 560. 14. Vidusek, D.A. (1989) Circuit World, 15, 6. 15. Shipley Company Inc. (1988) US Patent 4 751 172. 16. (a) Ciba-Geigy Co. (1988) US Patent 4 746399; (b) Ciba-Geigy Co. (1991)

US Patent 5 073 233. 17. Nippon Oil Co Ltd. (1991) European Patent 0441 308. 18. Ciba-Geigy Co. (1986) US Patent 4 632 900. 19. Kansai Paint Co. Ltd. (1990) European Patent 0435262. 20. Roffrey, e.G. (1989) in Photopolymerisation of Surface Coatings (ed. e.G.

Roffrey), Wiley, Chichester, p. 67. 21. Clements, S. (1988) in Plastics for Electronics (ed. M.T. Goosey), Elsevier, New

York, p. 210. 22. Ciba-Geigy Co. (1991) US Patent 5 002 858. 23. Rohm and Haas Co. (1991) US Patent 5066374. 24. Rychwalski, J.E. (1990) New developments and commercial applications in

moulded interconnect device technology. Paper presented at the 5th Printed Circuit World Convention, Glasgow, 12-15 June 1990.

25. Ciba-Geigy Co. (1991) US Patent 5073 478.