Excimer laser treatment of ¯uorocarbon resin forimproved adhesion

Feng Huang, Qihong Lou, Jingxing Dong*, Yunrong WeiShanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, PO Box 800-216, Shanghai 201800, PR China

Received 14 April 2000; accepted 28 October 2000

Abstract

We discuss a method of improving adhesion to ¯uorocarbon resin by irradiation with an XeCl excimer laser. The adhesive

force between the polymer and metal, and contact angles with water as measured by the shear test method and a Ne±He laser

system, respectively, are improved when the polymer surface is irradiated in the presence of water or solutions of boric acid,

sodium hydroxide, copper sulfate, and sodium aluminate. The results are interpreted in terms of a simple laser heating model.

# 2001 Elsevier Science B.V. All rights reserved.

Keywords: Excimer laser; Surface treatment; Polymer

1. Introduction

Excimer lasers can ef®ciently modify surface prop-

erties of materials as a result of their emission wave-

lengths in the ultraviolet, short pulse duration, and

high power. These lasers are widely used for marking,

lithography, doping, deposition, and annealing, and

great economical bene®t has been obtained. If the

laser intensity is properly controlled the resulting

reacted region can be very thin, which provides a

unique means for treating the surfaces of many mate-

rials without damage to the underlying material. In

recent years these have been used to treat polymers

and to alloy metals in order to improve adhesion [1,2].

Furthermore, this technique is important in the aero-

space and automotive industries.

Polymers such as ¯uorocarbon resins are not only

heat-resistant but also acidproof and relatively inert

chemically. As a result, it is dif®cult to change the

surface to improve the relatively poor adhesion to

other materials. Use of an excimer laser to break C±F

bonds in the presence of solutions such that the H� and

OHÿ ions in solution replace the ¯uorine atoms in the

¯uorocarbon resin results in an improvement in both

hydrophilic properties and adhesion with other mate-

rials. For example, Okamoto et al. [3,4] have improved

the adhesive force and contact angle to ¯uorocarbon

resin using a ArF laser (193 nm), and have achieved

adhesive forces of up to 98 kg/cm2 and contact angles

as small as 208. However, use of a 308 nm XeCl laser

has not been reported so far.

2. Adhesive force and contact angle with water

It is well known that photo-induced reactions are

highly dependent on laser intensity and number of

Applied Surface Science 174 (2001) 1±6

* Corresponding author. Tel.: �86-21-59534890-530;

fax: �86-21-59916703.

E-mail address: jxdong@mail.shcnc.ac.cn (J. Dong).

0169-4332/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved.

PII: S 0 1 6 9 - 4 3 3 2 ( 0 0 ) 0 0 8 7 9 - 5

laser pulses, so the use of a homogeneous laser beam is

necessary while treating the surface [5]. Here, we

focus the beam from a 308 nm excimer laser beam

by a lens �f � 20 cm�, then pass the beam through a

homogenizer that is formed from a silica microlens

array and a rectangular waveguide [6]. The laser beam

is multiply re¯ected in the rectangular waveguide,

then mixed at its end. The spatial uniformity of the

emerging beam is excellent, with an irradiation uni-

formity of less than 2%, as detailed in [6]. We next

form a uniform homogeneous layer of solution about

1 mm thick between a silica window and the surface of

the sample as shown in Fig. 1.

The laser beam is focused on the surface of the

¯uorocarbon resin with an irradiation area of

3� 4 mm2. After treatment, the adhesive force of

the surface is measured by forming a bond with

industrial epoxy resin 509 between the treated regions

of the surface and an Al bar. The adhesive force is

measured after the epoxy has been allowed to set for

24 h. The measuring system is shown in Fig. 2. Mea-

surements were performed by Standard Institute of

Shanghai.

Results are shown in Table 1. The adhesive force of

the ¯uorocarbon surface without treatment is less than

2 kg/cm2. Laser surface treatments in the absence of a

solution show some improvement, with adhesion

increasing to slightly less than four times that of an

untreated surface. However, after laser irradiation

with solutions such as water and water-containing

CuSO4, NaOH, H3BO3, NaAlO2, HCl, H3NO3 and

H2SiO3, the adhesive force is increased. Water alone

and solutions containing CuSO4, NaOH, H3BO3, and

NaAlO2 are particularly effective in improving adhe-

sion. The highest adhesive force is obtained with 1%

NaAlO2. Moreover, the higher the adhesive force, the

smaller the contact angle y.

The effectiveness of UV laser surface treatments is

related to many parameters, such as energy density

and number of pulses. Figs. 3(a) and (b) show the

relationships between the adhesive force and the

energy density and number of pulses, respectively,

where the solution is water. The adhesive force

increases with both energy and number of pulses. At

8 mJ/mm2 the adhesive force has increased 20 times

beyond that of the untreated surface. Furthermore,

the adhesive force increased with the laser pulse

number.

Fig. 1. Structure for surface treatment system.

Fig. 2. Measurement of the adhesion of ¯uorocarbon resin with an

aluminum bar.

Table 1

Adhesive forces and contact angles after laser treatments

Solution Shots Intensity (mJ/cm2) Adhesive force (kg/cm2) Contact angle (8)

HCl (1%) 1500 535 7.97

CuSO4 (1%) 1500 535 16.17

HNO3 (1 ml/200 ml) 1500 535 10.34

NaOH (1%) 1500 535 11.18 45

H2O 1500 535 22.9 28

H2SiO3 (<1%) 1500 535 6.80

H3BO3 (2 ml/200 ml) 1500 535 12.2 33

NaAlO2 (1%) 1500 535 26.2

No solution 1500 535 7.25

Untreated <1.95 110

2 F. Huang et al. / Applied Surface Science 174 (2001) 1±6

3. Laser heating model of liquid thin ®lm on solidsubstrate

The interaction mechanism between the laser beam

and the polymer surface is rather complex, including

both physical and chemical phenomena. We discuss

here mechanisms associated with laser heating and

photo-induced reactions. During the laser pulse dura-

tion of about 50 ns the incident laser energy is essen-

tially converted instantaneously into thermal energy.

Laser interaction with the liquid thin ®lm on the

¯uorocarbon surface is illustrated schematically in

Fig. 4.

We use the heat conduction equation (1) of a laser

interacting with a liquid thin ®lm on a polymer sub-

strate to estimate the surface and subsurface tempera-

tures of the polymer:

r�z�C�z� @T�z; t�@t

ÿ k�z� @2T�z; t�@z2

� Q�z; t�;

r�z� � rf � rs

2� rs ÿ rf

2sgn�zÿ hf�;

C�z� � Cf � Cs

2� Cs ÿ Cf

2sgn�zÿ hf�;

k�z� � kf � ks

2� ks ÿ kf

2sgn�zÿ hf�;

Q�z; t� � Iag�x; y�f �z�q�t� (1)

where ri �i � f; s� is the mass density, s and f refer to

substrate and ®lm, respectively, C the speci®c heat at

constant pressure, k the heat diffusivity, and sgn�x� a

function that has the value 1 when z is larger than zero

and ÿ1 when z is negative. Q�z; t� indicates the heat

produced by the laser, g�x; y� describes the distribu-

tion of the laser energy intensity in the xÿy-plane. f �z�is the absorption function in the z-direction, and q�t�the time dependence of the laser pulse, which is

Gaussian in our experiment.

The liquid ®lm is 1 mm thick, and hf ! hs. The heat

diffusion length LT � 2�Dt�1=2 ! hf for a pulse dura-

tion of 50 ns and a water thin ®lm. In this case the heat

equations for the thin ®lm and solid substrate are

independent. Therefore, the heat equations of the

liquid thin ®lm on a solid substrate are as follows:

rfCf@T�z; t�@t

ÿ kf@2T�z; t�@z2

� Qf�z; t�;Qf�z; t� � Iag�x; y�f �z�q�t� (2)

rsCs@T�z0; t0�@t0

ÿ ks@2T�z0; t0�

@z0� Qs�z0; t0�;

Qs�z0; t0� � Ia eÿaf hf g�x; y�f �z0�q�t0� (3)

Fig. 3. The relationship between (a) the adhesive force and the laser energy density with 1500 pulse, and (b) the laser pulse number with

10 mJ/mm2 energy density.

Fig. 4. Schematic diagram of uniform irradiation of a substrate

covered with a water thin ®lm.

F. Huang et al. / Applied Surface Science 174 (2001) 1±6 3

where z0 � zÿ hf and t0 � t ÿ hf=c. The temperature

rise of the thin ®lm and solid substrate, can be

expressed as follows:

DT�z; t� � Ia

kLTierfc

z

LT

� �ÿ 1

aexp�ÿaz�

�� 1

2aexp

aLT

2

� �2X�

� exp��az�erfcaLT

2� z

LT

� �� ���t < t�

(4)

DT�z; t� � 2IaD1=2

kt1=2ierf

z

2�Dt�1=2

!"

ÿ�tÿt�1=2ierf

z

2D1=2�tÿt�1=2

!#�t < t�

(5)

For a water thin ®lm k � 5:97� 10ÿ3 W=�cm K�;r � 0:998 g=cm

3; C � 4:18 J=�g K�; a � 0:00066

cmÿ1, and for the ¯uorocarbon substrate k � 1:54�10ÿ3 W=�cm K�; r�2:15 g=cm

3; C� 0:935 J=�g K�;a � 46 cmÿ1, the laser pulse duration is 50 ns, and the

laser energy intensity is Ia � 535 mJ=cm2. The calcu-

lated temperature rise of the thin water ®lm and

substrate are shown in Fig. 5(a).

The temperature of the liquid thin ®lm is above

1008C, so the water is vaporized and the temperature

of the liquid thin ®lm decreases rapidly along the z-

direction. When the water vaporizes, the feedback

force roughens the substrate surface. The feedback

force is expressed as [7]:

Prec � xIahu2i1=2

DHv � xhu2i � xrKBTfs

m

� �1=2

v0 exp ÿFv

Tfs

� �(6)

where x is a function of adiabatic coef®cient g, r the

mass density, m the mass of the molecular group, v0

the velocity, Fv the molecular potential energy. When

the laser energy intensity I0 increases, the surface

temperature and the feedback force both increase

and the surface becomes rougher, which yields the

stronger adhesive force as shown in Fig. 3(a). Because

the repetition of the excimer laser is 5, the period time

0.2 s is much longer than the time laser acted (100 ns),

then the temperature of the substrate can be calculated

by the single pulse laser model.

The surface temperature of the substrate is calcu-

lated to be 3818C, which is well above that of the melt

temperature 3278C of the ¯uorocarbon, so the surface

melts as well. The time dependence of the ¯uorocar-

bon temperature is also calculated and is shown in

Fig. 5(b). At time t1 the substrate melts and is rough-

ened by vaporized water. After 4 ns it re-solidi®es.

During melting and re-solidifying the surface is also

roughened as shown in Fig. 6. The resulting contact

angle changes from 110 to 288.

4. Laser-induced chemical reactions

Table 2 shows SIMS (secondary ion mass spectro-

metry) data of ¯uorocarbon under four conditions:

Fig. 5. Temperature rise of the (a) water thin ®lm and ¯uorocarbon substrate along the z-direction, and (b) surface temperature of the substrate

with time.

4 F. Huang et al. / Applied Surface Science 174 (2001) 1±6

(1) untreated, (2) laser treated without solution, (3)

water being used as the reaction solution, (4) boric

acid as the solution, and (5) sodium aluminate as the

solution, respectively. After treatment the concentra-

tions of clusters CF, CF2, CF3, C3F3, C2F4, C3F5, C4F6

and C4F7 decreased and CH2, C3H5, C4H3 increased.

For example, the C3F3 atomic group decreases from

2.05 to 1.01, 0.39, 0.94 and 0.83 and the CH2 atomic

group increases from 0.085 to 0.38, 0.27, 0.46 and

0.85 under treatment conditions 2, 3, 4 and 5, respec-

tively. The C±F bond atom group ratio decreased more

when water is used, which indicated a larger adhesive

force. These results indicate that the C±F bonds are

partially broken and C±H bonds are created, with ions

such as H� replacing the F atoms in the ¯uorocarbon

resin. On the other hand, the C±H epoxy resin bond is

harder than C±F epoxy resin bond [8].

The laser-induced chemical reactions and the

photolysis of the ¯uorocarbon can be expressed

schematically as follows:

The C±F bond is broken when the laser irradiates,

and C±H bonds are formed if precursor species are

available. For pulse irradiation the average rate is [9]

hWs�x�i � 1

pti

hsF

x2 � h2s

sABNABFhn

(7)

where F is the cross section of the laser beam, f the

¯uence per pulse, and NAB the concentration of the

¯uorocarbon species. sAB is the dissociation cross sec-

tion. The density of the product species is as follows:

F �Z

FWs�x� (8)

When the pulse number increases, F increases accu-

mulatively, F also increases, and the adhesive force

increases accordingly, which is shown in Fig. 3(b).

In summary, the hydrophilic character of ¯uorocar-

bon resin can be improved by excimer laser irradiation,

Fig. 6. Surface after irradiation by (a) 1500 pulses with 535 mJ/

mm2 energy density laser H2O is used, and (b) 1500 pulses of

535 mJ/mm2 in NaAlO2 solution.

Table 2

The intensity ratios, masses and structures of the clusters

Mass Formula Intensity ratio

1a 2b 3c 4d 5e

1 H 9.06 34.35 23.22 28.01 28.11

12 C 54.4 41.06 66.64 48.41 49.65

14 CH2 0.08 0.38 0.27 0.46 0.85

27 C2H3 0 1.76 0.35 1.55 1.94

31 CF 22.08 11.94 3.49 12.71 10.87

41 C3H5 0 1.83 1.13 1.80 1.89

50 CF2 0.81 0.49 0.08 0.44 0.35

51 C4H3 0.03 0.17 0.07 0.18 0.11

65 CH5O3 0.01 0.08 0.09 0.11 0.09

69 CF3 5.74 3.11 3.05 2.13 3.01

92 C3F3 2.05 1.01 0.39 0.94 0.83

100 C2F4 1.74 1.021 0.16 0.88 0.95

131 C3F5 3.46 2.40 0.72 2.09 0.95

162 C4F6 0.23 0.16 0.05 0.12 0.09

181 C4F7 0.25 0.23 0.28 0.16 0.25

a Untreatment.b Laser treatment without solution.c Solution is water.d Solution is boracic acid (H3BO3).e Solution is NaAlO2.

F. Huang et al. / Applied Surface Science 174 (2001) 1±6 5

and adhesion to other materials improved. Different

solutions such as water, boric acid, sodium hydroxide,

copper sulfate, and sodium aluminate improve both

adhesion and wetting angle beyond that attainable with

laser irradiation alone. The mechanism of this

improvement can be understood in terms of a simple

heating model and laser-induced chemical reaction.

Acknowledgements

This work is supported by State Key Lab of Laser

Technology, Huazhong University of Science and

Technology, China.

References

[1] F. Hennari, W. Blau, Excimer laser surface treatment of

metals for improved adhesion, Appl. Opt. 34 (1995) 581.

[2] H. Niino, A. Yabe, Surface modi®cation and metallization of

¯uorocarbon polymers by excimer laser processing, Appl.

Phys. Lett. 65 (1993) 3527.

[3] K. Hatao, T. Okamoto, K. Toyoda, Development of a

continuous surface modi®cation system of ¯uorocarbon resin

for strong adhesion, Laser Sci. Progr. Rep. RIKEN 19 (1997)

64.

[4] T. Okamoto, T. Shimizu, K. Toyoda, Development of even

exposure system for improvement of dyeing property of

¯uorocarbon resin surface, Laser Sci. Progr. Rep. RIKEN 19

(1997) 67.

[5] M. Olfert, R.E. Muller, W. Duley, T. North, J. Hood, D. Sakai,

Enhancement of adhesion in coated steels through excimer

laser surface, J. Laser Appl. 8 (1996) 79.

[6] H. Gao, Q. Lou, J. Dong, D. Ning, Z. Ye, Y. Wei, Beam

homogenizer for XeCl excimer laser and its application, Acta

Opt. Sinica 16 (1996) 1379.

[7] A.D. Zweig, A thermo-mechanical model for laser ablation, J.

Appl. Phys. 70 (1991) 1684.

[8] J. Breuer, S. Metev, G. Sepold, Laser-induced photochemical

adherence enhancement, Appl. Surf. Sci. 46 (1990) 336.

[9] D. Bauerle, Laser Processing and Chemistry, 2nd Edition,

Springer, Berlin, 1996.

6 F. Huang et al. / Applied Surface Science 174 (2001) 1±6