Electroless Plating of Copper on Polyimide Film

Japanese Journal of Applied Physics 48 (2009) 036501

REGULAR PAPER

Electroless Plating of Copper on Polyimide Film Modified by 50 Hz Plasma Graft Polymerization with 1-Vinylimidazole Chiow San Wong, Hon Pong Lem1 , Boon Tong Goh1 , and Cin Wie Wong1 Plasma Research Laboratory, Department of Physics, University of Malaya, 50603 Kuala Lumpur, Malaysia 1 nanoFLEX Sdn Bhd, E308, 16/11, Phileo Damansara 1, Jalan Damansara, 46350 Petaling Jaya, Selangor, Malaysia Received August 6, 2008; accepted December 22, 2008; published online March 23, 2009 This paper reports on the proof of concept work on the novel process of producing metalized polyimide (PI) film by coating a layer of copper (Cu) thin film on the surface of the PI film without using any adhesive. The method which is employed to produce a metalized PI film used in flexible printed circuit (FPC) is based on plasma graft polymerization of 1-vinlyimidazole (VIDz) on plasma pre-treated PI surface. The plasma grafted PI film (VIDz-g-PI) surfaces are characterized by X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), and scanning electron microscopy (SEM). AFM results show that the PI film surface has been successfully treated and grafted with VIDz. As post-thermal treatment is known to promote adhesion strength between the metallic film and the PI surface, the effects of post-thermal treatment environment and temperature on the adhesion property of Cu plated VIDz-g-PI (Cu/VIDz-g-PI) are evaluated. Post-thermal treatment in air shows better adhesion strength than in vacuum. The adhesion strength decreases as the post-thermal treatment temperature is increased. In the present development work, the adhesion strength obtained has met the initial market targeted 9 –10 N/cm adhesion strength. Samples obtained at a pre-selected plasma power and time window are able to maintain their adhesion strength after being subjected to ageing at 100 C for 168 h. # 2009 The Japan Society of Applied Physics DOI: 10.1143/JJAP.48.036501

1.

Introduction

Electroless plating of copper (Cu) onto polyimide (PI) films has attracted considerable attention especially for applications in flexible printed circuit and flexible packaging technology in the microelectronics industry due to its capability to produce good adhesion between metallic surface and PI film.1–6) In general, Cu metallization can be done by various methods, such as physical vapour deposition (PVD), chemical vapour deposition (CVD), electroplating, and electroless plating.7,8) Electroless deposition of metal from solution on PI film is one of the most frequently used industrial processes of metallization. However, the adhesion strength of the electrolessly deposited metal, such as Cu and nickel (Ni), to the pristine PI film is weak for practical application. Modification of PI surfaces by various methods such as chemical treatment,9–11) plasma treatment,12) and laser treatment13) to enhance the adhesion with the electrolessly deposited metals has been widely explored. Apart from these methods, surface modification of plasma-pretreated PI films via ultraviolet (UV) or thermally induced graft copolymerization with functional monomers has been reported.14) Plasma polymerization is a technique used to modify surface by depositing a thin polymer film on the substrate, without affecting the bulk properties of the substrate. The process is solvent-free, which makes this technique becoming more attractive for packaging technology in recent years.15–17) It was shown that the adhesion strength between the electrolessly deposited Cu and the PI surface had been improved as compared to the conventional electroless plating method. The adhesion strength of about 7 N/cm was achieved by using argon (Ar) plasma pretreated PI surfaces and modified by plasma graft copolymerization with 4-vinylpyridine (4VP) as a monomer.14) The strong adhesion between the electrolessly deposited Cu with the Ar plasma pre-treated PI surfaces and modified by plasma graft copolymerization was attributed to the strong interaction of the pyridine functional groups of the pre

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treated and plasma grafted 4VP with Cu. The improved adhesion strength was also attributed to the spatial distribution of the grafted 4VP polymer chains on the PI surface and into the metal layer. In this work, 1-vinlyimidazole (VIDz) monomer is used to produce the adhesion promotion layer for electroless plating of Cu on PI films modified by alternating-current plasma graft polymerization. Oxygen (O2 ) plasma is used for both pre-treated process and plasma graft polymerization. The effects of different plasma pre-treatment and grafting conditions on the adhesion strength between Cu layer and PI surface are investigated. The VIDz-g-PI surface properties are characterized by X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and atomic force microscopy (AFM). The effects of post-thermal treatment environment and temperature on the adhesion strength of Cu/VIDz-g-PI are also studied. 2.

Experimental Methods

2.1 Materials The PI film used in this study is Techpoly IH which is supplied in a roll of 70 mm width and 75 mm thickness. The film is cut into strips of about 7 3 cm2 in size. The surface of the PI films are cleaned with acetone and dried by nitrogen gas. These films are kept in oven at 40 C before use. The monomer used is VIDz. The chemical structures of PI and VIDz are shown in Fig. 1.14) 2.2

Plasma treatment and plasma graft polymerization of VIDz on PI surface The alternating-current (AC) plasma polymerization system is set up at the Plasma Research Laboratory, University of Malaya, Malaysia. The physical geometry of the system is shown in Fig. 2. The AC power supply provides electrical power from 0 to 50 W and is operated at a frequency of 50 Hz. The plasma deposition process occurs between two circular parallel plate electrodes of 5 cm in diameter with inter-electrode distance of 10 cm, in a Perspex cylindrical chamber. The PI sample is placed between the two circular parallel plate electrodes and subjected to the O2 plasma for a

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# 2009 The Japan Society of Applied Physics

Jpn. J. Appl. Phys. 48 (2009) 036501

O

O

C

C

C

C

O

O

N

C. S. Wong et al.

surface are determined by XPS. The XPS measurements are performed on a Kratos AXIS HSi spectrometer using a monochromatic Al K X-ray source (1486.6 eV photons) at a constant dwell time of 100 min and pass energy of 40 eV. The sample is mounted on the standard sample stud using double-sided adhesive tape. The core-level signals are obtained at a photoelectron take-off angle (, measured with respect to the sample surface) of 90 . The X-ray source is run at a reduced power of 150 W (15 kV and 10 mA). The pressure in the analysis chamber is maintained at 108 Torr or lower during each measurement. The binding energy (BE) referenced to the carbon (C) is the hydrocarbon peak at 284.6 eV. Surface elemental stoichiometry is determined from XPS spectral area ratio, after correcting for the experimentally determined sensitivity factor and is reliable to within 10%. The elemental sensitivity factor is calibrated using stable binary compounds of well-established stoichiometries.

O

N

n

(a)

CH=CH2 N N (b) Fig. 1. Chemical structures of (a) PI and (b) VIDz.

Pressure gauge Wire

2.4

Electrode plate

To vacuum pump

O2 + monomer

Electrode holder Fig. 2.

Wire

Schematic diagram of the plasma polymerization system.

pre-treatment process at an AC power of 18 W, an O2 flow rate of 5 sccm and the chamber pressure is kept at 1 mbar for 10 min. This set of plasma conditions has been found to be optimum for activation of the PI surface. The VIDz monomer was introduced into the reactive chamber by the O2 carrier gas flowing through a monomer reservoir. The monomer-carrier gas mixture is allowed to flow into the chamber evenly from a distributor located in the entry electrode. The carrier gas stream is assumed to be saturated with the VIDz monomer, as dictated by the partial pressure of the latter. The reactive chamber is maintained at a pressure of 1.0 mbar, while the O2 gas flow-rate is fixed at 5 sccm. For the deposition of polymer layer on the PI film surface, the PI substrate is pre-treated or pre-activated by O2 plasma at 18 W and for a fixed treatment time. After the treatment, the PI substrate will undergo the plasma polymerization process at a discharge power of 5 W and for a fixed grafting time. After the grafting process, the VIDz-g-PI film is washed thoroughly with distilled water for about 1 hour to remove any residual amount of the physically adsorbed VIDz monomer and homopolymer which may be adsorbed on the grafted layer.18) The grafted layer of VIDz also acts to prevent further oxidation at the surface but the absorbing homopolymer have to be removed through solvent extraction. 2.3 Surface characterization by XPS The chemical composition of the PI surface and VIDz-g-PI

Surface morphologies properties by AFM and SEM The morphology of the VID-g-PI film surface is studied by AFM using a Nanoscope IIIa AFM from Digital Instrument. In each case, an area of 50 50 mm2 is scanned using the tapping mode. The drive frequency is 330 50 kHz, and the voltage is between 3.0 and 4.0 V. The drive amplitude is about 300 mV and the scan rate is 0.5 to 1.0 Hz. An arithmetic mean of the surface roughness (Ra ) is calculated from the roughness profile determined by AFM. The PI surface and VIDz-g-PI surface properties are analyzed by SEM model PHI Quantum 2000 XPS/ESCA System, which include surface and cross sectional analyses. 2.5

Surface activation and electroless plating of Cu on the VIDz-g-PI surface The grafted PI (VIDz-g-PI) film is activated by immersing into an activator solution where palladium (Pd) catalyst is absorbed for the subsequent electroless deposition of Cu. In the activation process, the film is immersed for 6 min in a solution containing 1 mg/mL of PdCl2 and 10 mg/mL of HCl (37 wt %), followed by rinsing thoroughly with distilled water. Next, the Pd-activated film is sensitized by SnCl2 , through the immobilization of Pd catalyst. The surface activated VIDz-g-PI substrate is then immersed in an electroless Cu plating bath for 10 min. The composition of the solution in the plating bath is the following: 7 mg/mL of CuSO4 5H2 O, 25 mg/mL of potassium sodium tartrate, 4.5 mg/mL of sodium hydroxide and 9.3 mg/mL of formaldehyde.19) A Cu layer of about 100 nm is deposited. The Cu plated VIDz-g-PI (Cu/VIDz-g-PI) is then rinsed thoroughly with distilled water. The metalized PI film is subsequently electroplated with a Cu layer to about 25 mm thick. This additional Cu layer serves as an anchoring layer for peeling off the underlying electroless deposited metal during the adhesion strength measurement. The electroplating is carried out in the standard electroplating solution containing Cu(II) sulfate (0.75 M), sulfuric acid (0.6 M), and glucose (0.2 M).18) The electroplated substrate is rinsed thoroughly with distilled water.

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C. S. Wong et al. 10

Peel Strength (N/cm)

9

Smooth

8

Rough

7 6 5 4 3 2 1 0 2

Fig. 3. Schematic diagram of the peel test equipment.

8

16

10 9

Smooth

8

Rough

7 6 5 4 3 2 1 0 2

4

8

16

Grafting time (min)

Fig. 5. Peel strength of Cu to PI at different plasma grafting time. 11 0 min

Results and Discussion

3.1 Evaluation of pre-treatment time and grafting time Plasma pre-treatment and grafting times are determined at a fixed plasma power. The power is established to be 18 W for pre-treatment and 5 W for grafting. The low grafting power is chosen based on previous work.20) Plasma pre-treatment time is varied while the plasma grafting time is fixed at 3 min with O2 flow rate of 5 sccm. It is found that pretreatment times of 8 to 16 min seem to result in the highest peel strengths for both smooth and rough surfaces of PI as shown in Fig. 4 and hence 10 min pre-treatment time is used for the subsequent evaluations. In order to determine the optimum plasma grafting time it is varied at fixed pre-treatment conditions where the power and pre-treatment time are fixed at 18 W and 8 min respectively, while the grafting power is fixed at 5 W. It is found that 2 min grafting time results in the best peel strength for both rough and smooth surfaces of PI as shown in Fig. 5. In Fig. 6, the samples obtained at different grafting time are subjected to heat over a period of 168 h. It is observed that 6 min grafting time produces the best result, with peel strength of about 8 N/cm after ageing for 168 h at 100 C in Ar. Table I shows the peel strength of samples produced by plasma grafting process using different type of gases, namely O2 and Ar. Three tests have been carried out at different ageing times for each type of gas. O2 gas used

Peel strength of Cu to PI at different plasma pre-treatment

Avg. peel strength (N/cm)

2.6 Adhesion strength measurement The adhesion strength is determined by performing the T-peel adhesion strength measurement using the setup as shown in Fig. 3. The pre-peeled Cu is clamped using a clip and the clip is hooked onto the measuring unit. A vertical force is applied hence causing a 90 pull upwards while the sample is moving forward. The load is recorded from the measuring device to calculate the peel strength. Each adhesion or peel strength reported is the average of at least two sample measurements.

Fig. 4. time.


Finally, the rinsed and dried electroplated substrate is subjected to post-thermal treatment at 120 C for 3 h in an air oven. Post-thermal treatment has been known to promote further interaction of the deposited metals with the graft chain which can result in improved adhesion. The thermally treated sample is allowed to cool slowly to room temperature over a period of about 4 h to minimize any thermal stress at the metal-polymer interface.

3.

4

Pre-treatment time (min)

10

2.5 min

9

4 mins 6 mins

8 7 6 5 0

24

48

72 96 120 144 Ageing time (hour)

168

192

Fig. 6. Dependence of the peel strength on the ageing time of the Cu/VIDz-g-PI films. All the samples are subjected to post-thermal treatment at 100 C in Ar.

for both pre-treatment and grafting seems to give better adhesion as compared to when Ar gas is used for grafting. This can be due to more carbonyl and carboxyl species on the plasma treated PI surface produced by the oxidation of the active species on the PI surface in O2 plasma.14) The result here can be explained in more detail by referring to the XPS results in the following section. 3.2

Surface characterization by XPS analyses for plasma grafting using different gases The surfaces of the samples obtained are investigated using XPS. In Table II, pristine is the as-prepared PI sample.

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Table I. Peel strength at different grafting plasma after ageing process. Peel strength (N/cm) Process condition (Pre-treatment/grafting)

Test 1

Test 2

Test 3

0 h ageing

24 h ageing

0 h ageing

48 h ageing

0 h ageing

10.6

7.6

—

—

7.5

3.2

O2 /O2

>11:0

4.0

10.0

8.3

10.6

3.8

O2 /Ar

8.6

6.0

9.7

4.8

9.4

3.2

Control

168 h ageing

Control: Sample with pre-treatment process by O2 O2 /O2 : Sample with pre-treatment and grafting processes by O2 O2 /Ar: Sample with pre-treatment process by O2 and grafting process by Ar

Table II. Elemental composition (wt %) of the samples.

8000

6000

Sample Cl

Cu

Zn

Total

‘‘Pristine’’ ‘‘Control’’

67.0 52.7

7.9 7.7

24.2 26.6

— 0.6

0.9 0.2

— 2.9

— 2.6

— 6.7

100.0 100.0

‘‘Process 1’’

37.5

28.2

31.5

—

—

1.0

1.8

—

100.0

‘‘Process 2’’

51.2

21.1

23.2

—

—

3.6

0.9

—

100.0

Pristine: PI sample without plasma pre-treatment and grafting processes Control: PI sample with O2 plasma pre-treatment process only Process 1: O2 /O2 , PI sample with O2 plasma pre-treatment and grafting processes Process 2: O2 /Ar, PI sample with O2 plasma pre-treatment process and Ar plasma grafting process

Control is the PI sample that has been pre-treated by O2 plasma. ‘‘Process 1’’ sample is the PI sample that has been both pre-treated and grafted by O2 plasma. ‘‘Process 2’’ sample is the PI sample that has been pre-treated by O2 plasma and grafted by Ar plasma. From Table II, sample that has been plasma treated is observed to contain chlorine, Cu, and zinc as compared to pristine sample. However the amount is minimal or small. This can be due to the presence of materials such as poly(vinyl chloride) (PVC) and brass used to fabricate the electrodes. ‘‘Process 1’’ sample seems to contain greater amount of O2 as compared to ‘‘Process 2’’ sample. It is believed that O2 promotes the formation of free radical which enhances polymerization process.21,22) Surface composition of surface-treated and grafted PI films Figure 7 shows the C 1s core-level spectra of (a) as-prepared pristine PI film, (b) PI film treated by O2 plasma, (c) PI film pre-treated by O2 plasma and grafted by O2 plasma, and (d) PI film pre-treated by Ar plasma and grafted by O2 plasma. The C 1s core level spectrum of the pristine PI film consists of three components which are C–H (CI ), C–O and C–N (CII ), and N(C=O)2 (CIV ) species at 284.6, 285.8, and 288.4 eV respectively.23,24) An additional peak indicating component with a binding energy at 287.4 eV attributed to the C=O species (CIII ) is found in the O2 plasma pre-treated PI surface. This is attributed to the oxidation of active species on the PI surface induced by the O2 plasma in pretreatment process. No additional peak at that binding energy is observed in the Ar plasma pre-treated PI surface since the Ar plasma prevents formation of peroxide and hydroperoxide. Furthermore, the PI film surface treated by O2 plasma shows higher intensity in N 1s and O 1s as compared to the

6000

4000

Intensity (Arb. Units)

Si

287.6 288.4

2000

0

294

290

286

282

4000 287.6

2000

0

278

294

4000

(c)

288.4

4000

284.6 2000

1000

290

282

278

284.6

C 1s

3000

294

286

285.8

(d)

C 1s

0

290

Binding energy (eV)

Binding energy (eV)


P


O


N

3.3

C 1s

C 1s

C

284.6

(b)

284.6

(a)

286

282

Binding energy (eV)

278

3000

2000

1000

0

294

290

286

282

278

Binding energy (eV)

Fig. 7. XPS C 1s core level spectra of (a) as-prepared pristine PI film, (b) PI film treated by O2 plasma, (c) PI film pre-treated by O2 plasma and grafted by O2 plasma, and (d) PI film pre-treated by Ar plasma and grafted by O2 plasma.

PI film surface treated by Ar plasma in the pre-treatment process as shown in Fig. 8. This result suggests that O2 plasma treatment introduces higher concentration of O- and N-containing groups on the PI films. The VIDz-g-PI film surface exhibits the characteristic C 1s core level peak components of the functional groups in the imidazole ring, namely the C–N species (CV ) at 285.4 eV and the N–C=N species (CVI ) at 286.4 eV.18) The minor peak component at the higher binding energy of 288.4 eV, attributable to N(C=O)2 species, is associated with the imidazole groups of the underlying PI film. However, domination of N(C=O)2 species for VIDz-g-PI film surface treated by O2 plasma shown in Fig. 8(c) may help to enhance the adhesion between Cu and grafted PI films and make it capable of withstanding higher temperature. 3.4 Post-thermal treatment process Post-thermal treatment has been known to promote further interaction between the deposited metals with the grafted chain to result in improved adhesion. The process can be carried out after electroless or electrolytic plating. Various post-thermal processes and conditions including heating sequence, post-thermal environment and temperature are

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C. S. Wong et al. Table III. Comparison of peel strength of the Cu/VIDz-g-PI films at different condition.

(Control) 3500 9000

(a)

(b)

531.4 3000



O 1s 7000

5000

3000

399.4

390.0 2500

2000

1500

1000

1000 540

536

532

528

524

408

404

400

396

(Process 1) 8000

(c) O 1s

399.4

N 1s Intensity (Arb. Units)


10000

(d)

531.6

8000

6000

4000

Post-thermal condition

Surface

Single-sided Cu plated

Double-sided Cu plated

Electroless ! electrolytic

Smooth

4.2

1.3

Electroless ! electrolytic

Rough

4.2

1.3

Electroless ! electrolytic ! 140 C for 3 h in air

Smooth

9.2

0.4

Electroless ! electrolytic ! 140 C for 3 h in air

Rough

7.1

1.2

392

Binding energy (eV)

Binding energy (eV) 12000

Peel strength (N/cm)

N 1s

6000

4000

2000

2000

540

536

532

528

12

0

524

408

Binding energy (eV)

404

400

396

392


0

Binding energy (eV)

(Process 2) (e)

531.4

5000

O 1s



7000

5000

3000

399.4

(f) N 1s

4000

3000

smooth side 10

rough side

8 6 4 2

2000

0 1000

1000 540

536

532

528

Binding energy (eV)

524

408

404

400

396

120

392

140

150

160

Temperature (°C)

Binding energy (eV)

Fig. 8. XPS O 1s and N 1s core level spectra of PI film pre-treated by O2 plasma and grafted by O2 plasma (Process 1) and PI film pretreated by Ar plasma and grafted by O2 plasma (Process 2).

Fig. 9. Average peel strength at different post-thermal treatment temperatures (for 3 h in air).

investigated and reported here. In Table III, samples with post-thermal treatment show better adhesion property for both surfaces (single-sided) as compared to samples without the post-thermal treatment process. It is also found that single-sided Cu shows better adhesion than double-sided Cu. This is probably due to the difference in stress released on the two sides or surfaces. Furthermore, the average peel strength shows decreasing trend with increase in postthermal treatment temperature as shown in Fig. 9. At higher temperature, it does not result in improving the adhesion but instead it starts to weaken the bond between the metal and PI film resulting in the decrease of the peel strength. In this work, good peel strength of better than 10 N/cm is obtained with post-thermal treatment for 3 h in air at a temperature of 120 C. 3.5

130

Analysis of interface condition of Cu/VIDz-g-PI treated at different post-thermal temperature by SEM The adhesion condition at the Cu–VIDz interface of the Cu/ VIDz-g-PI sample after treatment at different post-thermal temperatures are investigated by observing the Cu surface after peel off using SEM as shown in Fig. 10. The SEM images are taken on the side of the Cu which is plated to the VIDz. From Fig. 10(a), which is for sample that has been

(a)

(b)

(c)

(d)

Fig. 10. SEM images of the peeled off Cu surface for the Cu/VIDzg-PI treated at post thermal temperatures of (a) 120, (b) 140, and (c) 160 C. Image (d) shows the PI surface for the Cu/VIDz-g-PI treated at post-thermal temperature of 160 C.

treated at post-thermal temperature of 120 C, a rough surface can be seen on the Cu surface. This rough surface is due to the peeling effect. On the other hand, the peeled off surface of Cu for the Cu/VIDz-g-PI treated at post-thermal

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C. S. Wong et al. 13 Control 6 mins 9 mins 12 mins


11

(a)

9 7 5 3

Post-thermal treatment in air

1 0

24

48

72

96

120

144

168

Ageing time (hr)

(a) 8 Control 7

(b)

6 mins 9 mins


6

12 mins 5 4 3 2 1

Post-thermal treatment in vacuum

0

(c)

0

24

48

72

96

120

144

168

Ageing time (hr)

Fig. 11. SEM cross sectional images of Cu/PI interface for the Cu/ VIDz-g-PI treated at post-thermal temperatures of (a) 120, (b) 140, and (c) 160 C.

temperatures of 140 and 160 C [Figs. 10(b) and 10(c)] are clearly smoother. This indicates that the peel strengths of Cu/VIDz-g-PI treated at post-thermal temperatures of 140 and 160 C are lower as compared to the Cu/VIDz-g-PI treated at post-thermal temperature of 120 C. Furthermore, notable blisters are observed on the surface of Cu for samples treated at post-thermal temperatures of 140 and 160 C. This is not seen on the surface of Cu for sample treated at post-thermal temperature of 120 C. Blisters are more dominant on both surfaces of Cu and PI for the Cu/ VIDz-g-PI treated at post-thermal temperature of 160 C as compared to that of 140 C [Fig. 10(d)]. The interfaces of Cu/VIDz-g-PI have also been investigated by taking crosssectional images as shown in Fig. 11. As can be seen from this figure, a good adhesion of interlayer between the Cu and the VIDz-g-PI is found for the Cu/VIDz-g-PI treated at postthermal temperature of 120 C. This has contributed to the higher peel strength observed for this sample. However, a lot of particles and defects were observed on the interlayer of the Cu/VIDz-g-PI treated at post-thermal temperatures of 140 and 160 C. This may be due to the diffusion of O2 into the interface causing re-polymerization at higher postthermal temperature. The re-polymerization formed defects which can reduce the adhesion of the Cu/VIDz-g-PI.25) It can be observed in the SEM image for the Cu/VIDz-g-PI treated at post-thermal temperature of 160 C that the interface shows very clear gap between the Cu and PI layers

(b) Fig. 12. The dependence of the grafting time and peel strength of electroless Cu plated VIDz-g-PI on (a) the post-thermal treatment in air condition and (b) the post-thermal in vacuum condition. The represents the electroless Cu plated on PI sample while the , , and represent the electroless Cu plated on VIDz-g-PI samples prepared at different grafting times (6, 9, and 12 min).

causing poor adhesion of the Cu/VIDz-g-PI. This agrees with the change of the peel strength at different post-thermal treatment temperatures. 3.6

Investigation on post-thermal treatment environment The effect of post-thermal treatment environment has been investigated with the condition in air and in vacuum at temperature of 100 C for electroless Cu plated VIDz-g-PI samples and the results are shown in Fig. 12. There are two possible types of reactions involved when the sample is under-going post-thermal treatment in air: chemical (presence of O2 ) and heat (aid in stress relaxation). The presence of O2 for the post-thermal treatment in air results in more free radicals being formed. This can lead to the occurrence of more chain reactions or polymerisation.14,26–28) This is evident from the results shown in Fig. 12, where the initial peel strength for the samples treated in air is much higher than that of the samples treated in vacuum. However, the adhesion property drops by an average of 40% after it is subjected to ageing test for 168 h. The presence of excess O2 in the sample can cause weakening of bond instead of

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Table IV. Average peel strength of samples after post-thermal treatment at different temperature in vacuum.

—

120 C for 24 h

3.5

130 C for 3 h

5.3

130 C for 24 h

—

140 C for 3 h

2.3

140 C for 24 h

3.0

improving the adhesion.25) In other words, the sample has reached its maximum degree of polymerization or curing before going through ageing test. When a material is 100% cured or polymerized, the properties are most likely to be more brittle and weaker when subjected to other tests as compared to when it is 90% cured or polymerized. On the other hand, sample that has been subjected to post-thermal treatment in vacuum shows lower initial peel strength but its adhesion property increases as ageing time is increased. During vacuum post-thermal treatment, only heat is involved and it is suspected that the sample has not reached its maximum curing property. Hence it may perform better during ageing test. The variation of post-thermal treatment temperature in vacuum does not seem to improve the adhesion strength as compared to post-thermal treatment in air. This can be seen in Table IV where lower peel strengths are recorded for the samples treated at higher temperature in vacuum. More studies are required in this area to understand what are the reactions involved during vacuum post-thermal treatment. 3.7 Morphology of surfaces by AFM The changes in surface morphology of the PI film after modification by plasma pre-treatment, plasma graft polymerization with VIDz and then Cu plating are studied by using AFM. The AFM images of the pristine PI film surface, plasma pre-treated PI, VIDz-g-PI and electroless Cu plated VIDz-g-PI are shown in Fig. 13. The image in Fig. 13(b) is observed to be rougher than the image in Fig. 13(a), due to the effect of plasma treatment. In Fig. 13(c), the VIDzgrafted surface is smooth as compared to pre-treatment surface due the formation of homogenous and film-like surface. This suggests that the film is deposited uniformly over the entire PI surface. The image in Fig. 13(d) is the surface of VIDz-g-PI film plated with Cu, which is smoother than the other surfaces shown in this figure. 4.

400nm (a)

Conclusions

The layer of VIDz monomer can be readily deposited by plasma polymerization on PI substrates using a 50 Hz alternating current glow discharge system. AFM images reveal that the PI surface is successfully treated and grafted with VIDz and it also suggests that a homogenous thin layer of grafted VIDz is obtained. The optimum plasma discharge powers for pre-treatment and grafting determined in the present work are 18 and 5 W respectively. The durations for these two processes are 10 and 6 min respectively. The gas flow-rate for O2 which is used as

(b)

50× ×50µm

50×50µm 150nm

120 C for 3 h

200nm


300nm

Post-thermal condition

50×50µm

50×50µm

(c)

(d)

Fig. 13. AFM image of (a) pristine PI film, (b) plasma pre-treated PI film, (c) VIDz-g-PI film, and (d) Cu plated VIDz-g-PI film. The images of (b), (c), and (d) are the films treated by O2 plasma for both pretreatment and grafting process.

the carrier gas is 5 sccm. An average adhesion strength of 9 to 10 N/cm is obtained when a single-layer Cu/VIDz-g-PI film is produced and subjected to post-thermal treatment at 120 C for 3 h in air. Ageing of the sample at 100 C for 168 h in Ar may result in a drop of the adhesion strength by about 12%. Acknowledgements This work was supported by the Cradle Investment Programme (CIP) grant under Malaysia Venture Capital Management Bhd (mavcap). We would like to acknowledge Dr. C. Q. Cui, Chief Technology Officer from Compass Technology Ltd., NT, Hong Kong for valuable guidance and support. We would also like to acknowledge Dr. Raymon Chan from Hong Kong Science and Technology Park (HKSTP), Hong Kong, for XPS measurement. Lastly, we would like to acknowledge Professor Gan Seng Neon from University of Malaya (UM) and Mr. Ng Kim Hooi from Tunku Abdul Rahman College (TARC) for their valuable comments in this work. Last but not least, we are grateful to the reviewer for his critical but constructive comments and suggestions to improve the contents as well as the writing of this paper.

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