Spin Coating Polyimide Film on Hydrogenated DLC Film Surface Prepared by Ion Beam Deposition Method

Fong-Cheng Tai

_{, Shih-Chin Lee}

_{and Che-Hung Wei}

1

Department of Materials Science and Engineering, National Cheng-Kung University, Tainan 701, Taiwan

2_{Department of Mechanical Engineering, TATUNG University, Taipei 104, Taiwan}

The present work describes the hybrid layers consisting of 1-mm-thick hydrogenated diamond-like carbon (DLCH) film deposited by ion beam deposition (IBD) method as the bottom layer and spin coating polyimide (PI) film at a thickness of 6mmas the top layer. Optical microscopy shows that all DLCH film, PI film and PI/DLCH film have smooth surface and are free from cracks or wrinkles. Water contact angle measurement shows that the annealed DLCH film has the higher relative adhesive tension and adhesive work than that of PI film. Raman spectra show that the DLCH film can avoid severe graphitization & degradation at 300_{C during annealing treatment, and FTIR spectra also exhibit that}

the PI film will reach 95% imidization during the curing step at 300_{C. PI film can act on DLCH film as the stress buffer. This is demonstrate}

from warpage measurement showing that top PI film with tensile stress can partially reduce the compressive stress over bottom DLCH film with 6.7% warpage reduction. The electrical resistivity of PI/DLCH/Si is similar to that of PI/Si is resulted from the PI passivation effect. [doi:10.2320/matertrans.47.1869]

(Received February 13, 2006; Accepted June 14, 2006; Published July 15, 2006)

Keywords: hydrogenated diamond like carbon film, Polyimide film, Raman spectra, Fourier transform infrared spectrometer spectra

1. Introduction

It is well known that DLCH film has outstanding mechanical property (high hardness) and chemical property

(high chemical corrosion resistance).1)On the other hand, PI

film has good thermal property (high Tg) and chemical attack

durability due to five-fold type of imide ring.2) Generally,

DLCH film has two major drawbacks: poor adhesion due to higher compressive residual stress during deposition and worse thermal stability due to meta-stable amorphous structure. These can be solved with F or N or Si elements doping into pure DLCH film to form the complex a-C:H:X

film.3–6)_{Another solution is the annealed treatment on DLCH}

film in order to get the transformation from amorphous to graphitic structure. However, such treatments will cause hardness significantly decreasing when annealed temperature

is above certain critical transition point.7–9) _{Other methods}

use the additive layer under the DLCH film, such as the interlayer structure (DLCH/Ti/substrate), multilayer struc-ture (DLCH/TiCN/TiN/Ti/substrate) and special graded

structure to improve the DLCH adhesion.10) In addition,

many studies have been reported about the property in DLCH

over polymer structure.11–21) Several test results show that

DLCH/polymer can significantly reduce the oxygen gas penetration into polymeric film. From our knowledge, no study about spin coating polymer on DLCH structure is reported. The purpose of this work is to evaluate the thermal stability, buffer stress and interfacial adhesion on this PI/ DLCH/Si hybrid structure.

2. Experimental Details

DLCH film was deposited by ion beam deposition method using hydrocarbon gas on 6’’ (100)-oriented p-type silicon

wafer with electrical resistivity 30–60-cm (WAFER

WORKS CORP.). During the DLCH deposition, the base

pressure of the ion beam chamber reached 5104_Pa

within 40 minutes by rotary pump and turbo pump, and the working pressure is 0.1 Pa. The hydrocarbon gas is used as the reactive gas, and the DLCH growth temperature was

controlled at 200_{C and the final deposited thickness was}

1.0mm. To evaluate the cured degree of PI film, the liquid

polyimide precursor was synthesized with PMDA and ODA monomers. PMDA-ODA type of polyimide film was fab-ricated on bare Si and DLCH surface with spin-on method. The liquid PI precursor is dispensed at wafer center with the spin speed up to 600 r.p.m to spread the PI precursor over wafer surface within 10 seconds, and then the PI film is operated under 2500 r.p.m for 30 seconds, at last the PI/Si

wafer was cured at 300, 350 and 400_{C for one hour in the N}

2 gas, respectively. The PI/DLCH wafer was also cured at

300_{C for one hour in the N}

2gas. With cured treatment, the

liquid polyimide precursor film was converted into solid polyimide for better electronical, mechanical and chemical properties and the final thickness of cured polyimide film was

6.0mm.22)The chemical structure of PMDA-ODA polyimide

unit is shown in Fig. 1. This type of polyimide (C22H12N2O5)

consists of two major parts: one is the modified PMDA resin

(Pyro-mellitic dianhydride: C10H2O4) containing one

ben-zene ring, four C=O bonds, and two N atoms; the other is due

to modified ODA resin (Oxydianiline: C12H10N2O)

contain-ing two benzene rcontain-ings and one O atom.2)

The thickness of DLCH and PI films were measured by FE-SEM (Philips, XL-40FEG). Water contact angle (OP-TEM) was also measured by using Laplace-Young equation.

Raman spectroscope (488 nm wavelength of Arþ _{laser and}

probe aperture 10mm) was used to identify the amorphous

structural of DLCH film. Fourier Transform Infrared (FTIR) spectrometer was applied to evaluate the curing degree of PI film. The ATR contact probe mode was used to avoid the interference fringes resulted from the uniformity of PI film thickness. IR spectra were taken from Perkinelmer Spectrum

2000 at a resolution of 4 cm1and the scan number was 16

units. Residual stress was determined by the profilometer

with the contact probe and was calculated from the conven-tional Stoney’s formula based on the radius curvature differences of the substrates between before and after coated with DLCH or PI film. The thermal stress of the film was calculated from the following formula:

th¼EFT ðFSÞ ð1Þ

whereEF is the Young’s modulus of the film,T was the

temperature difference between treatment and room

temper-ature,F,Swere the coefficients of thermal expansion of the

film and the substrate, respectively. Tape test used crosscut pattern tool (ASTM D3359-02) was used to evaluate the adhesion strength between PI and DLCH interface. MIS (Metal-Insulator-Semiconductor) sandwich structure was used to measure the electrical resistivity by using Cu as top and bottom electrode, the measuring tool is Agilent 4156 C precision semiconductor parameter analyzer, the test fre-quency is set at 1 MHz.

3. Results and Discussion

3.1 Surface morphology and cross section

The morphology of the 1.0mmthick DLCH film, 6.0mm

thick PI film and PI on DLCH hybrid films was made by optical microscope with a magnification factor 1000X. The DLCH film is free from any surface crack even with 1.67 GPa compressive stress measured by surface profilometer. PI surface is smooth and crack-free with low tensile stress; the stress value for PI film is 90 MPa and is about 5.4% of DLCH

film. Figure 2 shows the cross section of PI/DLCH hybrid structure and there is no crack existed at the DLCH/Si or PI/ DLCH interfaces. The measured thickness of DLCH film is

1.0mmand the thickness of PI film is 6.0mm.

3.2 Contact angle test

According to the Young’s equation on three phase

boundary (air-substrate-liquid), the adhesive tension

(Lcos) can be derived from the simple difference

(S{SL) between the surface tension of the solid (S) and

the interfacial tension at solid/liquid (SL).23)The definition

of adhesive work is L (1þcos). It is well known that

DLCH has the larger contact angle because of the

hydro-phobic C-H or CH3 chemical groups measured by FTIR

spectrum, as a result, DLCH contains high non-polar part.24)

However, in this study, annealed DLCH film seems to be more polar than the cured PI film. Possible reasons are the

loss of hydrogen evolution, graphitization from sp3 _{to sp}2

carbon bonding and oxidized surface after the N2 gas heat

treatment or surrounding contamination. Unlike DCLH film,

PI is a strongly polar polymer due to imide ring.1)_{Table 1}

shows the water contact angle test results based on the surface free energy of distilled water is 72.1 mN/m. The measured contact angle of DLCH film is consistent with the

Ostrovskaya’s result.25) The water contact angle on the

annealed DLCH is less than that of cured PI, but the water contact angle of the PI sample is similar to that of the PI/ DLCH. As a result, the adhesive tension on DLCH surface is

Fig. 2 The SEM image for cross section of PI/DLCH hybrid structure.

Table 1 The water contact angles, adhesive tensions and adhesive works.

Sample type DLCH/Si wafer PI/Si wafer

Color bright black dark brown

Water droplet

Contact angle 52.6 _61.3

larger than that on PI surface at a value of 9.2 mN/m. From these observations, we can hypothesize PI on DLCH system is a polar on more polar system, therefore surface roughness and surface chemical groups (hydrophilic or hydrophobic type) will affect the measured values during contact angle test.

3.3 Raman spectra of DLCH film

Figure 3 shows the typical Raman spectra of DLCH and annealed DLCH film. The D band means the disorder carbon

bonding centered at around 1350 cm1_{and G band means the}

graphite carbon bonding located at around 1580 cm1_.

Table 2 lists the D band position, G band position, full width high maximum (FWHM) of D band and G band, and

ID=IG ratio measured by Gaussian curve fitting under

different annealed condition. The clear trend is that the D

band peak position, G band peak position andID=IGpeak area

intensity ratio increase with increasing annealed temperature. These annealed characteristics indicate that an annealed DLCH film would lose the hydrogen and induce

graphitiza-tion reacgraphitiza-tion from C-C sp3_{bonded type transformation to}

C-C sp2_{bonded type and cause a small reduction of the residual}

stress.26,27)_{Table 2 also implies that the DLCH is more stable}

and has slight graphitization when heating up to 300_{C. This}

critical temperature is in agreement with the study that thermal annealing treatment would result in the conversion from amorphous carbon into graphite and soften the DLCH

structure.7–9)

3.4 FTIR spectra of PI film

The thermal imidization step of PI film consists of two major reactions: imidization and dehydration. The former is to form the imide ring with five fold type in relative to

general benzene ring (six fold type), and the latter is to evaporate the water molecule because one unit of PMDA and ODA monomer will produce two unit of water molecule during the polymerization reaction. The so-called ‘‘curing degree of PI’’ is the same meaning as conversion percentage of PI. The curing degree can be determined by the

conven-tional conversion equation. If one peak at 1380 cm1_{band is}

assigned to C-N stretching vibration of imide ring, and the

other peak at 1500 cm1 _{band is assigned to C-C stretching}

vibration of benzene,28)then the curing degree can be defined

as:

Curing degree

¼ bðA1380=A1500ÞT=ðA1380=A1500Þ400Cc 100% ð2Þ

whereAIis the intensity of absorbance at I wavelength.

The previous study has shown the PMDA-ODA PI

precursor will begin to convert to full cured PI at 250_C

and its initial decomposition temperature (Td) is 470C when

the curing temperature is 400_{C. Figure 4 shows the FTIR}

absorbance spectra of spin on PI film cured at 100, 300, 350

and 400_{C for one hour, and the relative curing degree is 47,}

95, 97 and 100%, respectively. From these results, cured

temperature 300_{C is the most critical temperature in order to}

get enough curing degree. This implies that the imidization reaction has almost finished at this critical temperature. Generally, the higher the curing temperature will result in

higher Tg and higher fracture strength for PI film.29)

3.5 Residual stress

The concave warpage of bare Si wafer is 3mmas shown in

Fig. 5(a). It shows during the polished step of wafer fabrication process, the wafer was in tensile residual stress state. Figure 5(b) shows that the convex warpage of DLCH

on Si wafer is about 30mm, this is caused by the energetic ion

bombardment during the DLCH deposition process. The compressive residual stress of as-deposited DLCH films range from 1.53–1.82 GPa was measured by warpage test. Many studies have reported that the high compressive stress

of DLCH film ranging from 5.0 to 12.5 GPa.30) The

compressive stress can be drawn from the fact that the coefficient of thermal expansion (CTE) of DLCH is smaller than that of Si wafer. It is very difficult to measure the

1000 1200 1400 1600 1800

Intensity (a.u)

Raman shift, R/cm-1

400o

C

350o_C

300o_C

RT

Fig. 3 Raman spectra of annealed DLCH films.

Table 2 Deconvolution for Raman spectra of DLCH films. Annealed

Temp. (_C)

D band FWHM (cm1₎

G band FWHM (cm1₎

D band (cm1₎

G band (cm1₎

ID=IG

ratio RT 331 142 1367 1555 1.72 300 318 135 1374 1559 1.77 350 319 128 1378 1562 2.02 400 308 123 1385 1566 2.04

800 1000 1200 1400 1600 1800

400O

C

350O

C 300O

C 100O_C

Intensity (a.u)

Wavelength, W/cm-1

thermal stress of DLCH film without accurate measurement of DLCH’s CTE and Young’s modulus. Figure 5(c) shows

that the concave warpage of PI on Si wafer is 4.5mmresulted

from the cured process of PI and the CTE of PI (35 ppm/k) is larger than that of Si wafer (2.3 ppm/k). The measured tensile residual stress of PI film at cuing temperature 300,

350 and 400_{C are 90, 95 and 114 MPa, respectively, the}

calculated thermal stress are 41, 48 and 56 MPa, respectively. The thermal residual stress is estimated near 50% of total residual stress for PI film. Figure 5(d) shows that the convex

warpage of PI/DLCH/Si wafer is around 28.0mm. In

comparison with Fig. 5(b), it has 6.7% warpage reduction, this means that PI has certain effect on DLCH film surface warpage and can be used as stress-buffer layer. The combination of PI’s residual tensile stress and thermal annealing relief effect on DLCH film can partially release

some DLCH’s residual compressive stress.7–9)

Adhesion strength can be evaluated by using the crosscut tool and tape test method. The results show a white and small area, which indicates the crosscut paths for remained DLCH surface due to PI film have been scratched out. On the other hand, the large and gray area shows the non-crosscut for PI/ DLCH combination. Figure 6(a) shows that for the PI on DLCH hybrid films, there is no network-like crack on the surface. The report from G.A. Abbas indicated 148-nm thick DLCH film on PET substrate had cracking all around the coating surface due to high compressive stress with

3.40 GPa.31)_{Another study from Y. Ohgoe showed that the}

0.3mmthick DLCH film on polymeric diaphragm of artificial

heart device will induce severe wrinkling from high compressive stress of DLCH film corrugates the lower

tensile stress of polymeric film.20)_{In contrast to the easily}

crosscut soft PI film, it is difficult to crosscut the hard DLCH film due to its high hardness. Figure 6(b) indicates that at the interface of PI/DLCH hybrid structure, the adhesion strength is high and this phenomenon is confirmed by the tape test.

3.6 Electrical resistivity

Figure 7 shows the typical current-voltage curve for the MIS structures on Cu/DLCH/Si/Cu, Cu/PI/Si/Cu and Cu/

PI/DLCH/Si/Cu system.32)_{The electrical resistance resulted}

from I–V feature curve show that the electrical resistivity of

DLCH is 1.23*1012-cm and is 2.72*1011-cm at 300_C

annealing state due to slight graphitization during annealing treatment. These values are in agreement with the previous

reports.33,34)The electrical resistivity of PI film is three orders

of magnitude higher than DLCH and is about 5.30*1014

-cm. The electrical series resistance of the PI/DLCH/Si

0 10 20 30 40 50

-200 -150 -100 -50 0 50 100 150

(d)

(c)

(b)

(a)

Wafer warpage,

Λ

/um (X10)

Scanning length, Λ/mm

Fig. 5 Warpage test for (a) Si wafer, (b) DLCH/Si, (c) PI/Si and (d) PI/ DLCH/Si hybrid structure.

Fig. 6 Tape test for PI/DLCH/Si hybrid structure with (a) before and (b) after test with a 50X magnification.

-10 -5 0 5 10

1E-14 1E-13 1E-12 1E-11 1E-10 1E-9 1E-8 1E-7 1E-6 1E-5 1E-4 1E-3

Current, I/A

Voltage, V/V DLC_RT

DLC_300o_C PI_300o

C PI/DLC_3000_C

Experimental results show that for PI/DLCH/Si hybrid

structure, the optimal curing step is 300_{C for one hour with}

N2 protective gas. This condition can get enough curing

degree for PI film during thermal imidization reaction and avoid severe graphitization resulted from the conversion of

sp3into sp2within DLCH film. PI/DLCH film have smooth

surface and are free from cracks or wrinkles due to high compressive stress of DLCH. The residual tensile stress of PI film can partially release DLCH’s residual compressive stress at PI/DLCH/Si hybrid structure. No obvious delamination occurs at the PI/DLCH/Si hybrid structure by tape test with crosscut pattern and this implies there is enough adhesion in the PI/DLCH interface. The electrical series resistance of the PI/DLCH/Si has increased significantly due to the PI passivation effect.

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