Wet-Process Surface Modification of Polyimides - ACS Symposium

Chapter 13

Wet-Process Surface Modification of Polyimides Adhesion Improvement

Downloaded by NANYANG TECHNOLOGICAL UNIV on August 25, 2015 | http://pubs.acs.org Publication Date: November 9, 1990 | doi: 10.1021/bk-1990-0440.ch013

Kang-Wook Lee and Steven P. Kowalczyk T. J. Watson Research Center, IBM Corporation, Box 218, Yorktown Heights, NY 10598

Polyimide surface modification by a wet chemical process is described. Poly(pyromellitic dianhydride-oxydianiline) (PMDA-ODA) and poly(bisphenyl dianhydride-paraphenylenediamine) ( B P D A - P D A ) polyimide film surfaces are initially modified with KOH aqueous solution. These modified surfaces are further treated with aqueous HC1 solution to protonate the ionic molecules. Modified surfaces are identified with X-ray photoelectron spectroscopy (XPS), external reflectance infrared ( E R IR) spectroscopy, gravimetric analysis, contact angle and thickness measurement. Initial reaction with K O H transforms the polyimide surface to a potassium polyamate surface. The reaction of the polyamate surface with HC1 yields a polyamic acid surface. Upon curing the modified surface, the starting polyimide surface is produced. The depth of modification, which is measured by a method using an absorbance-thickness relationship established with ellipsometry and E R IR, is controlled by the K O H reaction temperature and the reaction time. Surface topography and film thickness can be maintained while a strong polyimide-polyimide adhesion is achieved. Relationship between surface structure and adhesion is discussed.

Polymer surfaces are modified to obtain the desired surface properties without altering the bulk properties (jj. One of the most desired properties is adhesion between polymers and other materials such as polymers, metals or ceramics (2). There are many techniques for polymer surface modification, but they can be divided into two major categories. One is a dry process in which the polymers are modified with vapor-phase reactive species that are

0097-6156/90/0440-00179$06.00/0 © 1990 American Chemical Society In Metallization of Polymers; Sacher, E., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.


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METALLIZATION OF POLYMERS

often plasma excited (3). The other is a wet process in which the polymers are modified in chemical solutions (4,5). The reactive species of the dry process are ions, radicals or electrons generated by electrical discharge, electron beam or laser irradiation. Since the radicals and the electrons are involved in the modification, the reactions are complicated and the products are not well understood. O n the other hand, the reactive species of the wet process are acids, bases, electron donors, or electron acceptors in a solvent. If the reaction with small molecules in homogeneous solution is well-defined, there is a chance that the corresponding reaction occurs at the polymer sur face. If a single functional group can be introduced to the surface, the re lationship between the surface structure and the surface properties can be studied. The dry process can be applied to any polymer, but the wet process can be applied only to polymers which are insoluble in solvent. In this work we are interested in polyimide surface modification by wet processes. Polyimides are employed as insulating layers in the fabri cation of chips and chip carriers since they have good processability and excellent bulk properties such as low dielectric constant, high thermal sta bility, low moisture absorption, low thermal expansion, good mechanical properties, etc (6). It is useful to change only the surface properties without altering these bulk properties. It is known that the imide ring can be opened with a base such as an amine or an hydroxide (7). A polyimide such as poly(pyromellitic dianhydride-oxydianiline) ( P M D A - O D A ) reacts with K O H or N a O H to give a polyamate (potassium or sodium salt of polyamic acid) which is subsequently protonated with acid to give the corresponding polyamic acid (8). Upon curing at 230 ° C or higher, the polyamic acid is converted back to polyimide.

SCHEME ι

PMDA-ODA

Polyamate

Polyamic Acid

PMDA-ODA

In Metallization of Polymers; Sacher, E., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.


13. LEE AND KOWALCZYK

Wet-Process Surface Modification ofPolyimides 181

If the concentrated K O H solution is employed at high temperature, the base hydrolyzes the amide to cleave the chains. Thus the surface of the polyimide is etched (9). If the etching occurs up to a few microns of the surface, adhesion of metal to polymer is improved (10). The depth of polymer surface modification generally ranges from a few tens A to a few microns depending on the application purposes. For some industrial applications such as microelectronics and photoimaging, it is useful to maintain the surface topographical integrity within a few hundred A. Thus the maximum modification depth should be within a few hundred A. The depth is usually measured by X-ray photoelectron spectroscopy (XPS), Rutherford back scattering (RBS), forward recoil spectrometry ( F R E S ) or infrared spectroscopy (IR). However, the depths in the range of 100 A - 300 A cannot be measured by either of these techniques since the sensitivities are out of range. The X P S sampling depth is less than 100 A and the R B S sensitivity is around 300 A. Thus there is an inaccessible zone between 100 A and 300 A. We have been able to measure the depth of modification from 100 A up to 1000 A using an absorbance-thickness relationship established with ellipsometry and external reflectance infrared spectroscopy ( E R IR). Here we report a wet surface modification of P M D A - O D A and poly(bisphenyl dianhydride-/?ûrû-phenylenediamine) ( B P D A - P D A ) with K O H or N a O H solution. The modified surfaces are identified with contact angles, X P S spectra and E R IR spectra. Polymer thickness and weight changes are also studied. The depth of modified layer is measured by a non-destructive technique using E R IR and ellipsometry. Relationship between surface structure and adhesion strength is discussed.

Experimental Materials. Kapton H ( P M D A - O D A ) films (25 um), P M D A - O D A polyamic acid and B P D A - P D A polyamic acid were obtained from D u Pont. Upilex S ( B P D A - P D A ) films were purchased from Ube Chemicals. K O H , N a O H , HC1, l-methyl-2-pyrrolidinone ( N M P ) and isopropanol were obtained from Aldrich.

Methods. Polyamic acid in N M P was spin-coated onto a Si or Quartz wafer (diameter = 2.25 inches) coated with C r , and then cured to polyimide at 400 ° C . The purpose of the 500-750-A-thick layer of chromium is to enhance wettability and to give good reflectance to the Quartz wafer. Kapton H ( P M D A - O D A ) and Upilex S ( B P D A - P D A ) films were employed for gravimetric analysis. Around 5-um thick layers were used to measure the thickness change. The 100-1000-A-thick layers were employed to obtain X P S and E R IR spectra. The samples for contact angle measurement, X P S and E R IR were dried under vacuum at ambient temperature for 12-24 h and the samples for gravimetric analysis were dried at 85 ° C for 12 h. The samples for film thickness measurement were fully re-cured to polyimide.




182

Contact angle measurements were obtained with a Rame-H art telescopic goniometer and a Gilmont syringe with a 24 gauge flat-tipped needle. Dis tilled water was used as the probe fluid. Dynamic advancing and receding angles were determined by measuring the tangent of the drop at the inter section of the air/drop/surface while adding (advancing) and withdrawing (receding) water to and from the drop. External reflectance infrared ( E R IR) spectra were obtained under nitrogen using a Nicolet-710 F T I R spectrometer with a Harrick reflectance attachment. X-ray photoelectron spectra (XPS) were taken with a Surface Science Laboratories SSX-100 spectrometer with ΑΙ Κ α excitation. Thicknesses of polyimide were meas ured with a Dek-Tak for thicknesses greater than 1000 À and with a Waferscan ellipsometer equipped with a HeNe laser (λ = 6328 À ) for 100-1000 À . The refractive index employed for P M D A - O D A is 1.73. Modification O f P M D A - O D A . The polyimide samples were treated with 1 M K O H aqueous solution at 22 ° C for 1 - 1 2 0 min depending on the purposes, followed by washing with water ( 2 x 3 min) and isopropanol ( 2 x 3 min). The samples were dried under vacuum at ambient temperature for 12 h. The resulting modified surface is potassium polyamate as identified by X P S and E R IR. The X P S survey spectrum displays a K2s peak at 379 eV and K2p doublet at 294 eV, and the E R IR spectrum exhibits the bands at 1668 (s), 1608 (s), 1540 (m), 1512 (m, sh), 1502 (s), 1411 (m) and 1369 (w) cm in the range of 2000-1300 c m . The potassium polyamate samples were treated with 0.2 M HC1 aqueous solution at 22 ° C for 5 min followed by washing with water ( 2 x 3 min) and isopropanol ( 2 x 3 min). The samples were dried under vacuum at ambient temperature for 12 h or at 80 ° C for 12 h for gravimetric analysis. This modified surface is polyamic acid as identified by contact angle, X P S and E R IR. Thickness and weight changes of the films compared with the starting polyimide were also studied. The potassium peak in the X P S survey spectrum disappeared and the E R IR spectrum shows the bands at 1727 (s), 1668 (s), 1608 (m), 1540 (m), 1512 (m, sh), 1502 (s) and 1414 (m) c m - in the range of 2000-1300 cm" . Curing of the polyamic acid produces polyimide as identified by contact angle measurement, X P S and E R IR. A l l the analytical data are identical to those of the polyimide starting material. - 1

- 1

1

1

Modification O f B P D A - P D A . The samples were treated with 1 M K O H aqueous solution at 22 or 50 ° C for 1-90 min. The modified surface is potassium B P D A - P D A polyamate which is converted to polyamic acid by treatment with 0.2 M HC1 aqueous solution at 22 ° C for 5 min. The sample washing and drying procedures are similar to those for the P M D A - O D A modification. Adhesion Measurement. The solution of polyamic acid was spin-coated onto a chromium-coated Si wafer and cured at 400 ° C for 40 min. Thickness of the polyimide layer is approximately 6 um. A thin layer (200 A ) of gold was





sputter-coated onto one side (20% of the total area) of the polyimide sample to initiate peel (polyimide has poor adhesion to gold). The surface of the polyimide in the exposed area was modified to polyamic acid as described above. PMDA-ODA polyamic acid in NMP was spin-coated to the surface-modified polyimide film, and subsequently cured at 400 °C under nitrogen. Thickness of the adherate layer (peel layer) after curing is ap proximately 20 um and the width of the peel layer is 5 mm. The peel strengths were measured by 90° peel using an MTS with 25 um/sec peel rate. The reported values are the average of at least three measurements. Results And Discussion Surface Modification Of PMDA-ODA With KOH. Polyamic acid solution in l-methyl-2-pyrrolidinone (NMP) was spin-coated onto a Si or Quartz wafer and subsequently cured to polyimide. Since the polyimides cured at high temperature (350-400 °C) have good properties such as relatively low moisture absorption, low solvent swelling and good tensile properties (6), all thefilmsin this work were cured at 400 °C under nitrogen. For the purpose of gravimetric analysis (1_1), Kapton Hfilms,the chemical content of which is PMDA-ODA, were used. PMDA-ODA films were treated with 1 M KOH aqueous solution at 22 °C to give the corresponding potassium polyamate (potassium salt of polyamic acid). The excess of KOH was re moved by washing with water (2x3 min). These samples without further washing and drying were used for the protonation reaction (discussed be low). To identify the potassium polyamate surface by contact angle, XPS and ER IR, the samples were further further washed with isopropanol (2 χ 3 min) and dried under vacuum. The water contact angles (advancing (0a)/receding (€>r)) decreased from 85°/38° (on polyimide) to 23°/5°. The XPS survey spectrum displays new peaks due to potassium (Atomic Ratio Calcd for K/C: 0.09. Obsd: 0.02). Figure 1 shows the XPS Cl s regions of polyimide. and modified surfaces. The absolute binding energies are shifted due to charging and have not been corrected. We are interested in changes of the characteristic line shapes. There is only one carbonyl carbon peak (highest binding energy peak) of polyimide starting material (Figure la) since the polyimide carbonyls have the similar nuclear environments. But the spectrum (Figure lb) corresponding to potassium polyamate surface exhibits two carbonyl carbon peaks since the binding energies of carboxylate carbon and amide carbon are different. Changes in the Ols spectra are consistent with the changes in the CIs spectra. Thin layers (100-1000 À) of polyimide on the substrate were prepared for external reflectance IR spectra and to measure the average depth of modification, which will be discussed in detail. Figure 2 displays the ER IR spectra of the 870-A-thick polyimide and the modified samples in the range of 1900-1300 cm which provide the most useful information for this re-1



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290

285

Binding Energy (eV) Figure 1. C l s X P S core-level spectra of (a) P M D A - O D A starting ma terial, (b) potassium polyamate, (c) polyamic acid and (d) re-cured polyimide. The takeoff angle of electrons was 35° from the sample sur face.

ι 1800

.

.

,

"*

I

1600

1400

WAVENUMBERS (cm* ) 1

Figure 2. External reflectance IR spectra (a) P M D A - O D A polyimide, (b) potassium polyamate and (c) polyamic acid. The starting polyimide is 870 A thick and the whole layer is modified. The IR incidence angle is 3 7 ° from the sample surface.




action. The external reflectance IR spectrum of PMDA-ODA (Figure 2a) displays the bands at 1778 (w), 1740 (vs), 1726 (w, sh), 1598 (vw), 1512 (m, sh), 1502 (s) and 1381 (m, br) cm . The intensities of the external reflectance IR bands are slightly different from those of transmittance IR because of the orientation effects of vibrational modes (12). As shown in Figure 2a the strongest carbonyl stretching at 1740 cm is the strongest absorption while in transmittance IR the ODA phenylene stretching at 1502 cm is the strongest (the strongest carbonyl stretching band in transmittance IR appears at 1725 cnr ). The whole layer of 870-A-thick polyimide was modified to obtain a good IR spectrum. As shown in Figure 2b the imide carbonyl stretching at 1740 cm (imide I band) and the imide II band at 1381 cm completely disappeared. The peaks corresponding to PMDA-ODA polyamate are located at 1668 (s), 1608 (s), 1540 (m), 1512 (m), 1502 (s), 1411 (m) and 1369 (w) cm". The peaks at 1668 and 1540 cm correspond to the amide I and amide II bands, respectively. The peaks at 1608 and 1369 cnr are due to the carboxylate (asymmetric and symmetric stretching). It is noteworthy that Linde and Gleason (13) have observed the carboxylate bands at 1600 and 1390 cm in the transmittance IR spectrum of the samples prepared by the reaction of polyamic acid (baked at 120 °C) with KOH. The peaks at 1512 and 1502 cm", which are also observed in the polyimide spectrum, correspond to the phenylenes. The strong C-O-C (of ODA) stretching band appears at 1248 cnr (not shown in the figures). -1

-1

-1


1

-1

-1

1

_1

1

-1

1

1

Reaction Of Potassium PMDA-ODA Polyamate With Hydrochloric Acid.

The samples treated with KOH and washed with water were protonated by treating with 0.2 M HC1 aqueous solution at 22 °C for 5 min to yield a polyamic acid surface. The samples were washed with water (2x3 min) and isopropanol (2x3 min) and dried under vacuum. This modified surface was extensively analyzed since the polyamic acid surface is of great interest in electronic applications Q4). Treatment with HC1 does not modify the polyimide, but acidifies all of the potassium polyamate surface to the polyamic acid surface. The water contact angles increased from 23°(©a)/5°(©r) to 58°/8°, indicating that the surface became less polar. The polyamic acid surface is less polar than the potassium polyamate surface, thus these results are consistent with the proposed reaction shown in Scheme I. A kinetic study using the water contact angles of the modified (to polyamic acid) surfaces was performed by changing the KOH reaction conditions since the contact angle provides us with information on the top monolayer (5 A). The smaller the water contact angle is, the greater the wettability of the polyimide surface. The greater the wettability is, the stronger the polyimide adhesion. When the surface of the polyimide film was modified to polyamic acid, the water contact angles decreased to the



186


lowest values within a minute of K O H reaction and by subsequent acidification (0a/0r): 8 5 ° / 3 8 ° (on polyimide), 5 6 ° / 8 . 6 ° , 5 8 ° / 7 . 5 ° , 6 2 ° / 9 . 4 ° , and 6 0 ° / 8 . 6 ° for control, 1,5, 10 and 30 min reaction with K O H , respec tively). The control reaction only skipped the treatment with K O H . Further reaction up to 4 h does not alter the receding contact angle, but it provides inconsistent advancing contact angle because the extensive reaction makes the surface rough (IS). The receding contact angles, which are shown in Figure 3, are indicative of the surface wettability. A s illustrated in Figure 3, the outermost layer of P M D A - O D A has been modified within a minute of K O H reaction, while the surface modification of B P D A - P D A is at least ten times slower at room temperature than that of P M D A - O D A (further discussed below). Gravimetric analysis was performed on 6-cm films after the two-step reaction (KOH-HC1). The condition of an HC1 reaction is not a factor. The condition described here are on the K O H reaction. The weight keeps de creasing (less than 1 % for 10 min reaction with K O H and by subsequent acidification and 16 % for 4 h reaction) as the reaction progresses, but the thickness of the thin film layer does not change as much as the weight does. The thickness of the films are 4.87, 4.87, 4.82, 4.77 and 4.76 urn for control, 10, 30, 60, and 240 min reaction with K O H , respectively (Figure 4). These results suggest that hydrolysis and etching of Kapton H films, which are used for gravimetric analysis, may proceed faster into the X - Y direction of the film than into the Ζ direction probably since the edge of the film may be more labile to saponification. The potassium peaks in the X P S survey spectrum disappears. The shape of the X P S C l s spectrum (Figure lc) is similar to that of polyamate and consistent with that of polyamic acid as is the Ο Is spectrum. The E R IR spectrum (Figure 2c) exhibits the bands at 1727 (s), 1668 (s), 1608 (m), 1540 (m), 1512 (m, sh), 1502 (s) and 1414 (m) cm" . The strong band at 1727 c m corresponds to the carbonyl stretching of carboxylic acid. The peaks at 1668 and 1540 c m , which are also observed with polyamate, cor respond to amide I band (carbonyl stretching) and amide II band (coupling of C - N stretch and N - H deformation), respectively. The bands at 1512 and 1502 c n r , which also appear in the spectra of polyimide and polyamate, are due to the phenyl groups. Assignment of the 1414 c m peak (also observed in the polyamate) is not clear, but it seems related to the C - N stretching (imide II band) at 1381 c m . The intensity and the shape are similar. The vibrational mode of imide II band has been assigned (16,17). The C - O - C stretching of O D A has the similar wavenumber (1248 c m ) for polyimide, polyamate and polyamic acid. Leary and Campbell (]8) have reported that both carboxylic acid and amide of P M D A - O D A polyamic acid, which is baked at 120 ° C to remove the N M P solvent, react with K O H . They have observed changes of carbonyl binding energies in the X P S spectra. O n the other hand, in the 2

1

- 1

- 1

1

- 1

- 1

-1




present work neither the carbonyl binding energies in the X P S spectra nor the amide carbonyl stretching band in the E R IR spectra have changed, indicating that no salt of amide is present at the modified surfaces. Even if the amide of polyamic acid reacts with K O H to give amide salt, the salt may be protonated in the washing stage (refer to experiment).


Surface Modification Of PMDA-ODA With NaOH And Acetic Acid. P M D A - O D A samples were treated with a 1.25 M N a O H aqueous solution at 22 ° C for 1-10 min followed by protonating with 0.1 M acetic acid. The samples were washed and dried as previously described. Water contact angles, X P S spectrum and E R IR spectrum of the modified surface are typical of a polyamic acid surface, indicating that the reaction proceeds in the same way as described above. Upon curing the polyamic acid surface, the starting polyimide surface is produced. Contact angles and X P S C l s spectrum (Figure Id) of the re-cured surface are exactly same as those of the starting polyimide.

Measurement Of Modification Depth With ER IR And Ellipsometer. For some industrial applications such as microelectronics and photoimaging, it is useful to modify the polymer surface deep enough to achieve good adhesion while minimizing the loss of polymer and maintaining the surface topography. A s shown in Figure 4, the thickness of P M D A - O D A polyimide (5 urn) remains unchanged when the polyimide was treated with 1 M K O H aqueous solution at 22 ° C for 10 min. The topography of the modified surface also remains unchanged within the limit of S E M sensitivity while a strong polyimide-polyimide adhesion is achieved. T o observe the loss of polymer thickness within a few tens A, a very thin layer (260 À) of polyimide was prepared by spin-coating a dilute P M D A - O D A polyamic acid solution in N M P onto a chromium-coated Si wafer. Thickness was measured with an ellipsometer. The thickness of P M D A - O D A , which is modified and re-cured to polyimide, remains unchanged. We are interested in measuring the modification depth for the samples treated with 1 M K O H aqueous solution at 22 ° C for 10 min since surface topography and film thickness remain unchanged under these reaction conditions. The modified layer is thicker than the X P S sampling depth (approximately 100 A) since the X P S spectrum only displays the peaks due to the product. The R B S spectrum does not give a peak corresponding to potassium, so the modified layer is probably thinner than the limit of R B S sensitivity (approximately 300 A). These results indicate that the modification depth is in the range of 100-300 A which is an inaccessible zone for thickness measurement with usual techniques. McCarthy and co-workers (19) have modified poly(chlorotrifluoroethylene) ( P C T F E ) and could estimate the modification depth using X P S and UV-Visible. First P C T F E is modified within the X P S sampling depth


188


50


40

PMDA-ODA BPDA-PDA

30 "g 20 10

10 20 Time of Reaction with KOH (min)

30

Figure 3. Receding contact angle vs. reaction time with K O H . T h e contact angles were measured on the samples modified with K O H at 22 ° C and then treated with HC1.

5.3 weight change thickness

4.8 g

-1250

-2500

60 120 180 Time of Reaction with KOH (min)

4.3 240

Figure 4. Variations of P M D A - O D A film weight and thickness by the reaction with K O H at 22 ° C followed by acidification with HC1. The condition of the acidification reaction is not a factor.





and the modification depth is calculated by angle-resolved X P S technique. Then an UV-Visible absorbance corresponding to a modified surface with a known depth is measured. Assuming that the modification depth has a lin ear relationship with absorbance, they have estimated the depth by meas uring an absorbance of a modified polymer. This technique can apply only to the case in which the modified surface has an isolated UV-Visible absorbance. However, the modified surface in this work does not have any isolated absorbance peak. We have employed a method to measure the modification depth using an IR absorbance-thickness relationship established with ellipsometry and external reflectance infrared ( E R IR) spectroscopy. Thin and uniform layers (100-1000 A) of polyimide were prepared on the metal (chromium is used here to get good wettability of polyimide precursor) substrates, and then the film thicknesses and the absorbances of the imide carbonyl stretching were measured by ellipsometry and E R IR, respectively. The refractive index of P M D A - O D A employed here is 1.73. A s shown in Figure 5, there is a linear relationship between the film thickness and the absorbance of carbonyl stretching at 1740 c m . If the imide carbonyl absorbance of modified film is measured, the thickness of unmodified layer can be calculated. The depth of modification can be obtained by subtracting the thickness of remaining polyimide from the thickness of starting polymer. Figure 6 shows the E R IR spectra of the 260 Α-thick P M D A - O D A polyimides modified in an 1 M K O H aqueous solution for 0, 1 and 10 min. When the polymer reacted for 30 min, the imide carbonyl band completely disappeared (the IR spectrum is similar to one in Figure 2b). When the reaction time was 10 min, the absorbance of imide carbonyl is 0.0012 which corresponds to 30 A in the thickness-absorbance relationship shown in Figure 5. Thus the modification depth for the 10 min reaction is approximately 230 (subtracted 30 from 260) À . A s long as an IR absorbance corresponding to a polymer can be measured, the thickness-absorbance relationship can be established using ellipsometry and E R IR. Thus the average depth of modification can be calculated by measuring the absorbances of starting polymer and modified polymer. We have not found a previous report on this method. - 1

Polyimide-Polyimide Adhesion. T o study polyimide-polyimide adhesion (10), a thin layer (200 A) of gold was sputter-coated onto one side (20% of the total area) of the polyimide sample to initiate peel (polyimide has a poor adhesion to gold). The surface of the polyimide in the exposed area was modified to the polyamic acid surface. P M D A - O D A polyamic acid in N M P solvent was spin-coated to the surface-modified (to polyamic acid) polyimide film, and subsequently cured at 400 ° C under nitrogen. Thickness of the adherate layer (peel layer) after curing is approximately 20 urn and the width of the peel layers is 5 mm. The peel strengths were measured by 9 0 ° peel of the top polyimide layer. The failure occurs at the interface between



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Wet-Process Surface Modification of Polyimides - ACS Symposium

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