Langmuir 1991, 7,2450-2453
2450
Surface Modification of BPDA-PDA Polyimide Kang-Wook Lee,* Steven P. Kowalczyk, and Jane M. Shaw IBM Research, Thomas J. Watson Research Center, Yorktown Heights, New York 10598 Received July 13,1990. I n Final Form: December 5, 1990 Poly(bipheny1 dianhydride-p-phenylenediamine) (BPDA-PDA) polyimide film surfaces are initially modified with KOH aqueous solution to yield a potassium polyamate surface. The reaction of the polyamate surface with acid gives a polyamic acid surface. The starting polyimide surface is reproduced upon curing the modified surface. Modified surfaces are identified with X-ray photoelectron spectroscopy, external reflectance infrared (ER IR) spectroscopy, and contact angle measurement. The average depth of modification, which is measured by a method using an absorbance-thickness relationship established with ellipsometry and ER IR, is controlled by the reaction temperature and time. Surface topography and film thickness can be maintained while a strong polyimide-polyimide adhesion is achieved. A relationship between surface structure and adhesion is established.
Introduction Polyimides are employed as dielectric layers in microelectronic applications such as printed circuit boards, module thin film wiring, chip carriers, and chip multilevel metal interconnects,' since they have low dielectric constant, high thermal stability, good mechanical properties, and processability.2 Since the polyimides were first developed for electronic applications, several chemistry types have been introduced. The first generation polyimides, which were used for film technology and hightemperature insulation, are poly(pyromellitic dianhydride oxydianiline) (PMDA-ODA), poly(benzophen0ne tetracarboxylic acid dianhydride-oxydianiline-m-phenylenediamine) (BTDA-ODA-MPD), and PIQ polyimide (isoindoloquinazoline diamine). The second generation polyimides, which were developed to simplify processing and to improve planarity, include photodefinable polyimides. A typical third generation polyimide is poly(bipheny1 dianhydride-p-phenylenediamine)(BPDA-PDA) which has low dielectric constant, low moisture absorption (less than l % )low , thermal expansion ((5-10) X lo6 K-9, and excellent toughness that are the important characteristics for microelectronic applications. Good adhesion between two polyimide layers is also a requirement for most microelectronic applications.' However,BPDA-PDA has poor self-adhesion. I t is possible to modify polymer surfaces to obtain the desired surface properties such as adhesion without altering the bulk proper tie^.^ 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 often plasma enhanced? The other is a wet process in which the polymers are modified in chemical solutions."7 (1) Tummala, R. R.; Rymaszewski, E. J. Microelectronics Packaging Handbook; V. N. Reinhold: New York, 1989. (2) (a) Bessonov, M. I.; Koton, M. M.; Kudryavtaev, V. V.; Laius, L. A. Polyimides: Thermally Stable Polymers; Consultante Bureau: New York, 1987. (b) Mittal, K. L., Ed. Polyimides; Plenum: New York, 1982; VOl. 1 & 2. (3) (a) Clark, D. T.; Feast, W. J. Polymer Surfaces; John Wiley & Sons: New York, 1978. (b)Feast, W. J.; Munro, H. S. Polymer Surfaces and Interfaces; John Wiley and Sons: New York, 1987. (4) Chapman, B. Glow Discharge Processes;John Wiley & Sons: New York, 1980. (5) Lee, K.-W.; Kowalczyk, S. P.; Shaw, J. M. Macromolecules 1990, 23, 2097.
PdY.m* Add
BPDA-PDA
We have reported5 that PMDA-ODA, the first generation polyimide, reacts with KOH in aqueous solution at 22 "C to give a polyamate (potassium salt of polyamic acid) which is subsequently protonated with acid to give the corresponding polyamic acid. The reactions are confined to the film surface by adjusting the reaction conditions and the solvent. As shown in Scheme I, the corresponding reactions of BPDA-PDA are expected to follow the same reaction route as PMDA-ODA. However, the reaction of BPDA-PDA with KOH a t room temperature is much slower than that of PMDA-ODA, probably since BPDA-PDA has lower absorption of moisture and solvents. It turned out that the peel strength for polyimide-polyimide adhesion is very sensitive to the reaction temperature. Here we report the surface modification of a third generation polyimide, BPDA-PDA, and the surface structure-adhesion relationship.
Experimental Section Materials. BPDA-PDA polyamic acid (PI 2611D) was obtained from Du Pont. KOH, HCl, acetic acid, l-methyl-2pyrrolidinone (NMP), and 2-propanol were obtained from Aldrich. Methods. Polyamic acid in NMP was spin-coated onto a Si wafer (2.25 in. diameter) coated with chromium, and then cured _ _ _ _ _ _ ~ ~ ~
(6) Whitesides and co-workers have studied surface modification of polyethylene by a wet method. Holmea-Farley, S. R.; Reamey, R. H.; McCarthy, T. J.; Deutch, J.; Whitesides, G. M. Langmuir 1986,1,726, and references therein. (7) McCarthy and co-workers have extensively investigated surface modification of polypropylene and flourinated polymers. (a)Diae, A. J.; McCarthy, T. J. Macromolecules 1985,18,1826. (b) Costello, C. A.; McCarthy, T. J. ~ a c r o m o ~ e c u f1987,20,2819. es (c) Lee, K.-W.; McCarthy, T. J. Macromolecules 1988, 21, 309. (d) Lee, K.-W.; McCarthy, T. J. Macromolecules 1988,21, 2318.
0 1991 American Chemical Society
Langmuir, Vol. 7, No. 11,1991 2451
Surface Modification of BPDA-PDA Polyimide to polyimide by slowly ramping the temperature to 400 "C. The purpose of the 500-750-A-thicklayer of chromium is to enhance wettability. The 100-1000-&thick layers of polyimides were employedto obtain X-rayphotoelectron and external reflectance infrared (ER IR) spectra. The surface-modified samples for contact angle measurement, X-ray photoelectron spectroscopy (XPS), and ER IR spectroscopy were dried under vacuum at ambient temperature for 12-14 h. Contact angle measurements were obtained with a h e - H a r t telescopic goniometer and a Gilmont syringe with a 24-gauge flat-tipped needle. Distilled water was used as the probe fluid. Dynamic advancing and receding angles were determined by measuring the tangent of the drop at the intersection of the air/drop/surface while adding (advancing)and withdrawing (receding) water to and from the drop. ER IR spectra were obtained under nitrogen by using a Nicolet-710 FTIR spectrometer with a Harrick reflectance attachment. XPS were taken with a SurfaceScienceLaboratories SSX-100 spectrometer with A1 Ka excitation. Thicknesses of polyimide were measured with Dek-Tak for thicknesses greater than 1000 A and with a Waferscan ellipsometer equipped with a HeNe laser (A = 6328 A) for the thickness of 100-1O00 A. The refractive index employed for BPDA-PDA is l.Wia Modification of BPDA-PDA. The polyimide sampleswere treated with 1 M KOH aqueous solution at 50 O C for 1-30 min dependingon the depth of surfacemodification required, followed by washing with water (2 x 3 min) and 2-propanol (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 contact angles, XPS, and ER IR spectroscopy. The potassium polyamate samples were treated with 0.2 M HCl aqueous solution at 22 O C for 5 min followed by washing with water (2 X 3 min) and 2-propanol (2 x 3 min). This modified surface is polyamic acid as identified by contact angle, XPS, and ER IR spectroscopy. Curing of the polyamic acid at greater than 230 "C produces polyimide as identified by contact angle measurement,XPS, and ER IR spectroscopy. All the analytical data are essentiallyidentical with those of the polyimide starting material. Adhesion Measurement. The solution of polyamic acid was spin-coated onto a chromium-coated Si wafer and cured at 400 "C for 40 min. Thicknessof the polyimide layer is approximately 6 pm. A thin layer (200 A) of gold was sputter-coatedonto 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. BPDA-PDA polyamic acid in NMP was spincoated to the surface-modified polyimide film and subsequently cured at 400 "C under nitrogen. Thicknessof the adherate layer (peel layer) after curing is approximately 20 pm and the width of the peel layer is 5 mm. The peel strengths were measured by 90" peel using an MTS with 25 pm/s peel rate. The reported values are the average of at least three measurements.
Results and Discussion S u r f a c e Modification of BPDA-PDA with KOH. BPDA-PDA polyamic acid solution in NMP was spincoated onto a Si wafer coated with a 500-A-thick layer of chromium and subsequently cured to polyimide under nitrogen at 400 "C for 40 min to obtain good mechanical properties. The reaction of the cured BPDA-PDA with KOH a t room temperature is very slow; thus the reactions were performed at 50 "C. The relationship between the reaction temperature and the adhesion strength will be discussed later. The BPDA-PDA sample was treated with 1M KOH aqueous solution a t 50 "C for 1-30 min to give the corresponding potassium polyamate. The excess of KOH was removed by washing with water (2 X 3 min). These samples without further washing and drying were (8)The refractive index of BPDA-PDA employed is 1.848 at 6328 A, which was obtained by W. A. Pliskin and J. D. Chapple-Sokol at IBM GTD with several sampleswith different thicknesses. The film thickness is quite uniform in the range of 40-1000 A and the experimental error is within 5 X .
;; i
/ /
1 .I'
C
\
\ \
d
Binding En0rgl.r
(.v)
Figure 1. C lsXPS core-levelspectra of (a)BPDA-PDA starting material, (b) potassium polyamate, (c) polyamic acid, and (d) recured polyimide. The takeoff angle of electrons is 55" (35" from the sample surface). used for the protonation reaction (discussed below). To analyze the modified surface by contact angles, XPS, and ER IR spectroscopy,the samples were further washed with 2-propanol (2 X 3 min) and dried under vacuum at the ambient tempreature for 12 h. The advancing/receding contact angles decreased from 79"/41° for BPDA-PDA polyimide to 9"/4". The small water contact angles are consistent with the carboxylate surface. The XPS survey spectrum displays new peaks at 379 and 294 eV, which are corresponding to K 2s and K 2p, respectively. Figure 1 shows the XPS C 1s regions of polyimide and the 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 l b ) 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 0 1s spectra are consistent with the changes in the C 1s region. ER IR spectra of thin and uniform layers (100-1OOOA) of polyimide on chromiun-coated Si wafers were obtained to analyze the products and to measure the depth of modification (which will be discussed in detail). Figure 2 displays the ER IR spectra of an 800-A-thicks polyimide film and the modified samples. The spectra in the range of 1900-1300 cm-l provide the most useful information for this reaction. The ER IR spectrum of BPDA-PDA (Figure 2a) displays the bands a t 1776 (w), 1739 (vs), 1709 (w, sh), 1621 (vw),1517 (m), 1423 (w), and 1358 (m, br) cm-l. The bands at 1776and 1739 cm-l correspond to the carbonyl. The peaks at 1621 and 1517 cm-l are due to BPDA and PDA phenyl groups, respectively. The peak at 1358 cm-1 is the imide I1 band which is related to the C-N stretching. When the whole layer of 800-&thick polyimide was modified to obtain a good IR spectrum, as shown in Figure 2b, the imide carbonyl stretching at 1739 cm-l
2452 Langmuir, Vol. 7, NO.11,1991
Lee et a1.
0
200
400 600 800 loo0 Polyimide Thickness (Angstrom)
1200
Figure 3. Relationship between BPDA-PDA polyimide thickness and imide carbonyl IR (1739cm-l) absorbance. The angle of IR incidence is 7 5 O .
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"
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I
.
.
"
.
'
'
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1400 Wavonumbor (an")
Figure 2. External reflectance IR spectra of (A) BPDA-PDA polyimide, (B)potassium olyamate,and (C)polyamic acid. The starting polyimide is 800A)thickand the whole layer is modified. The angle of 1R incidence is 7 5 O (15' from the sample surface). (imide I band) and the imide I1 band at 1358 cm-l completely disappeared. The peaks corresponding to BPDA-PDA polyamate are located at 1664 (s), 1613 ( 8 , sh), 1591 (s), 1516 (s), 1404 (s), 1391 (m), and 1372 (w) cm-l. The peak at 1664 cm-l corresponds to the amide I band. The peaks at 1591,1391(m),and 1372cm-l are due to the carboxylate asymmetric and symmetric stretchings. Reaction of Potassium BPDA-PDA Polyamate with Hydrochloric Acid. The BPDA-PDA samples treated with KOH and washed with water were protonated by treating with 0.2 M HC1 (or 1 M acetic acid) aqueous solution at 22 OC for 5 min to yield a polyamic acid surface. The samples were washed with water and 2-propanol, and dried under vacuum at ambient temperature for 12 h. The potassium peak in the XPS survey spectrum disappears and the shape of the XPS C 1s spectrum (Figure IC) is similar to that of polyamate and consistent with that of polyamic acid as is the 0 1s spectrum. The ER IR spectrum (Figure 2c) exhibits the bands at 1726 (s), 1670 (s), 1607 (m), 1547 (m), 1517 (s), 1408 (s), and 1322 (m, br) cm-l. The bands due to the carboxylate (1591 (s), 1391 (m), and 1372 (m, sh) cm-*) disappeared and the band at 1726 cm-l corresponding to the carbonyl stretching of carboxylic acid has appeared. The peaks at 1670 and 1547 cm-l correspond to amide I band (carbonyl stretching) and amide I1 band (coupling of C-N stretch and N-H deformation), respectively. The bands at 1607 and 1517 cm-l are probably due to the phenyl groups. The peak at 1408cm-' corresponds to the C-N stretching. These surface-modified films were employed to study polyimide-to-polyimide adhesion. Contact angle measurements were used to investigate wettability, a factor in adhesion strength. The water contact angles decrease from 79'141" (advancingcontact anglelreceding contact angle) for BPDA-PDA to 4B0/8", indicating that the modified surface has become more hydrophilic and wettable than the BPDA-PDA surface. Thus better adhesion on the modified film is expected. The starting polyimide surface is reproduced upon curing the polyamic acid surface. Contact angles (74O/ 399, XPS C 1s spectrum (Figure Id), and the ER IR
spectrum of the recured surface are essentially indistinguishable from the starting polyimide. Measurement of Thickness of the Modified Layer. The relationship between surface structure and surface property has been poorly understood. Adhesion is one of the most important surface properties in industry. Both chemical structure and thickness of a modified layer are related to adhesion strength. Even though the interface between the modified layer and unmodified layer is not sharp, it is useful to obtain an approximate depth of modification to investigate the thickness-adhesion relationship. A method to measure the average depths of modification has been previously reported.6 Ellipsometry and ER IR spectroscopy are employed and the same method is used in this work. Thin and uniform layers (1WlOOO A) of polyimide were prepared on chromium-coatedSi wafers (2.25 in. diameter), and then the film thicknesses and the absorbances of the imide carbonyl stretching were measured by ellipsometry and ER IR, respectively. As shown in Figure 3, there is a linear relationship between the film thickness and the absorbance of the carbonyl stretching at 1739 cm-1. By use of this relationship, the thickness of the unmodified layer can be calculated by measuring the imide carbonyl absorbance of the modified (to polyamate) film. The average depth of modification can be obtained by subtracting the thickness of unmodified polyimide from the thicknesa of starting polymer. When a 435-A-thick BPDAPDA film was treated with 1M KOH aqueous solution at 50 "C for 10 min, the absorbance of imide carbonyl (corresponding to unreacted polyimide) is 0.0690, which corresponds to 225 A in the thickness-absorbance relationship shown in Figure 3. Thus the modification depth for the 10 min reaction is 210 (subtracted 225 from 435) A. Surface Structure-Adhesion Relationship. The surface of the DPDA-PDA polyimide was modified to the polyamic acid surface, and then BPDA-PDA polyamic acid in l-methyl-2-pyrrolidinonesolvent 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 pm and the width of the peel layers is 5 mm. The peel strengths, which are indicative of adhesion strengths and measured by 90° peel of the top polyimide layer, depend on the temperature and time of the reaction with KOH, but not on the acidifying condition. The reactions of BPDA-PDA were carried out at the three different temperatures for 10min and the polyimide-polyimide adhesion was studied, Figure 4 shows the dependence of peel strengthg on the reaction temperature. As (9) The unit employed here is J/m2 (1 J/mz = 0.102 g/mm).
Surface Modification of BPDA-PDA Polyimide I
Langmuir, Vol. 7, No. 11, 1991 2453
0
. 0 10
35 KOH Reaction Temperature
60
('C)
Figure 4. Dependence of adhesion on the reaction temperature.
the reaction temperature is raised, the peel strength increases. The reactions were also carried out a t 50 OC for 5 and 10 min, and the adhesion is evaluated. The peel strengths are 23,480, and 950 J/m2 for control, 5 and 10 min reaction with KOH, respectively. There is no visible adherate polymer left on the adherend after peeling. The more deeply modified layer results in the stronger adhesion. These results suggest the following polymer surface structure-adhesion relationship. If the modified surface has a similar chemical structure as an incoming polymer (adherate), affinity between two phases is good and thus adhesion is strong. Polyamic acid is known to be amorphous,*Othus it is reasonable to assume that the polyamic acid surface layer on crystalline polyimide is amorphous. (10) Polyimide precursors (polyamicacids) have unordered structures
until they are cured at high temperature. The unordered polyamic acid and polyimide diasolve well in NMP and other solventa can easily diffuse into the unordered layer. X-ray scattering study does not show any evidence for a crystalline structure. Private communication with Dr. Ravi Saraf at IBM M a r c h .
If the modified surface layer is amorphous like the polyamic acid layer in this work, an adherate polymer quickly diffuses into the amorphous layer (modified region) of an adherend. Subsequent curing induces interlocking of the polymer chains of adherend and adherate, and thus a strong adhesion is obtained. For some industrial applications, 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. When the polyimide films (5 pm or 223 A thick) were treated with 1M KOH aqueous solution a t 50 "C for 10 min, the thickness of polyimide and topography of modified surface remain unchanged within several angstroms and within the limit of SEM sensitivity, respectively. However,the polyimidepolyimide adhesion strength increases by 40 times. Summary. BPDA-PDA polyimide surface modification with KOH aqueous solution is well defined. The reaction initially gives a potassium polyamate surface which is protonated with acid to yield a polyamic acid surface. The average depth of modification is nondestructively measured by a method using an absorbancethickness relationship established with ellipsometry and ER IR. The surface topography and the film thickness can be retained while a strong polyimidepolyimide adhesion is achieved. A relationship between polymer surface structure and polymer surface properties is proposed. Acknowledgment. We acknowledge Eleftherios Adamopoulos for technical help. Registry No. (BPDA)(PDA) (copolymer), 71329-95-8; (BPDA)(PDA) (SRUimide), 32197-39-0; (BPDA)(PDA) (SRUamic acid), 29319-22-0; KOH,1310-58-3; HC1, 7647-01-0.
