Improvement in Copper-Resin Bond Strength after High-Temperature Testing Using C–H–Si Thin Film

Abstract

The bonding between copper and epoxy resin is investigated using a C–H–Si thin film to achieve highly adhesive and reliable bonding between the resin mold and the copper substrate in power modules. In this study, high-temperature (473 K) testing is performed to improve the reliability of this junction after high-temperature operation. The bond strength decreases and delamination occurs at the film–copper interface after high-temperature testing. The decrease in bond strength may be due to the decrease in the film–copper interfacial strength because of the decrease in the binding force between copper and oxygen upon heating. When Fe and Cr, which have high oxygen bond disassociation energies, are intercalated at the film–copper interface, the bond strength improves after high-temperature testing and a bond strength above 30 MPa was retained after 1000 h.

This study is a novel method of dissimilar bonding based on chemical and physical effects of C–H–Si film existing between copper and resin. Further, it is an effective method of bonding to achieve high-temperature operation of a resin-molded power module, which can contribute to future advancements in power electronics products.

1. Introduction

To achieve the aim of carbon neutrality, in recent years, there has been rapid progress in the development of electric and hybrid vehicles. The Si devices used in the inverter power module of the power control unit (PCU), which controls the power of such automobiles, have been improved in terms of both loss and miniaturization. A succession of higher power densities and further miniaturization have been achieved each year by increasing the operating temperature; however, the characteristics of the Si device are approaching their theoretical limits. Hence, research and development on wide-bandgap (WBD) power semiconductors, such as SiC, have been actively pursued in recent years.^1,2) While Si is typically used at temperatures in the range of 423–448 K or below, SiC can operate at temperatures above 473 K as a standalone device. However, considering the entire power module, there are various problems related to high-temperature operation, including the heat life of mounting materials, such as junction and sealing materials, heat dissipation, and ensuring the reliability of different material junctions.^3,4)

The objective of this study was to develop a highly adhesive and highly reliable bond between the copper substrate and the resin mold, which was used for protecting the power module from the external environment. To achieve high reliability, a resin-metal interface, which is highly reliable even after the above-mentioned high-temperature operation, is required. Conventional methods to improve the bond strength of the epoxy resin-copper interface include chemical oxidation treatment of the copper plate,^5,6) roughening of the Ni plating film,^7,8) and laser roughening of the copper plate.⁹⁾ Anodization on aluminum plates¹⁰⁾ and laser roughening of the steel plates¹¹⁾ have also been investigated for bonding of resin-metals other than copper. All of these methods aim at an anchor effect by roughening the surface of the metal base material. To realize more efficient surface treatment than the conventional techniques, we attempt to improve the epoxy resin-copper bonding process using a surface treatment layer deposited through a dry process, in this study. We examine a previously reported bonding method for the copper substrate and mold resin using an amorphous C–H–Si thin-film,^12,13) which is formed by plasma chemical vapor deposition (CVD).^14,15) The high hardness and low coefficient of friction¹⁶⁾ of the C–H–Si thin-film enable its application in tools, molds, and automotive parts. Because C–H–Si film includes both inorganic and organic characteristics, we aim to achieve strong adhesion based on its affinity with resin, and high corrosion resistance based on its density and uniformity by intercalating it between the copper substrate and resin. Previously, we had reported¹²⁾ a high bond strength (>30 MPa) in a copper–epoxy resin bond using a C–H–Si thin film and had speculated that this was caused by the anchor effect due to nanoscale unevenness on the film surface and the chemical affinity effect due to surface functional groups. In a different paper,¹¹⁾ we had reported that the film grain coverage depends on the unevenness and oxidation state of the copper substrate, and that the larger the film grain coverage, the better was the adhesion to the resin.

In this study, high-temperature (473 K) testing was performed to improve the reliability of the copper–C–H–Si film–epoxy resin junction after high-temperature operation. The results indicated improvement in bond strength after high-temperature testing, achieved by the introduction of iron and chromium, which have high oxygen bond disassociation energies, to the copper substrate surface.

2. Experimental

2.1 Deposition of C–H–Si film on copper substrate

A copper substrate of dimensions 35 mm × 18 mm × t3 mm was subjected to microblasting pretreatment for achieving a ten-point average roughness (R_ZJIS) of 1 µm. Microblasting was performed using green carborundum (SiC) samples of different shapes as the media, and adjusting the media size, injection volume, pressure, injection distance, scanning speed, and number of operations to realize the specified roughness. The copper substrate was acid washed with 10% hydrochloric acid for 60 s at 298 K. After these pretreatments, the copper substrate was placed on a copper stage or stainless steel (SUS304) stage of dimensions φ230 mm × t10 mm, and the film was deposited by plasma CVD (JEOL P-CVD continuous deposition system JPE-767-HVRF-X05) using a DC power supply. After the temperature was increased to 623 K by introducing the dilution gas, the C–H–Si film was applied under conditions of 3 kV, 10 Pa, and 180 s using tetramethylsilane (Si(CH₃)₄) as the precursor gas.

2.2 Preparation of copper substrate–C–H–Si film–resin bond specimens

Copper substrate–C–H–Si film–resin bond specimens were prepared by transferring the molding epoxy resin to the copper substrate, on which the C–H–Si film had been deposited. Cresol novolac epoxy resin containing 80% SiO₂ filler was used as the resin, and the three pieces were transfer-molded into a conical shape with a basal diameter of 3.7 mm, height of 4 mm, and inclination angle of 76°. These were then bonded. Transfer molding was performed using a small electric press (Orihara G-12RS 500) at a molding temperature of 453 K, under a molding pressure of 10 MPa, and for a curing time of 60 s. The specimens were then cured for 14.4 ks at 448 K in air atmosphere. The copper substrate–C–H–Si film-resin bond specimens are shown in Fig. 1.

Fig. 1

Copper substrate–C–H–Si film–resin bond specimens.

2.3 Evaluation of the bond strength of the bond test specimen

The strength of the resin bond with the copper substrate was tested according to SEMI G69-0996.¹⁷⁾ Using a bond strength evaluation device (Dage 4000 Bondtester), a shear load was applied at a shear rate of 0.05 mm/s and a test height of 100 µm, and the maximum load for rupturing was divided by the bottom area of the cup to obtain the shear bond strength of the adhesive interface (hereafter referred to as bond strength). One bond test specimen with three bonding sites was prepared for each condition, and the average value of the bond strength at the three sites was determined.

2.4 High-temperature testing

The fabricated bond specimens were placed in a thermostatic bath at 473 K, in air or nitrogen atmosphere, and subjected to high-temperature testing for different durations (100 h, 200 h, 250 h, 500 h, and 1000 h).

2.5 Morphological observations

The resin delamination surface after bond strength evaluation, and those before and after high-temperature testing was observed under a stereomicroscope. The bond specimens were cross-sectioned by ion-polishing and subjected to scanning electron microscopy (SEM) observation and energy-dispersive X-ray analysis (EDS).

2.6 Depth and surface composition analysis of C–H–Si films

The depth profile of the C–H–Si films was obtained by Auger electron spectrometry (AES, JEOL JAMP-9500F). At a primary electron beam acceleration voltage of 10 keV, an irradiation current of 10 nA, and a probe beam diameter of φ1 µm, etching was performed using Ar ions at 2 keV, 10 nm/min, and a 2.5 nm/cycle for analysis. The depths were calculated based on the SiO₂ etching rate.

The composition of the C–H–Si film surface was determined by X-ray photoelectron spectroscopy (XPS, KRATOS/SHIMADU AXIS-165) using monochromated Al Kα (1486.6 eV) as the X-ray source, and analysis was performed at a photoelectron extraction angle of 90° and area of 700 × 300 µm².

3. Results and Discussion

3.1 Change in bond strength and delamination form after high-temperature testing

Figure 2 shows the change in bond strength after the copper substrate–film–resin bond specimens with C–H–Si film deposited on a copper stage were subjected to high-temperature (473 K) testing in air or nitrogen atmosphere. In air, the bond strength decreases after 100 h of high-temperature testing, while in nitrogen, the bond strength decreases after 500 h. Figure 3 depicts the delaminated surface of the resin after the bond-strength test. In both air and nitrogen atmospheres, the delaminated surface changes from having a large quantity of resin residue on the film before high-temperature testing to having a large area of exposed copper substrate at the point where the bond strength decreases. This shows that the fracture point, which is at the resin–film interface before high-temperature testing, moves to the film–copper interface after high-temperature testing, i.e., the weak point moves from the resin–film interface to the film–copper interface. This indicates that the degradation of the film–copper interfacial strength is responsible for the decrease in bond strength after high-temperature testing; therefore, it is necessary to improve the film–copper interfacial strength for maintaining the bond strength after high-temperature testing.

Fig. 2

Change in bond strength of bond specimens with film deposition on a copper stage after high-temperature (473 K) testing in air or nitrogen atmosphere.

Fig. 3

Delaminated surface of the resin after bond strength testing.

To determine the factors contributing to the reduction in film–copper interfacial strength, the composition in the depth direction, including the interface of the film–copper before and after 500 h of high-temperature testing, was analyzed by AES. Figure 4 displays the results of AES analysis. Compared to pre-high-temperature testing (Fig. 4(a)), the oxygen content in the film–copper interface increases during high-temperature testing in air (Fig. 4(b)), but remains almost unchanged in nitrogen (Fig. 4(c)). The bond strength decreases after both 100 h of high-temperature testing in air and 500 h of high-temperature testing in nitrogen. The faster decrease in bond strength in air suggests that the increase in oxygen content at the interface reduces the interfacial strength. This decrease in bond strength may have been suppressed in nitrogen because the increase in oxygen content at the film–copper interface is suppressed. However, the bond strength decreases after 500 h of high-temperature testing even in nitrogen, suggesting that the binding force between copper and oxygen and the film–copper interfacial strength are reduced upon heating.

Fig. 4

Results of AES analysis of the specimen in the depth direction of (a) pre-high-temperature testing, (b) post-500 h high-temperature testing in air atmosphere, and (c) post-500 h high-temperature testing in nitrogen atmosphere.

In plasma CVD, dilution gas and precursor gas are introduced and deposited on the copper substrate after vacuuming; however, oxygen is present in the copper substrate surface during the microblasting process and exists on the film–copper interface. An initial bond strength of 35 MPa has been reported¹³⁾ for a specimen with resin bonded to the deposited surface after oxidizing the copper substrate at 473 K for 900 s, which is sufficient. However, when oxidized for 3600 s, some exposed copper was clearly observed on the delaminated surface after bond-strength evaluation, and there was a slight decrease in bond strength. Therefore, excess copper oxide film is brittle and may cause delamination. However, when copper oxide grains cover the entire surface after oxidizing the copper surface for up to 900 s, these grains serve as nuclei for film formation; the number of film grains increase and the bond strength of the resin–film–copper is high. This suggests that oxygen up to a certain quantity at the film–copper interface does not inhibit bonding; rather, a bond is formed between the copper substrate and C–H–Si film via the oxygen at the interface.

3.2 Improvement in film–copper interfacial strength after high-temperature testing

Based on the results described in Section 3.1, we decided to introduce elements with high oxygen bond disassociation energies at the copper substrate to improve the film–copper interfacial strength. Table 1 lists the oxygen bond disassociation energy of each element.¹⁸⁾ Since the bond disassociation energies of Fe–O and Cr–O are higher than that of Cu–O, Fe and Cr were selected as the elements to be introduced. As shown in Fig. 5, a copper substrate was placed on an SUS304 stage (Fe: 72%, Cr: 18%), and it was assumed that Fe and Cr would adhere to the copper substrate via sputtering of the stage components using dilution gas during the increase in temperature.

Table 1 Bond dissociation energies (BDEs).

Fig. 5

Introduction of Fe and Cr in the C–H–Si film–copper interface by deposition on an SUS stage.

Figure 6 depicts the change in bond strength after the specimens with film deposition on an SUS stage were subjected to high-temperature (473 K) testing in air. A bond strength above 30 MPa is maintained even after 1000 h of high-temperature testing. Figure 7 shows the delamination surface of the resin after bond strength testing. Prior to the high-temperature test, the delamination condition was good, with large amounts of resin remaining. However, as time passed under the test condition, the remaining resin content was reduced and increasing amounts of the copper substrate became gradually exposed. However, even after 1,000 h, resin and film were retained in some sections, with no significant decrease in bond strength.

Fig. 6

Change in bond strength of bond specimens with film deposition on an SUS stage after high-temperature (473 K) testing in air.

Fig. 7

Delaminated surface of the resin after bond strength testing.

Figure 8 displays the cross-sectional image of the bond specimen with film deposition on an SUS stage. Grains with diameters ranging from 5 to 10 nm were observed at the film–copper interface, and Fe and Cr were detected by EDS analysis.

Fig. 8

Cross-sectional image of the bond specimen with film deposition on an SUS stage.

AES analysis was performed to determine the composition in the depth direction, including the film–copper interface, before and after 500 h of high-temperature testing, and the results are shown in Fig. 9. Fe and Cr were detected at the film–copper interface in both before and after the testing. Compared to that before the high-temperature testing (Fig. 9(a)), the oxygen content at the film–copper interface increases after high-temperature testing in air (Fig. 9(b)), similar to the observation for the bond specimen with film deposition on a copper stage. However, unlike this bond test specimen, the bond strength is not reduced. This suggests that the interposition of Fe and Cr particles at the film–copper interface maintains the binding force with oxygen and suppresses bond strength reduction at the copper–film interface during high-temperature testing.

Fig. 9

AES analysis results in the depth direction (a) before and (b) after high-temperature testing for 500 h.

3.3 Retention of resin–film interfacial strength after high-temperature testing

In the bond specimen with film deposition on a copper stage, the fracture point was at the resin–film interface before high-temperature testing; after high-temperature testing, it moved to the film–copper interface, suggesting that the resin–film interface retained more interfacial strength than the film–copper interface after the high-temperature test. Accordingly, we discussed the bonding of the resin–film interface.

We had previously reported¹³⁾ that film grain coverage depends on the unevenness and oxidation state of the copper substrate, and that the larger the film grain coverage, the stronger was the adhesion to the resin. Table 2 depicts the results of surface composition analysis by XPS for films with large and small grain coverages. Figure 10 shows the Si 2p spectrum and the surface SEM image. It is determined that the larger the film grain coverage, the higher is the oxygen content on the film surface; in particular, there is a large quantity of Si oxide functional groups (SiOx). Based on this, it is inferred that the chemical affinity between SiOx and the resin contributes to the resin–C–H–Si film bond strength. In addition, O is involved in the bonding of the resin–film interface; however, as shown in Table 1, the bond disassociation energy is greater for Si–O than those for Cu–O, Fe–O, and Cr–O, suggesting that the bond strength does not degrade even after high-temperature testing.

Table 2 Surface compositions obtained by XPS analysis.

Fig. 10

Si 2p spectrum of the C–H–Si film surface obtained by XPS analysis and surface SEM analysis: (a) film with small film grain coverage, (b) film with large film grain coverage.

4. Conclusion

The following conclusions were drawn regarding the bonding of copper and resin by a C–H–Si film, by examining the factors contributing to the reduced bond strength after high-temperature testing and attempting to improve the bond strength.

1. (1)    The bond strength decreased after high-temperature testing and delamination occurred at the film–copper-substrate interface. The decrease in bond strength was attributed to the decrease in the film–copper interfacial strength because of the reduction in binding force between copper and oxygen upon heating.
2. (2)    When Fe and Cr, which have high oxygen bond disassociation energies, were intercalated at the film–copper interface, the bond strength improved after high-temperature testing and a bond strength of more than 30 MPa was retained after 1000 h.
3. (3)    The resin–film interface was bonded mainly through the Si oxide functional group in the C–H–Si film, and the strong Si–O bond could have maintained the interfacial strength after high-temperature testing.
4. (4)    This study is an effective dissimilar bonding method to achieve high-temperature operation of resin-molded power modules, which make a contribution to the evolution of future power electronics products.

Acknowledgments

We would like to express our gratitude to Toyota Central R&D Labs for providing the bonding test specimens.

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