AlN ceramics substrates by using a CuO interlayer

Accepted Manuscript A preparation method for Al/AlN ceramics substrates by using a CuO interlayer

Meng Fei, Renli Fu, Simeon Agathopoulos, Jun Fang, Caixia Wang, Haitao Zhu PII: DOI: Reference:

S0264-1275(17)30548-8 doi: 10.1016/j.matdes.2017.05.067 JMADE 3091

To appear in:

Materials & Design

Received date: Revised date: Accepted date:

29 December 2016 21 May 2017 23 May 2017

Please cite this article as: Meng Fei, Renli Fu, Simeon Agathopoulos, Jun Fang, Caixia Wang, Haitao Zhu , A preparation method for Al/AlN ceramics substrates by using a CuO interlayer, Materials & Design (2017), doi: 10.1016/j.matdes.2017.05.067

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ACCEPTED MANUSCRIPT

A preparation method for Al/AlN ceramics substrates by using a CuO interlayer

Meng Fei a, Renli Fu a,*, Simeon Agathopoulos b,*, Jun Fang a, Caixia Wanga, Haitao

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Zhua

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College of Materials Science and Technology, Nanjing University of Aeronautics and Astronautics, Nanjing

Materials Science and Engineering Department, University of Ioannina, GR-451 10 Ioannina, Greece

*

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210016, P. R. China

: Corresponding author Tel.: +86-25-84236198; fax: +86-25-52112626

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E-mail address: [email protected] (Prof. Renli Fu) b,*

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E-mail address: [email protected]

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: Co Corresponding author Tel.: +30 - 26510-0-9007; fax: +30 26510 9097

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Abstract A novel method for preparing aluminum/aluminum nitride ceramics (Al/AlN) substrates is proposed in this paper. The method included two processes: (1) the surface of AlN was coated with a CuO thick film and sintered; (2) Al foil was put on the pretreated (in process 1) AlN surface and bonded to AlN through

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pre-heating at 400 oC and final heating at 660 oC in N2-H2 reduction atmosphere. The experimental results

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obtained by optical and scanning electron microscope (SEM) observations as well as by X-ray diffraction

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analysis at the cross-sections of the joints and the fracture surfaces, suggest that the reaction mechanism starts with the reaction of CuO with AlN to form CuAlO2, when the AlN coated with CuO is heat-treated at high

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temperature. A Cu layer is produced by the reduction of CuO and Cu diffuses in the Al foil, forming strong

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Al/AlN joints. Elongated crystals of Al2Cu are developed in the reaction zone at the interface between Al and AlN. The results of mechanical tests showed that the peeling-off strength of the Al foil from the surface of the

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AlN substrate reached a value of 15.4 MPa for the Al/AlN couples produced after 30 min of heat treatment at

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660 oC.

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Keywords: Al/AlN interface, bonding, joining, Al-Cu eutectic, microstructure, mechanical properties.

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1. Introduction Power electronics are widely used in energy conversion systems, such as those involved in renewable energy production, motor drivers, transportations, and power transmission. Generally, the operation temperature of power semiconductor devices may reach 250 oC or even temperatures higher than 300 oC [1].

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Circuits with high power density in automotive applications achieve 135 W/in3 in an inverter [2], and 82

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W/in3 in a battery charger [3]. Due to the increase of thermal loading of power semiconductors, thermal

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management is now a serious challenge in microelectronic packaging [4, 5].

Apparently, the issue of thermal management is closely related to the thermally conductive package

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structure [6]. Therefore, direct bonded copper (DBC) substrates are utilized to improve the thermal

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conductivity without large heat sink. More recently, a DBC substrate based on aluminum nitride (AlN) was considered for wideband-gap power semiconductor devices, especially those based on silicon carbide (SiC)

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and gallium nitride (GaN) [7, 8].

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In power electronic packaging, reliability is the most critical requirement. More specifically, the most crucial issue about packaging substrates is their thermal conductivity and generally their thermal properties

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and behavior under realistic circumstances [9]. Aluminum nitride (AlN) has the structure of wurtzite phase

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(w-AlN). It exhibits high thermal conductivity (70 - 210 W·m−1·K−1 for polycrystalline material, and a high value, as 285 W·m−1·K−1, for single crystals), good dielectric properties, a low coefficient of thermal expansion (CTE), close to that of silicon, and it is non-reactive with normal chemicals and gases used in semiconductor process [10]. However, it is difficult to bond a metal layer to the surface of AlN ceramics, due to the poor wettability of AlN and this is the case for most metals. Furthermore, the mismatch of the CTEs between AlN and the metal layer induces thermal stresses at the metal/ceramic interface after cooling from the manufacturing

ACCEPTED MANUSCRIPT temperature to the room temperature or the operation temperature [11]. This mechanical stress at the metal/AlN interface jeopardizes the reliability of these joints. Various methods have been proposed to improve bonding strength and thermal fatigue in the AlN metallization, such as thin film, thick film, and direct metal bonding techniques [12, 13]. In the latter case, direct bonding technique is carried out with no use of any

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two surfaces of any material, which can possibly favor the joining [14].

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additional intermediate layer. The bonding technique is generally based on chemical bonds developed between

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It has been postulated that direct bonding of aluminum (DBA) substrate has better thermal cycle reliability than DBC because it results in joints with a lower elastic modulus, a more flat plastic strain rate,

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and lower yield stress, along with other particular properties which are also better, e.g. high reflectivity and so

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forth [7, 15-17]. However, when DBA is applied on an AlN ceramic substrate, one should bear in mind that the couple of AlN with aluminum features an intrinsic chemical inertia [18]. In general, there is always a

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specific inherent problem when Al is involved in these processes. It is related to the aluminum oxide layer,

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which spontaneously and inevitably forms on the aluminum surface. The aluminum oxide layer prevents the wetting and the diffusion of molten Al on the AlN ceramic surface.

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Several researchers have proposed comprehensive methods to remove this surface Al-oxide barrier [7,

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15, 17, 19-21]. Ksiazek et al. produced a highly reliable bond between Al and Al2O3 in Al2O3/Al/Al2O3 joints by introducing a titanium film as an active metal layer [17]. Peng et al. used die-casting method under N2 atmosphere at 670 - 750 oC. This method eliminated the oxide film from the surface of Al and effectively bonded Al to AlN substrate [20]. Imai et al. used ultrasonic vibration to disrupt the continuous oxide layer on metal surface and successfully achieved good wetting of ceramics (ZrO2, SiC, and Si3N4) by metals (Mg, Cu, Ti, and Al) [21]. Consequently, active metal interlayer, reductive atmosphere, and mechanical force have been used to remove the oxide layer from Al-surface, and improve the wettability of ceramics by Al. However, the

ACCEPTED MANUSCRIPT aforementioned methods usually require high temperature, high pressure, high vacuum, and complex pretreatment process [22, 23]. Hence, for large-scale production, a new aluminum-bonding method, which does not require high temperatures, pressure, and vacuum along with complex processing, is an urgent demand.

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In this paper, Al foil was bonded to AlN ceramics through a thick film method using CuO paste under a

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relatively simple processing condition. The phase composition, element distribution, microstructure, and the

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morphology of the phases produced across the Al/AlN interface were investigated. The interfacial bonding

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strength was also measured.

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2. Materials and experimental procedure

AlN ceramic substrates (10  10  1 mm3, Huaqing Electronic Ceramics Co. Ltd, China), Al foil (>99%,

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0.3 mm thick, Sinopharm Co. Ltd, China), and nano-powder of CuO (200 nm, 30 m2/g, Shanghai Sinpeuo

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Fine Chemical Co. Ltd, China) were used. For the peel-off mechanical tests, cyanoacrylate (Ergo 5800, Swiss) glue was used.

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The AlN substrates and the Al foil were ground by an 800 grit SiC paper to remove the superficial oxide

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film and then cleaned with ethanol in an ultrasonic bath for 300 s at 25 oC. The CuO paste was produced as follows: The CuO nano-powder was mixed (63.6 wt.%) with an organic carrier (36.4 wt.%), which contained alcohol (99.5%, Aladdin), where Castor oil, dibutyl phthalate (DBP, 99%, Aladdin), polyethylene glycol (PEG, average Mn 600, Aladdin), and polyvinyl butyral (PVB, MW 40000-70000, Aladdin) were dissolved (each of these components were dissolved in a quantity of 0.9 wt% in the final paste), using a mechanical mixer to form a homogeneous paste. Fresh CuO paste was applied in the experiments.

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Process 1

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Process 2

Figure 1. Schematic representation of the aluminum bonding technique to AlN substrate.

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The aluminum bonding technique is schematically represented in Figure 1. The preparation technique involved two processes: (1) the surface of AlN ceramics was coated with a CuO paste film by screen printing.

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Sintering followed at 1100 oC in air for 30 min. (2) Al foil was put onto the surface of AlN ceramics produced

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in process (1). Heat treatment at 400 oC for 60 min and then at 660 oC for 30 min in a reduction (N2-5%H2)

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atmosphere followed to fabricate the Al/AlN joints. The temperature profiles of the aforementioned process (1) and (2) are summarized in Figure. 2. For comparison purposes, experiments were also carried out at different

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bonding temperatures and dwell times (as far as the final heating plateau are concerned, Figure. 2b). The experimental results were obtained by analysis of cross-sections of the Al/AlN joints along with observations of the fracture surfaces after peeling-off tests, as well as by the examination (top view) of the materials during the stages of the process. The techniques of scanning electron microscopy (SEM, Sigma 300, Carl Zeiss Co. Germany) and electron dispersive spectroscopy (EDS, Quantax, Bruker Co, American) for elemental analysis, X-ray diffraction analysis (XRD, Rigaku/Ultima IV, Japan, Cu K = 0.5418 nm), and optical microscopy (OM, Leica Co, Germany) with a digital camera (DSLR, EOS 1200D, Canon, Japan) were

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(a)

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Figure 2. Temperature profiles of the two bonding process stages (a) for the process 1, and (b) for the process

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2, as represented in Figure 1.

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The peeling strength of the Al/AlN joints was measured by a home-made tensile tester, schematically represented in Figure. 3, and calculated by the equation (1): (1)

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(MPa)

The presenting values of tensile strength are the average values (and their standard deviation) from

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measurements from five different samples.

Figure 3. Schematic diagram of the home-made tensile tester to measure the peeling-off strength. In addition, residual weight analysis was carried out to quantitatively evaluate the fracture position after

ACCEPTED MANUSCRIPT peeling tests. The weight of each sample before (left-hand side assembly in Figure. 4) and after peeling-off test (right-hand side assembly in Figure. 4) was measured by a lab balance (± 0.1mg, BSA124, Sartorius AG, Germany). The ratio of residual weight (in wt%) was calculated by the equation (2):

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Moreover, the cross-section of the assembly after the peeling-off test was observed by optical microscopy.

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Figure 4.Schematic diagram of the residual weight analysis. 3. Experimental results

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3.1 Analysis of CuO/AlN joints (process 1)

The microstructure and the crystalline phase analysis of the cross-section of the produced (at 1100 oC)

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CuO/AlN joints are shown in Figure 5. According to the SEM image (Figure 5a), it might be suggested that the CuO film reacted with the Al-oxide, which inherently exists on the surface of AlN ceramics, and formed a

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continuous phase along the CuO/AlN interface. The X-ray diffractograms, which were obtained from this interphase zone (i.e. located at the CuO/AlN interface), shown in Figure 5b, revealed that CuAlO2 and Al2O3 also exist in the interfacial reaction zone, along with CuO and AlN phases. Some researchers have reported that they found CuAl2O4 being formed during bonding reaction [24, 25]. However, there was no evidence of formation of this phase in our experiments.

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Figure 5. SEM image (a) and X-ray diffractograms (b) of the cross-section of CuO/AlN joints produced at 1100 oC.

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The top view (by SEM observation) and X-ray diffraction analysis, shown in Figure.6, suggest that the

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surface layer of the CuO/AlN joints consists of CuO phase, which has a reticulate structure with many pores

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randomly distributed.

Figure 6. Top view, by SEM (a), and X-ray diffractogram (b) of the surface of CuO joined to AlN at 1100 oC. 3.2 Characterization of Al/AlN joints The aluminum foil was successful joined to the AlN substrates through process 2, described in Figures 1 and 2. In order to investigate the influence of temperature and the dwell time (which correspond to the plateau at the maximum temperature in Figure 2b), in this section we shall present the results of the samples produced

ACCEPTED MANUSCRIPT under the following four specific experimental conditions: case a: 660 oC, 0 min; case b: 660 oC, 15 min, case c: 660 oC, 30 min, and case d: 700 oC, 30 min. Hereafter, the letters a, b, c, and d will refer to the four cases

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above (in the figure designations, as well).

Figure 7. Cross-sections of Al/AlN joints for the four investigated cases (the letters of the images (a), (b), (c),

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and (d) correspond to these cases; see the text in 3.2). 3.2.1 Analysis of the of Al/AlN interface

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The typical microstructures of the cross-sections of Al/AlN joints for the four investigated cases are shown in Figure 7, and the corresponding elemental line scan analyses (by EDS) along the vertical arrows, which are plotted in Figure 7, are presented in Figure 8. Accordingly, across the cross-section and from the top to the bottom, the layers are indexed to aluminum, copper, CuAlO2, and AlN. However, the thickness of the Cu layer featured significant variations, depending on the holding time and the temperature. More specifically, the Cu layer had a wide thickness and a porous structure in the Al/AlN joints produced at 660 oC after short holding time, as shown in Figure 7a (and Figure 8a). The Cu layer became thinner when holding

ACCEPTED MANUSCRIPT time and temperature increased and finally disappeared, as shown in Figures 7b to 7d (and in Figures 8b to

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8d).

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Figure 8. Line scan element analysis (by EDS, along the vertical arrows) in the cross-sections of Figure 7 (the letters (a), (b), (c), and (d) correspond to the four investigated cases; see the text in 3.2).

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According to Figure 8, O mainly existed at the interface of Al/AlN joints in a zone with a width of ~10 μm [24, 25]. Meanwhile, when the thickness of Cu layer decreased, Cu was detected in the Al layer, as shown in Figures 8c and 8d. Thus, it might be suggested that the joint of Al with AlN should be accomplished through the Cu-Al eutectic formation, which follows the diffusion of Cu in Al.

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Figure 9. Fractured surface morphologies of samples for the four investigated cases (the letters (a), (b), (c),

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and (d) correspond to these cases; see the text in 3.2). The inset shows the EDS elemental spot analysis at the

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rod-like crystals, observed in (c) and (d) (which were attributed to Al2Cu phase).

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3.2.2 Analysis of the fracture surface Details of the phase composition and the morphology of the interfacial reaction zone were obtained by the analysis of the fractured surfaces after the peeling-off tests by SEM/EDS (Figure 9) and X-ray diffraction (Figure 10). The original surface of the reduced copper film is seen in Figure 9a, which corresponds to the experiment of case a (i.e. very short dwell time). When the holding time and the temperature increased, the pores in the fracture surface disappeared, while rod-like particles were developed (Figures 9b to 9d). EDS analysis (inset in Figure 9) indicated that the rod-like particles were composed of Al and Cu with an atomic

ACCEPTED MANUSCRIPT ratio close to 2:1. Meanwhile, XRD analysis showed that CuAlO2 and Al2Cu phases were recorded in the fracture surface of Al/AlN joints, along with Al, AlN, and Cu phases (Cu phase gradually disappeared with the increase of holding time and temperature). These results suggest that Cu dissolves in Al to form Al2Cu. In conjunction with the EDS results (inset of Figure 9), the XRD results (Figure 10) also confirmed that the

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rod-like particles were made of Al2Cu phase. In the Cu-Al alloy system, Al2Cu is usually considered as a hard

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and brittle phase [28-30]. Thus, its presence in the interfacial reaction zone is expected to affect the

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mechanical properties of the produced Al/AlN joints.

Figure 10. X-ray diffractograms recorded from the fractured surfaces of Figure. 9 (the letters (a), (b), (c), and (d) correspond to these cases, i.e. to the four investigated cases; see the text in 3.2).

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3.2.3 Fracture strength of Al/AlN joints The values of the peeling strength of the Al/AlN joints for the four investigated cases (with their standard deviations) are seen in the plot in Figure 11. The lowest value of 3.2 MPa was recorded in the sample of case (a) produced at 660 oC for 0 min. The highest value of 15.4 MPa was recorded in the sample of case (c) (660 C, 30 min). However, increase of temperature (at 700 oC, case d) resulted in samples with peeling strength

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higher than the samples of cases (a) and (b), but lower than the samples of case (c).

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Figure 11. Peeling-off strength of Al/AlN joints for the four investigated cases (the letters a, b, c, and d

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correspond to these cases; see the text in 3.2).

In multilayer composite structures, their mechanical properties are usually determined by the weakest part of the system. Hence, the fracture position of Al/AlN joints can shed light on the fracture mechanism. Therefore, optical micrographs of the cross-sections of the fracture surfaces were obtained, and residual weight analysis was conducted to determine the fracture position for the four investigated cases. Table 1 presents typical examples from each of the investigated four cases (similar results were obtained from all the samples tested). According to these results, the ratio of residual weight decreased with the increase of holding

ACCEPTED MANUSCRIPT time and temperature. The depth profile of Cu layer was similar to the element line scan analysis across the interface of Al/AlN joints, shown in Figure 8. Therefore, it is suggested that the fracture occurred in the Cu layer and the fracture position was near the interface between the Cu and the Al layers. With the increase of holding time and temperature, the fracture occurred progressively closer to the Cu/AlN interface. This

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suggests that the position of fracture moves towards the AlN ceramic side over increasing holding time and

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temperature.

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Table 1. Typical results of optical micrograph and residual weight analysis from Al/AlN joints after peeling-off test for the four investigated cases (the letters (a), (b), (c), and (d) correspond to these cases; see

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the text in 3.2).

Optical micrograph

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50μm

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Original weight

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0.4126

63.49

0.6615

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58.38

0.6522

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53.20

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(c)

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50μm

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(d) 50μm

4. Discussion

ACCEPTED MANUSCRIPT As far as the first process of preparation technique is concerned, according to the thermodynamics in Cu-O system [26], CuO decomposes to Cu2O at temperatures >1000 oC: 4 CuO→2Cu2O+O2↑

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Meanwhile, AlN can react with oxygen in air atmosphere: 4AlN+ 3 O2 → 2Al2O3+2N2↑

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The formation of CuAlO2 can be due to the reaction of Cu2O with Al2O3:

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(4)

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Cu2O+Al2O3 → 2CuAlO2

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4 CuAlO2 + 2Al2O3 + O2→4 CuAl2O4

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Finally, in an oxygen-rich environment, the CuAlO2 phase can be partially transformed to CuAl2O4 [24]: (6)

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However, the significant mismatch of the CTEs of the spinel phase CuAl2O4, CuO, and AlN ceramic [24, 27] should have resulted in developing residual stresses in the interfacial zone of the CuO/CuAl2O4/AlN assemble.

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However, the interface was dense and continuous, with no gaps or cracks. This provides further evidence (in

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conjunction with the aforementioned results from SEM/EDS and XRD analyses) that CuAl2O4 was not formed. In other words, it is suggested that at the interface between AlN and CuO, oxygen diffusion rate is lower than

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the formation rate of CuAlO2; thus, only a small (negligible) part of CuAlO2 might be transformed into

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CuAl2O4 [25]; indeed, this phase was not experimentally detected in this study. The Introduction stressed that one of the biggest problems in this type of process, where Al is involved, is the disruption of the superficial Al-oxide layer. Further to the reaction described by chemical equation (5), the pronounced mismatch among the CTEs of Al, Cu, and Al2O3 (26.5, 18.3, and 8.8×10-6/K at 660 oC, respectively [31]), should facilitate the disruption of the superficial aluminum oxide film formed on the surface of Al [32]. Hence, liquid aluminum can pass through the cracks of the fragmented oxide film, copper can gradually dissolve in aluminum liquid, and Al-Cu intermetallic compounds are formed in the interface.

ACCEPTED MANUSCRIPT The formation of these intermetallic compounds is expanded with the increase of dwell time and temperature. When the holding time is enough to make the copper completely dissolved in aluminum, the porous copper layer disappears and the grains of the Al2Cu intermetallic compound continue to grow. The CTEs are also different among CuAlO2, Al, and Al2Cu (4.3, 26.5 and 1.6×10-6/K, respectively [31]).

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Therefore, the fact that the influence of dwell time and temperature on the mechanical properties differs from

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one case to another (Figure 11, where the treatment at 700 oC (case d) resulted in weaker Al/AlN joints than

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those produced at 660 oC (case c), both for the same dwell time, 30 min) should be attributed to the size and the shape of the Al2Cu grains formed in the interfacial reaction zone of the Al/AlN joints. More specifically,

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the small-sized rod-like Al2Cu intermetallic compounds (Figure 9c) had a positive influence on the peeling-off

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strength of the joints produced at 660 oC, whereas big-sized Al2Cu crystals, produced at 700 oC (Figure 9d), reduced the peeling-off strength. In the latter case, the residual stresses should favor the formation of cracks in

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the interface of the intermetallic compounds and the matrix during peeling-off tests [33].

5. Conclusions

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Joints of Al/AlN were successfully fabricated by a thick film method. A Cu layer was produced from the

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reduction of CuO and then diffused into the Al foil to obtain strong Al/AlN joints via a relatively simple manufacturing process. The new bonding method does not require high temperatures, pressure and complex process. Hence, it meets the requirements for large-scale industrial production. The experimental results showed that CuAlO2 was formed in the interface. Part of CuO was reduced and formed a porous Cu layer. The superficial Al-oxide film, which inevitably forms over the liquid Al, was effectively disrupted at 660 oC. This allowed the diffusion of Cu in Al, which resulted in the formation of rod-like grains of Al2Cu intermetallic compound in the interfacial reaction zone.

ACCEPTED MANUSCRIPT The results of the mechanical strength tests showed that the fracture occurred at the porous copper layer. However, in prolonged experiments, this Cu layer completely disappeared and dissolved in the Al-phase. In these cases, fracture occurs in the interface between CuAlO2 and aluminum layer. The grains of Al2Cu intermetallic compounds, formed in the interface, influence the mechanical properties of Al/AlN joints.

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According to the experimental results, it is suggested that the large-sized rod-like Al2Cu particles have a

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negative effect on the mechanical properties of Al/AlN joints, possibly due to the mismatch of the CTEs of the

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phases developed in the interfacial reaction zone of the joint.

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Acknowledgements This work was supported by Science and Technology Projects of Jiangsu Province (BE2016050) and the

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Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), Nanjing, China.

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Graphical Abstract

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Highlight 1. Al foil was bonded to AlN ceramics through a thick film method using CuO paste. 2. The peeling-off strength of the Al foil from the surface of the AlN substrate reached a value of 15.4MPa while heat treatment at 660 oC.

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3. The large-sized rod-like Al2Cu particles at interface have a negative effect on the mechanical properties of

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Al/AlN joints.