High-Resolution Identification of Chemical States in Individual Metal

Apr 22, 2016 - The effectivity of cryo-scanning transmission electron microscopy-electron energy loss spectroscopy was demonstrated for nanoscale anal...
2 downloads 0 Views 9MB Size
Download PDF
Article pubs.acs.org/ac

High-Resolution Identification of Chemical States in Individual Metal Clusters in an Insulating Amorphous Polymer Yugo Kubo,*,† Akira Mizoguchi,† and Jun-ichi Fujita‡,§ †

Analysis Technology Research Center, Sumitomo Electric Industries, Ltd., 1-1-3, Shimaya, Konohana-ku, Osaka-shi, Osaka 554-0024, Japan ‡ Institute of Applied Physics, Graduate School of Pure and Applied Sciences, University of Tsukuba, 1-1-1, Tennodai, Tsukuba, Ibaraki 305-8573, Japan § Tsukuba Research Center for Interdisciplinary Materials Science, University of Tsukuba, 1-1-1, Tennodai, Tsukuba, Ibaraki 305-8573, Japan S Supporting Information *

ABSTRACT: The effectivity of cryo-scanning transmission electron microscopy-electron energy loss spectroscopy was demonstrated for nanoscale analysis of the cross-section of the Cu/polyimide interface. The nanoscale Cu/Cu2O/CuO layer structure at the interface was clearly observed for the first time. In addition, a Cu atom was identified, embedded in the polyimide matrix, and the average valence of diffusing Cu atoms or nanoclusters was determined using (cryo-)scanning transmission electron microscopy-electron energy loss spectroscopy. On the basis of these results, we have proposed a mechanism for the diffusion of Cu atoms in polyimide. To the best of our knowledge, this is the first report of the observation of a metal atom embedded in an insulating amorphous polymer.

(1.67 × 10−6 Ω·cm) and is an excellent wiring material, in addition to the advantages of PI as an insulative coating material. Therefore, to overcome disadvantages (1) and (2), numerous efforts have been made to investigate the Cu/PI interface and Cu particles diffusion layer.1−6 Disadvantage (1) causes pattern 1 peeling, shown in Figure 1b, after repetitive use in a high temperature environment. Pattern 1 peeling is strongly related to the Cu oxide layer structure and the chemical bonding states at the Cu/PI interface.5,6 Conventional experimental approaches to study the interfaces are roughly divided into the following two groups. One is where several atomic layers of Cu are formed on PI by physical or chemical vapor deposition and the interface is analyzed through the Cu layer.1−4 The other is where ultrathin PI films are prepared on Cu foils by spin- or dip-coating and the interface is analyzed through the PI layer.5,6 The most popular analytical tool for both approaches is X-ray photoemission spectroscopy (XPS). In addition, techniques such as Fourier transform infrared spectroscopy (FT-IR) and ultraviolet photoelectron spectroscopy (UPS) have been used. The common shortcoming of these techniques is the poorer in-plane spatial resolution. The obtained spectra are generally an average of signals detected in the range of the X-ray, infrared light, or ultraviolet light

U

biquitous computing allows on-demand access to information networks such as the Internet to instantly obtain information from anywhere in the world. In this ubiquitous society, portable terminals such as multifunctionalized cellular phones, smartphones, or wearable computers play a central role for end-users. Flexible printed circuits (FPC) are one of the most important components in portable terminals. FPCs have flexibility so that they can be deformed with a weak force while the electronic properties are maintained. The FPC structure is comprised of copper (Cu) circuits formed on polyimide (PI) substrates. PI is the generic name for polymers that have imide bonds in their main chains. Figure 1a shows the chemical reaction where a typical PI substrate is obtained by the condensation polymerization of polyamic acid, the reaction product from pyromellitic dianhydride (PMDA) and oxidianiline (ODA). The reaction involves dehydration and intramolecular ring closure between NH and COOH groups.1 PI has the highest heat resistance of the engineering plastics, in addition to excellent mechanical strength, electrical insulation, and both chemical and environmental resistance; therefore, PI has been widely used in various applications that require high reliability, such as electronic devices, and in the automotive and aviation industries. The well-known disadvantages in the use of PI are (1) the inferiority of adhesion to Cu2 and (2) the diffusion of Cu particles inside PI. 2−4 Despite these disadvantages, the combination of Cu and PI has been considered an extremely important material for practical applications. This is because Cu has very low conductivity © 2016 American Chemical Society

Received: January 23, 2016 Accepted: April 22, 2016 Published: April 22, 2016 5225

DOI: 10.1021/acs.analchem.6b00305 Anal. Chem. 2016, 88, 5225−5233

Article

Analytical Chemistry

Figure 1. (a) Chemical reaction for the preparation of PI by the condensation polymerization of polyamic acid, the reaction product from PMDA and ODA. (b) Typical peeling patterns for the Cu/PI interface after repetitive use in a high temperature environment. (c−e) Cross-sectional BF images of Cu/PI SA prepared by (c) ultramicrotoming, (d) Ga FIB processing at room temperature, and (e) Ga FIB processing under liquid nitrogen cooling (−110 °C).

there have been no reports on the observation or analysis of insulating and amorphous polymers with atomic resolution. In the present study, we demonstrate the effectiveness of cryoSTEM-EELS for the nanoscale cross-sectional analysis of the Cu/PI interface. Furthermore, a Cu atom embedded in a PI matrix was successfully identified, and the average valence of diffusing Cu atoms or nanoclusters was determined from (cryo) annular dark-field (ADF)-STEM observations and (cryo-) STEM-EELS analyses.

diameter, which is typically larger than several micrometers. For disadvantage (2), the diffusion of Cu particles has also long been problematic in the wiring materials of silicon semiconductors in electronic devices. This can cause insulation failure in such devices, and weakening of the PI layer due to Cu diffusion can result in pattern 2 peeling (Figure 1b) in many products where PI is used as an insulating coating material. To investigate this Cu diffusion, Ho and colleagues performed cross-sectional transmission electron microscopy (TEM) observations, Rutherford backscattering spectroscopy, and Monte Carlo simulations to elucidate the role of particle formation and its influence on the diffusion profile.2−4 They surmised that the diffusion of weakly bound single Cu atoms into the glassy PMDA-ODA PI appears similar to the Fickian diffusion of gases and solvents.4 In these early works, TEM observations merely confirmed an increase in the size of the diffused Cu particles with the sample preparation temperature.2,3 Nowadays, aberration-corrected (scanning) TEM ((S)TEM) enables observations with atomic resolution for inorganic materials such as ceramics.7 STEM-electron energy loss spectroscopy (EELS) is also recognized as the only available analytical tool to identify the atomic-scale chemical state.8−10 However, in the application of these techniques to the observation and analysis of organic materials, the main problem to overcome is sample damage due to electron irradiation.11 In spite of many pioneering studies, damage remains the main barrier for the observation and analysis of organic or polymeric materials rather than inorganic materials.12,13 Incentive studies to develop state-of-the-art (S)TEM instruments and maximize their capabilities have gradually removed the difficulties in the observation and analysis of organic materials. Recent studies by Horiuchi and colleagues have systematically demonstrated that energy filtering-TEM enables the observation and analysis of cross sections of various polymer/polymer or polymer/metal interfaces with a high resolution of 10 nm.14 However, to the best of our knowledge,

■

EXPERIMENTAL SECTION Samples. 4,4′-Oxidianiline (9.43 g) was dissolved in 1methyl-2-pyrrolidone (80.3 g), and 1,2,4,5-benzenetetracarboxylic anhydride (10.3 g) was added to the solution. The solution was stirred for 1 h at 25 °C under a nitrogen atmosphere and then for 20 h at 60 °C to obtain a PI precursor (polyamide acid) coating solution. The PI precursor coating solution was applied to a Cu substrate using the doctor-blade method. The PI precursor was then dried for 1 h at 120 °C under a nitrogen atmosphere. The polyamide acid coating was imidized by heat treatment for 3 h (Sample A (SA)) or 1 h (Sample B (SB)) at 300 °C to obtain a PI film with an average thickness of 10 μm. These samples were thinned perpendicular to the layer structure using (1) an ultramicrotome at room temperature and (2) a Ga focused ion beam (FIB) to thicknesses of approximately 50−80 nm. The thicknesses of the FIB samples were confirmed from cross-sectional secondary electron images. FIB sample preparation was performed at room temperature or under liquid nitrogen cooling (−110 °C). For typical FIB sample preparation, a sample was thinned to a thickness of 4−5 μm with an accelerating voltage (Vacc) of 30−20 kV and probe current (Iprob) of 0.5−0.1 nA and subsequently thinned to the thickness of approximately 50−80 nm with Vacc at 10 kV and Iprob at 0.01 nA. During the sample preparation process, polyamic acid was first coated on a Cu substrate and subsequently heated at 300 °C for 3 or 1 h. We consider 5226

DOI: 10.1021/acs.analchem.6b00305 Anal. Chem. 2016, 88, 5225−5233

Article

Analytical Chemistry

Figure 2. Cross-sectional BF images of (a) SA and (b) SB. Higher magnification cross-sectional BF images of (c) SA and (d) SB. Particle size distributions for (e) SA and (f) SB. ADF images of a Cu particle in SB (g) before and (h) after electron beam irradiation. Irradiation was performed for 4 s at room temperature (Iprob = 200 pA and d = 0.2 nm). Similar particle ADF images (i) before and (j) after irradiation, performed for 4 s at −70 °C (Iprob = 200 pA and d = 0.2 nm). ADF images of a similar Cu particle (k) before and (l) after irradiation at room temperature, where the electron beam (d = 0.1 nm) was scanned in a 10 × 10 nm2 area for 20 s with Iprob = 30 pA. (m,n) EELS spectra of the Cu oxide particles (≤5 nm) measured (m) at −70 °C (T = 4 s per point, Iprob = 200 pA, and d = 0.2 nm), using HD-2700, and (n) at room temperature using ARM200F, where the electron beam (d = 0.1 nm) was scanned in a 10 × 10 nm2 area for 20 s with an Iprob of 30 pA. EELS spectra for (o) CuO, (p) Cu2O, and (q) Cu standard powder samples are also shown for comparison.

SA, prepared by (1) ultramicrotoming, (2) Ga FIB processing at room temperature, and (3) Ga FIB processing under liquid nitrogen cooling in Figure 1c−e, respectively. The diffusion of spherical particles is observed in the PI layer of all samples, as schematically shown in Figure 1b. Method (2) induced significant deformation during sample preparation, as shown in Figure 1b, while both methods (1) and (3) provided good EELS samples. The thinned sample cross-section prepared by ultramicrotoming was more easily contaminated by electron beam irradiation during observations and analyses than that prepared by FIB. We consider that this could be attributable to a larger amount of surface hydrocarbons in the ultramicrotomed sample than in the FIB samples because ultramicrotoming is performed in the atmosphere while FIB processing is performed in a vacuum. However, this was not examined in detail, because it was outside the main scope of this investigation. In some cases, EELS analyses were performed with long acquisition times (e.g., ca. 300 s) to ensure good SN ratios. In these cases, the ultramicrotomed samples were very unfavorable. On the other hand, the main problem with the FIB samples was artifacts that were attributable to the high energy Ga ion beam. Therefore, throughout this study, STEM observations of both ultramicrotomed and FIB (−110 °C) samples were performed to confirm whether they give similar observation results. EELS analyses were performed using FIB (−110 °C) samples to investigate the chemical states of the microareas. For instance, the diffusing particles observed in Figure 1c−e were also observed in the ultramicrotomed sample, which indicates that they exist in the pristine sample and are not the artifacts induced by FIB.

that heating allows the simultaneous diffusion of Cu atoms and imidization of the polyamide acid. ADF-STEM Observations and STEM-EELS Analyses. To investigate the element distributions and chemical states of microsites in the samples, ADF-STEM observations and STEM-EELS analyses were performed using (1) a Hitachi High-Technologies HD-2700 microscope equipped with a Gatan 776 Enfina 1000 spectrometer system and (2) a JEOL JEM-ARM200F microscope (with a cold field emission gun (CFEG)) equipped with a Gatan Quantum ER spectrometer system. Both microscopes were operated at 200 kV. The electron spectrometers employed metal oxide semiconductor (MOS) detector arrays (HD-2700, Hitachi High-Technologies) and an MOS detector array (JEM-ARM200F, JEOL) with respective beam diameters of 0.2−0.4 and 0.1 nm. For observation with the HD-2700, both low angle (LA)-ADF and high angle (HA)-ADF observation modes were employed with detection angles of 35−190 and 70−370 mrad, respectively. ADF observations and EELS analyses using the HD-2700 were performed under liquid nitrogen cooling (−70 °C) to reduce the damage from electron irradiation, except for the results presented in Figure 2i,j. ADF observations and EELS analyses using the JEM-ARM200F microscope were performed at room temperature, and a data acquisition method reported by Bosman and Keast15 that reduces the effect of correlated noise was adopted for the EELS analyses.

■

RESULTS AND DISCUSSION First, we discuss the sample preparation methods for STEM observations and EELS analyses. To obtain EELS spectra with better signal-to-noise (SN) ratios requires very thin samples. We present bright field (BF) cross-sectional images of Cu/PI 5227

DOI: 10.1021/acs.analchem.6b00305 Anal. Chem. 2016, 88, 5225−5233

Article

Analytical Chemistry

study, the temperature increase is insignificant (ΔT = ∼0.3 K);11 therefore, electron-beam heating is not expected to cause hole formation. Another possible mechanism is a local density decrease at the electron irradiated point due to the radiolysis of Cu oxide. In the following, the radiolytic mechanism proposed by Knotek and Feibelman16 is adopted to explain the electron beam damage in the present study. After the ionization of a core electron of the metal cation (Cu+ in the present case) by an incident electron, a valence electron on the O anion decays to fill the hole on the cation, which releases sufficient energy to eject an electron on the O anion as an Auger electron. Thus, O2− is turned into O0 and the Cu−O chemical bond is broken. If this occurs at room temperature, then the neutral O can be desorbed from the particle surface, which leads to a reduction of the metal Cu oxide and a corresponding decrease in the local density at the irradiated point due to O atom desorption. Here, we suppose that sample cooling prevents or slows down O atom desorption. The O atom has high oxidizing power; therefore, O atoms that remain at the particle surface deprive the Cu metal atoms of an electron to produce Cu+ ions and reform the Cu−O chemical bond. EELS spectra of such particles (≤5 nm) is presented in Figure 2m,n, which were, respectively, obtained at −70 °C (T = 4 s per point, Iprob = 200 pA, and d = 0.2 nm) using HD-2700 and at room temperature using JEM-ARM200F, where the electron beam (d = 0.1 nm) was scanned in a 10 × 10 nm2 square area for 20 s with an Iprob of 30 pA. Figure 2o−q shows EELS spectra obtained using standard CuO, Cu2O, and Cu powder samples, respectively, for comparison. Both of the spectra in Figure 2m,n are identified as Cu2O because of the following two spectral features: (1) the sharp L3 white line and (2) sufficiently high intensity at 950− 1000 eV compared to L3. Here, do the spectra in Figure 2m,n represent the pristine material? Long and Petford-Long showed a series of EELS spectra recorded for a CuO particle during exposure to an electron beam, where the analysis was conducted using 100 kV FE-TEM operated at a current density in the order of 4 × 103 A/cm2.17 They reported that CuO can be gradually reduced to metallic Cu via an intermediate phase such as Cu2O. More specifically, the spectrum measured when the particle was exposed to an electron beam was identified as CuO, while those recorded after 12.5 and 50 s exposure to the beam were identified as Cu2O and Cu, respectively. The dose amounts for 1, 12.5, and 50 s exposure at a current density of 4 × 103 A/cm2 correspond to 4 × 103, 5 × 104, and 2 × 105 C/ cm2, respectively. For the present study, the dose amounts were calculated to be 3 × 106 (Figure 2m) and 6 × 102 C/cm2 (Figure 2n). The dose amount for Figure 2n is significantly lower than that used by Long et al.; therefore, it is reasonable to postulate that the spectrum in Figure 2n represents the pristine material (i.e., a Cu2O particle). Furthermore, the spectrum in Figure 2m is also identified as Cu2O, which indicates that sample cooling prevents the reduction of Cu oxides, even if the dose amount is as much as 3 × 106 C/cm2. Next, we focus on how precisely the oxidization state of the Cu/PI interface can be identified using STEM-EELS because the Cu/Cu2O/CuO layer microstructure at the interface is fundamentally involved in interface adhesion. In addition, we focus on the mechanism for Cu particle diffusion in PI, which can provide useful information on the prevention of Cu diffusion. A mechanism was proposed on the basis of the observations and analyses of ultrafine Cu particles in a PI matrix, as discussed later. We demonstrate the outstanding capability of cryo-STEM-EELS (HD-2700) for clarification of the oxidation state of nanoscale

Figure 2a,b shows BF cross-sectional images of Cu/PI SA and SB prepared by FIB (−110 °C). The diameters of the particles in SA PI are larger than those in SB PI, as noted from the higher magnification images in Figure 2c,d and by the particle size distributions in Figure 2e,f, respectively. This is attributed to the longer processing time at 300 °C for SA than SB. However, this difference in particle sizes was not examined in detail, because it was outside of the scope of this investigation. The larger particles (>20 nm) are Cu/Cu2O/ CuO core−shell particles (Cu metal particles with a surface oxide layer several nanometers thick), while the smaller particles (≤5 nm) are Cu2O. In addition, for both SA and SB, no particles were observed in the vicinity of the Cu substrate/PI interface, the areas between the two triangular arrows shown in Figure 2a,b. This is discussed later in relation to the Cu diffusion mechanism. Next, we explain the two approaches adopted in this work to reduce electron irradiation damage. EELS analyses are often performed to identify the chemical state of nanoscale areas such as the Cu/PI interface. In the present work, EELS analyses require measurement times (T) typically longer than 1−4 s per point and a Iprob of 200 pA with a spot size (d) of 0.2 nm to ensure a sufficient SN ratio for discussion of the chemical states of these nanoareas. However, electron irradiation caused severe damage to the sample. Figure 2g,h shows ADF images before and after electron irradiation of a Cu particle in SB, where irradiation was performed at room temperature (T = 4 s per point, Iprob = 200 pA, and d = 0.2 nm). In Figure 2h, a hole is observed at the irradiated point (the center of the particle). Figure 2i,j shows ADF images of a similar particle before and after irradiation performed at −70 °C (T = 4 s per point, Iprob = 200 pA, and d = 0.2 nm). No hole was observed at the center of the particle in Figure 2j, which suggests that cooling of the sample decreases the irradiation damage. The presently employed cooling system can maintain temperatures lower than −70 °C; however, the formation of ice can occur. Therefore, the sample stage was always set at −70 °C if sample cooling was necessary. Although cooling has a certain effect, sample drift during observations and analyses is notoriously troublesome, and stabilization of the temperature requires time. Another promising approach to reduce irradiation damage is the use of smaller dosages of irradiation. Figure 2k,l shows ADF images of a similar particle before and after irradiation at room temperature, where the electron beam (d = 0.1 nm) was scanned in a 10 × 10 nm2 area for 20 s with an Iprob of 30 pA. A comparison of these images indicates that the EELS analyses can be performed without deformation of the particle under certain measurement conditions, even when performed at room temperature. Thus, these two different approaches were employed. When EELS spectra of nanoscale areas were required, the measurements were performed at −70 °C; however, if spatial resolution was not required, then the electron beam was scanned in a square area at room temperature with an Iprob of 30 pA. The images in Figure 2g−j were obtained using the HD-2700 microscope, while those in Figure 2k,l were obtained using the JEM-ARM200F microscope. Here, we discuss (1) the hole formation mechanism at the electron irradiated point and (2) the prevention of hole formation by sample cooling. The first conceivable hole formation mechanism involves local dissolution at the irradiated point of the particle due to electron-beam heating.11 However, when electron probes with small Iprob are used, as in the present 5228

DOI: 10.1021/acs.analchem.6b00305 Anal. Chem. 2016, 88, 5225−5233

Article

Analytical Chemistry

Figure 3. LA-ADF images of (a) a Cu particle diffusing in PI and (b) the PI matrix of SA. (c) EELS spectra (Cu-L) measured at ten points with 1 nm step distances along a line perpendicular to the Cu particle/PI interface, as shown in (b). Reference spectra for Cu, Cu2O, and CuO are shown in the right-hand panel. (d) Schematic diagram of the core−shell structure of a diffusing Cu particle. (e) Line profile of the O-K signal intensity crossing the particle center. LA-ADF images of (f) the Cu substrate and (g) the PI matrix of SA. (h) EELS spectra (Cu-L) measured at ten points with 1 nm step distances along a line perpendicular to the Cu substrate/PI interface. (i) Schematic diagram of the oxide layer structure around the Cu substrate/PI interface. (j) Line profile of the O-K signal intensity around the Cu substrate/PI interface. HA-ADF images of (k) a diffusing Cu particle of SA and (l) the Cu substrate/PI interface of SB. Electron beam diffraction patterns of the core of a diffusing Cu particle shown in the (m) [112] and (n) [015] directions.

Thus, the EELS spectra for Cu-L provides nanometer scale chemical information. However, it is somewhat difficult to gain information from the spectral shape for oxygen (O-K) because the PI matrix also includes oxygen atoms, and the oxygen signal thus reflects the sum of that from the PI and Cu oxide layers. However, the line profile of the O-K signal intensity crossing the particle center supports a change in the chemical state at the Cu/PI interface (in 1 nm step distances). Figure 3e shows that the O-K intensity increases around the interface, where the Cu-L signal is also present (indicated by two arrows). The increase of the O-K intensity at the interface is easily understood by considering the number of oxygen atoms per unit volume in the material. The number of oxygen atoms in PI

areas around the Cu/PI interface. The probe used to obtain the EELS spectra and line profiles was 200 pA × 0.2 nm × 1 s per point, which corresponds to a dose amount of 6 × 105 C/cm2. Figure 3a,b presents LA-ADF images of a diffusing Cu particle (24 nm diameter) and the PI matrix of SA, respectively. EELS spectra (Cu-L) were obtained at ten points with 1 nm step distances along a line perpendicular to the Cu particle/PI interface, as shown in Figure 3b. The obtained spectra (Figure 3c) show a gradual change of the Cu oxidation state around the interface; the particle core is Cu metal, while the layer just outside the particle is 2 nm thick Cu2O, and the outermost layer is 3 nm thick CuO, as schematically shown in Figure 3d (standard spectra for Cu, Cu2O, and CuO shown in Figure 3c). 5229

DOI: 10.1021/acs.analchem.6b00305 Anal. Chem. 2016, 88, 5225−5233

Article

Analytical Chemistry

Figure 4. (a−d) Cross-sectional images of PI SB prepared by (a, b) ultramicrotoming, and (c, d) Ga FIB processing at low temperature, where (a, c) and (b, d) are HA-ADF and BF images, respectively, with the same field of view. (e) HA-ADF image of the enlarged area of the PI SB cross-section prepared by FIB. (f) Line profile of the contrast crossing the ultrafine particle.

is 0.019 × Avogadro’s number per cubic centimeter, which is calculated from the density and molecular weight, while those in Cu2O and CuO are, respectively, calculated by 0.042 × and 0.079 × Avogadro’s number per cubic centimeter. The thickness of the oxide layers can be estimated from the O-K intensity profile to be approximately 5 nm (i.e., the sum of the Cu2O and CuO layer thicknesses). Similar observations and analyses were performed for the Cu substrate/PI interface. EELS spectra were obtained from ten points at 1 nm step distances along a line perpendicular to the Cu substrate/PI interface, as shown in Figure 3g. The obtained spectra (Figure 3h) also represent the gradual change of the Cu oxidation state around the interface. However, compared to that for the Cu particle/PI interface, the change of the chemical states is not obvious. In particular, it is difficult to determine the Cu valence from spectrum 8 in Figure 3h, which corresponds to the outermost layer of the Cu substrate in contact with PI. Moreover, spectrum 8 appears to be a summation of the Cu2O and CuO spectra. The ambiguity of the EELS spectra for the Cu substrate/interface compared with those for the Cu particle/PI interface is attributable to the greater roughness of the Cu substrate/PI interface. In (S)TEM analysis, the obtained signal is a summation of the sample thickness in the direction parallel to the incident electron beam. Therefore, if the Cu/ Cu2O or Cu2O/CuO interfaces are uneven, as schematically shown in Figure 3i, then the signal probably includes information on Cu, Cu2O, and CuO. Furthermore, a line scan of the O-K intensity crossing the Cu substrate/PI interface was performed, which indicated that an oxide layer (4 nm thick) was present at the Cu/PI interface (Figure 3j), similar to the case for the diffusing particle. The Cu2O or CuO layer thicknesses can thus be estimated with an in-plane spatial resolution of 1 nm using STEM-EELS. LA-ADF observations (Figure 3a,f) did not reveal clear signs of the Cu2O or CuO layers at the Cu/PI interface for either the Cu particle/PI or Cu substrate/PI interfaces. Figure 3k,l shows HA-ADF images for

the interfaces between a particle for SA and PI and between a Cu substrate and PI for SB, respectively. The core (Cu metal) and shell (oxides) structure of the particle is evident (the shell is located between the two triangular arrows in Figure 3k), although it is difficult to distinguish the Cu2O and CuO layers. The uniformity (ca. 4 nm) of the shell (oxide layer) thickness was confirmed, and this is discussed later. While the Cu substrate/PI interface is very rough, no obvious contrast of the oxide layers was observed (Figure 3l). It should be emphasized that the thicknesses of the local CuO/Cu2O nanolayers were obtained here for the first time. To the best of our knowledge, there are no examples of direct cross-sectional observations of the nanoscale Cu/Cu2O/CuO layer structure of a Cu/PI interface, even though the state of the oxide layer is important for industrial applications. It is very important that STEMEELS analysis enables the thickness of each oxide layer to be estimated. This is because the balance of the Cu2O and CuO layer thicknesses is one of the key parameters that determines adhesion at the Cu/PI interface because the mechanical properties of the two oxides are significantly different; therefore, PI can be easily peeled from the Cu substrate at the Cu2O/CuO interface (i.e., pattern 1 peeling in Figure 1b). The electron beam diffraction patterns of the core (Figure 3m ([112] direction) and Figure 3n ([015] direction)) indicate crystallinity (the indices of the crystalline plane match those of Cu metal) and not an amorphous core. Electron diffraction patterns of the shell could not be obtained due to experimental difficulties; therefore, it is difficult to confirm whether the shell is crystalline or amorphous. It is important to investigate the Cu particle diffusion to facilitate understanding of the particle diffusion mechanisms and consider methods to prevent particle diffusion, which causes insulation failures in electronic devices and not only intralayer peeling (pattern 2 in Figure 1b). Furthermore, the effectiveness of STEM-EELS for analyzing the chemical state of extremely small Cu particles in amorphous and insulating PI was investigated. As shown in Figure 2m, the 5230

DOI: 10.1021/acs.analchem.6b00305 Anal. Chem. 2016, 88, 5225−5233

Article

Analytical Chemistry

Figure 5. (a, b) HA-ADF images of the cross-section of PI SB prepared by FIB. (c) Contrast profile across a particle in (b). (d) EELS spectrum (before background removal) measured for the entire area of (a) (20 × 20 mm2) containing many subnanometer particles. (e) EELS spectrum around the Cu-L edge after background removal.

contrast profile (Figure 5c) across a particle in Figure 5b has a half-width of approximately 0.14 nm. To verify that these subnanometer particles are composed of Cu atoms, EELS measurements were performed for the entire area in Figure 5a containing many subnanometer particles. The electron beam (d = 0.1 nm) was scanned in a 20 × 20 nm2 area for 300 s with an Iprob of 30 pA, which corresponds to a dose amount of 2 × 103 C/cm2. The EELS analysis was also performed on the basis of the data acquisition method suggested by Bosman and Keast to reduce the effect of the correlated noise.15 An obvious peak was confirmed at around 940 eV, which corresponds to Cu-L, as indicated by the red dotted line in Figure 5d. The Ga-L edge (ca. 1140 eV) was not confirmed, as indicated by a similar dotted line in Figure 5d. The EELS spectrum around the Cu-L edge after background removal (Figure 5e) shows a more obvious peak around 940 eV. The spectrum in Figure 5e was identified as Cu2O; i.e., the subnanometer particles are Cu2O. We can postulate that the particles indicated by the red circles in Figure 5b are the contrast equivalent to Cu atoms. Here, we propose a mechanism for the diffusion of Cu particles based on the results for the analyses of diffusing Cu particles. Figure 6 shows a schematic summary of the proposed mechanism. First, both imidization and the diffusion of Cu atoms (not particles with sizes of several nanometers) progress under a nitrogen atmosphere at 300 °C (Figure 6a,b). Here, polyamic acid has two COOH moieties per monomer and COOH may lose H+ to become COO−. Cu+ migrates in the polyamic acid matrix via COO− ions from polyamic acid, as

oxidation state of 3 nm particles could be specified. However, it was difficult to specify the oxidation state of particles smaller than 3 nm because the SN ratio of the EELS spectra was insufficient. However, both the HA-ADF and BF images enable the smaller particles to be observed. Figure 4a−d shows crosssectional images of PI SB prepared by ultramicrotoming ((a) and (b)) and FIB (−110 °C) ((c) and (d)). Figure 4a,b shows HA-ADF and BF images, respectively, with the same field of view. Figure 4c,d also shows HA-ADF and BF images with the same field of view. In PI SB, there are a large number of ultrafine particles with sizes of approximately 1 nm, other than the particles of several nanometers in Figure 4a,b. The ultrafine particles can be recognized at the same places in the HA-ADF and BF images within the same field of view. These ultrafine particles were observed in both the ultramicrotomed and FIB samples; therefore, they must be present in the pristine sample and are not artifacts. Figure 4e, in which a subnanometer particle (indicated with a red arrow) is observed, shows the HA-ADF image of the enlarged area of the PI SB cross-section prepared by FIB. A line profile of the contrast crossing the ultrafine particle is shown in Figure 4f. The profile indicates that the particle diameter is 0.74 nm, which is probably equivalent to several atoms of Cu and O. Figure 4a,b was obtained using the JEM-ARM200F, while Figure 4c−e was obtained using HD-2700. In addition, subnanometer particles could be observed in the HA-ADF images obtained with the JEM-ARM200F, for the cross-section of PI SB prepared by FIB (Figure 5a,b). The 5231

DOI: 10.1021/acs.analchem.6b00305 Anal. Chem. 2016, 88, 5225−5233

Article

Analytical Chemistry

Figure 6. Schematic diagram of the proposed mechanism for the diffusion of Cu particles in PI.

Cu particle surfaces and the Cu substrate at the Cu/PI interface. It is also possible that the Cu/Cu2O/CuO particles are a result of exposure of the thin samples to the atmosphere during transfer from FIB to TEM. However, we consider that the core/shell particles are formed mainly during the cooling process described in Figure 6c. This is because PI generally has very high oxygen transmissivity and the residual oxygen in the nitrogen atmosphere during the curing process is approximately 1 mass%. To the best of our knowledge, there have been no reports on this mechanism for Cu atom diffusion in PI based on such nanoscale observations, although Ho and colleagues previously described the migration and agglomeration of Cu atoms in PI using Monte Carlo simulations.3,4

shown in Figure 6d. Recently, Choi et al. suggested a similar diffusion mechanism for Cu ions by the formation of Cu2O nanoparticles in PI.18 Second, during the cooling process, the Cu atoms diffusing in PI coalesce to form Cu metal crystalline particles, and subsequent crystal growth occurs (upper left panel in Figure 6e). Cu atoms in the vicinity of the Cu substrate are then absorbed into the substrate (upper left panel in Figure 6e). Therefore, particles in the vicinity of the Cu substrate/PI interface (indicated by the two triangular arrows in Figure 2a,b) can not be observed. It is difficult to give another reasonable explanation for the space between the Cu substrate and the Cu particle precipitation layer. A significant number of ultrafine Cu particles (ca. 1 nm) observed in SB are considered to be Cu clusters and atoms left behind in the PI matrix, without the chance of being absorbed into Cu particles or the Cu substrate. In addition, it is reasonable that formation of the Cu/Cu2O/ CuO layer structure occurs at the Cu particle/PI interface after the precipitation of Cu particles, considering the uniformity of the oxide layer thickness on the particles shown in Figure 3k. Thus, isotropic growth of the oxide layer after particle precipitation can explain the uniform thickness. Chambers and co-workers investigated the change of the oxidation states of the Cu/PI interface with time using XPS.5,6 They reported that oxidation of the Cu surface begins within an hour of the curing process and that Cu in contact with the PI layer is completely oxidized to CuO after being stored in air for 10 days. On the basis of this report, it is reasonable to postulate that Cu at the interface is gradually oxidized by PI and converted to Cu2O and then finally to CuO. In the present study, the oxygen source was considered to be residual oxygen in the nitrogen atmosphere during the curing process. Residual oxygen that permeated through PI is considered to oxidize the

■

CONCLUSIONS Chemical state analyses of cross sections of the Cu/PI interface were achieved with a spatial resolution of 1 nm using cryoSTEM-EELS. For the first time, we have unequivocally revealed that the Cu/PI interface consists of a Cu/Cu2O/CuO nanolayer structure at both the Cu substrate/PI interface and at the interfaces between diffusing Cu particles and PI. HAADF-STEM observations enabled the visualization of a Cu atom embedded in the PI matrix. To the best of our knowledge, this is the first report on the observation of a metal atom embedded in an insulating amorphous polymer. In addition, we found that the valence of diffusing Cu atoms in the PI matrix could be identified using STEM-EELS. On the basis of these observations and analyses, we have proposed that the mechanism for the diffusion of Cu particles in the PI matrix consists of two stages: the diffusion of Cu atoms in polyamic acid and the precipitation of Cu particles. We have focused on the combination of Cu and PI as a material that is used in 5232

DOI: 10.1021/acs.analchem.6b00305 Anal. Chem. 2016, 88, 5225−5233

Article

Analytical Chemistry various engineering fields. The study of metal/polymer interfaces is critically important for many fields of engineering and material science and has also been of fundamental interest for a long time. We postulate that the atomic scale approach using (cryo-)STEM-EELS has significant potential to provide a comprehensive understanding of the nanostructure and chemistry in polymers along with information provided by other traditional surface analytical methods, such as XPS, UPS, and FT-IR.

■

(15) Bosman, M.; Keast, V. J. Ultramicroscopy 2008, 108, 837−846. (16) Knotek, M. L.; Feibelman, P. J. Phys. Rev. Lett. 1978, 40, 964− 967. (17) Long, N. J.; Petford-Long, A. K. Ultramicroscopy 1986, 20, 151− 160. (18) Choi, D. J.; Maeng, J. S.; Ahn, K.-O.; Jung, M. J.; Song, S. H.; Kim, Y.-H. Nanotechnology 2014, 25, 375604.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.analchem.6b00305. Information on the TEM BF image observations and nanobeam electron diffraction measurements. (PDF)

■

AUTHOR INFORMATION

Corresponding Author

*Tel.: +81-6-6466-5600. Fax: +81-6-6466-5712. E-mail: [email protected]; [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors thank Prof. S. Matsumura of Kyushu University, Dr. K. Kimoto and Dr. T. Nagai of the National Institute for Materials Science (NIMS), and Prof. T. Kizuka of Tsukuba University for fruitful discussions. The authors also thank Mr. K. Sugawara and Mr. T. Yamada of the Foundation for Promotion of Material Science and Technology of Japan and Mr. M. Hanafusa of Sumitomo Electric Industries, Ltd. for their continued support. A part of this work was supported by the NIMS microstructural characterization platform as a part of the “Nanotechnology Platform” program of the Ministry Education, Culture, Sports, Science and Technology (MEXT), Japan.

■

REFERENCES

(1) Haight, R.; White, R. C.; Silverman, B. D.; Ho, P. S. J. Vac. Sci. Technol., A 1988, 6, 2188−2199. (2) Tromp, R. M.; Legoues, F.; Ho, P. S. J. Vac. Sci. Technol., A 1985, 3, 782−785. (3) LeGoues, F. K.; Silverman, B. D.; Ho, P. S. J. Vac. Sci. Technol., A 1988, 6, 2200−2204. (4) Faupel, F.; Gupta, D.; Silverman, B. D.; Ho, P. S. Appl. Phys. Lett. 1989, 55, 357−359. (5) Chambers, S. A.; Chakravorty, K. K. J. Vac. Sci. Technol., A 1988, 6, 3008−3011. (6) Chambers, S. A.; Loebs, V. A.; Chakravorty, K. K. J. Vac. Sci. Technol., A 1990, 8, 875−884. (7) Kimoto, K.; Asaka, T.; Nagai, T.; Saito, M.; Matsui, Y.; Ishizuka, K. Nature 2007, 450, 702−704. (8) Suenaga, K.; Sato, Y.; Liu, Z.; Kataura, H.; Okazaki, T.; Kimoto, K.; Sawada, H.; Sasaki, T.; Omoto, K.; Tomita, T.; Kaneyama, T.; Kondo, Y. Nat. Chem. 2009, 1, 415−418. (9) Spence, J. C. H. Rep. Prog. Phys. 2006, 69, 725−758. (10) Egerton, R. F. Rep. Prog. Phys. 2009, 72, 016502. (11) Egerton, R. F.; Malac, P. L. M. Micron 2004, 35, 399−409. (12) Varlot, K.; Martin, J. M.; Quet, C.; Kihn, Y. Ultramicroscopy 1997, 68, 123−133. (13) Egerton, R. F. Ultramicroscopy 2013, 127, 100−108. (14) Horiuchi, S.; Hanada, T.; Ebisawa, M.; Matsuda, Y.; Kobayashi, M.; Takahara, A. ACS Nano 2009, 3, 1297−1304. 5233

DOI: 10.1021/acs.analchem.6b00305 Anal. Chem. 2016, 88, 5225−5233