This work focuses on the morphological and optical evolution of Al2O3 thick films grown by atomic layer deposition on Si-SiO2 substrates. Blister formation has been the subject of extensive research in the literature; our work fills a crucial gap in the optical characterization of areas inside and outside blisters. Morphological studies were carried out by scanning electron microscopy; we found a reciprocal relationship between the density of the blisters and their diameter. The thickness and refractive index were studied by ellipsometry, revealing a systematic increase in the refractive index with increasing annealing temperature. In addition, we observed the hydrophobic behavior in all films using the water contact angle technique, which suggests that even with blisters, this material can be used in waterproof coatings. Using Auger spectroscopy, we confirmed that delamination occurs completely once the blisters are broken. In this work, we perform cathodoluminescence measurements outside and inside the ampoules. In the area outside the blisters, we observe emissions attributed to the F centers, and the change from the main peaks of 2.8 and 3.4 eV for the as-deposited film to the dominance of emissions centered at 3.4 and 3.7 eV is clearly observed. Furthermore, we observed a strong increase in the cathodoluminescence signal at higher annealing temperatures. On the other hand, we also observed the evolution of the blisters through the cathodoluminescence spectra; in that area, we observed the radical change in the spectrum once the blister is broken, giving rise to the SiO2 signals. We also observed this rupture through a new absorption band in the attenuated total reflectance IR spectra.

Atomic layer deposition (ALD) has emerged as a transformative thin-film fabrication technique with applications in microelectronics, optics, catalysis, and beyond.1 Recent strides in ALD technology have propelled it beyond its traditional boundaries, ushering in a new era where ALD is harnessed for the deposition of thick films.2 This shift in focus raises questions about how these thicker ALD-grown films respond to thermal treatments, emphasizing the formation and behavior of blisters, as some fabricating processes for optical devices may necessitate high-temperature processing.

Among the materials that find their place in optical coatings, alumina or aluminum oxide (Al2O3) stands out as versatile candidates because of its high transparency from near-UV to the near-infrared regions.3–6 Its refractive index of 1.6 at 633 nm can be used to fine-tune various optical components such as bandpass optical filters and waveguides.5,7,8 It is also a dielectric material with a high k, which makes it an interesting material for dynamic random-access memories for MEMS devices, in emerging technologies for nanodevices, or as surface passivation layers for silicon solar cells.9–11 

One possible scenario for thick films grown by ALD is the propensity to develop surface irregularities, commonly known as blisters. These blisters, far from being superficial anomalies, exert a substantial influence on the visual and functional attributes of the films. The above is mainly due to residual gases or functional groups that are generated and accumulate on the surface during each ALD cycle. In the first instance, these gases or molecules must accumulate within the film and then exert pressure on it and be released to the outside, generating blisters. One way to activate this phenomenon is by annealing the film.

The pioneering work of Beldarrain et al.12 indicates that thicker layers of Al2O3 desorb more H2 that can accumulate and be released, forming blisters in Al2O3. Vermang et al.13 concluded that blisters form under tensile stress. While in the work of Broas et al.14 covered with a bilayer of AlN-Al2O3 and concluded that the blisters are formed by two different types of mechanisms: compressive stress of AlN and diffusive stress of Al2O3. All three studies mention the presence of hydrogen in the blistering process. Vermang specifically discusses the desorption of H2 and H2O from the Si/Al2O3 interface and the Al2O3 bulk during annealing. Broas also highlights the role of hydrogen diffusion in the interface. Additionally, Yameng Bao and his research group revealed15 that thicker films offer more space for gas to be trapped, accelerating the appearance of blisters. On the other hand, there are cathodoluminescence (CL) studies of highly crystalline Al2O3 films, as in the case of Ghamnia et al.16 where they carry out CL studies at three different annealing temperatures: 1000, 1500, and 1700 °C.

While prior research has offered valuable insights into the factors influencing blistering in postdeposition annealing processes, there exists a notable gap in systematic studies of luminescent properties in films with blister packs. Our study bridges this divide, comprehensively exploring the material morphology and optical properties both within and outside the blisters. In this work, we employ cathodoluminescence and ATR-IR measurements. By combining these analyses with morphological observations, refractive index measurements, and transmittance spectra, we gain a more holistic understanding of how blistering affects the optical functionality of these films.

Thick films of Al2O3, measuring 1 μm in thickness, were deposited using the thermal atomic layer deposition technique (Beneq TFS-200 model). These films were grown on Si (100) substrates with 300 nm of thermally grown SiO2 (University WAFER ID 1432). It was decided to use a substrate with SiO2 to prevent the formation of Si-induced blisters.17 In the ALD process, we utilized trimethylaluminum (TMA-Strem Chemicals) as the precursor and de-ionized water as the oxidizing agent. The reactor was maintained at a constant temperature of 250 °C throughout the deposition process, with a working pressure inside the reactor of 1 mbar. Figure 1 visually represents the dosing and purging steps during the ALD process, where the growth rate per cycle (GPC) was 1.09 Å.

FIG. 1.

Schematic representation of the ALD cycle for Al2O3 film deposition, subsequent evolution during thermal treatments, and the phases of blister formation and rupture.

FIG. 1.

Schematic representation of the ALD cycle for Al2O3 film deposition, subsequent evolution during thermal treatments, and the phases of blister formation and rupture.

Close modal

Following the fabrication process, the Al2O3 films underwent a series of six thermal annealing temperatures: 550, 650, 750, 850, 950, and 1050 °C. Each treatment involved a gradual heating ramp of 10 °C per minute in an N2 atmosphere. Figure 1 also illustrates the sequence of thermal treatments and their relevance in our investigation of blister formation.

1. Scanning electron microscopy (SEM)

SEM images were acquired using a JEOL FIB-4500 system, with an electron beam accelerated to 15 keV. To minimize sample charging, a conductive silver paint contact was applied.

2. Spectroscopic ellipsometry

Spectroscopic ellipsometry measurements were performed using a J.A. Woollam Co. M-2000 ellipsometer. The ellipsometric parameters (Ψ and Δ) were collected over a wavelength range from 240 to 1100 nm, with an incidence angle of 75°. Thickness and refractive index values were determined by fitting these parameters to the Cauchy mathematical model. The simplicity of the Cauchy equation makes it very convenient, and it is the most widely used model for transparent dielectric materials.18 The Cauchy function is written as
n ( λ ) = A + B λ 2 + C λ 4 + ,
(1)
where parameter A sets the index amplitude, while B and C add curvature to produce normal dispersion. When the wavelength is limited to a small range, two terms (A and B) are often adequate. The C term helps to define the index curvature over a broader wavelength range.

3. Water contact angle measurements (WCA)

The wettability properties of the Al2O3 surface were measured using a lab-built goniometer. A controlled volume of 5 μl of de-ionized water was dropped onto the Al2O3 surface.

4. X-ray diffraction (XRD)

XRD analysis was conducted using a PANnalytical X’pert Pro MRD unit, utilizing an incidence angle of 1° and a resolution of 0.02° within the 20°–60° range.

5. Auger electron spectroscopy (AES)

Auger electron spectroscopy (AES) was performed using an XPS SPECS system, incorporating a PHOIBOS 150 WAL detector.

6. Cathodoluminescence (CL)

Cathodoluminescence spectra were recorded within the same SEM setup, operating in the panchromatic mode and employing a Gatan monochromator. The wavelength range was from 200 (6.2 eV) to 900 nm (1.37 eV), with magnifications of ×2000.

7. Attenuated total reflectance infrared spectroscopy (ATR-IR)

ATR-IR measurements were performed in the spectral range of 580–1000 cm−1 using a Bruker Tensor-27 instrument equipped with a VariGATR accessory. The measurements were taken at an angle of 65°, utilizing a germanium crystal.

Figure 2 provides SEM panoramic views of the Al2O3 film surface, captured at 100× magnification, both in its as-deposited state and after annealing in an N2 atmosphere for 1 h at varying temperatures. These images vividly illustrate the formation and progression of blisters. Initially, in Figs. 2(a) and 2(b), the SEM micrograph of the Al2O3 film deposited and annealed at 450 °C presents a homogeneous surface with no discernible alterations or irregularities.

FIG. 2.

SEM micrographs of the evolution of the surface morphology in Al2O3 films before and after thermal annealing in an N2 atmosphere. (a) As-deposited film; thermal annealing at different temperatures: (b) 450, (c) 550, (d) 650, (e) 750, (f) 850, (g) 950, and (h) 1050 °C; and (i) density of the blister as a function of diameter. The black squares represent the data from our work compared to others in the literature.

FIG. 2.

SEM micrographs of the evolution of the surface morphology in Al2O3 films before and after thermal annealing in an N2 atmosphere. (a) As-deposited film; thermal annealing at different temperatures: (b) 450, (c) 550, (d) 650, (e) 750, (f) 850, (g) 950, and (h) 1050 °C; and (i) density of the blister as a function of diameter. The black squares represent the data from our work compared to others in the literature.

Close modal

As the annealing temperature increases, a remarkable transformation occurs. At 550 °C [Fig. 2(c)], we begin to observe the inception of blister formation, albeit appearing as subtle reliefs in the film’s contour. Progressing to 650 °C [Fig. 2(d)], a greater number of blisters materialize, displaying well-defined contours. By 750 °C [Fig. 2(e)], the blisters’ definition intensifies, yet their diameter diminishes. It is at this juncture that we first notice instances of blister rupture.

After the stages of blister breakage, a pivotal occurrence transpires at 850 °C shown in Fig. 2(f). At this temperature, along with blister rupture, the film undergoes delamination and cracking, marking a crucial phase in its structural evolution. Advancing to 950 °C, the prevalence of surface delamination and blister formation reaches its zenith. Finally, at 1050 °C, the process culminates with the unequivocal rupture of the blisters and a substantial reduction in their diameter.

We have conducted a detailed analysis of the SEM micrographs to calculate the density of blisters for each film with ImageJ software. Furthermore, we determined the average diameter of the blisters at each annealing temperature. Our findings reveal a clear trend: as the annealing temperature increases, the density of the blister packs also rises, while, simultaneously, the average blister diameter decreases. Figure 2(i) clearly represents this correlation, with blister pack density plotted against their diameter. Our data (black squares) are plotted alongside density and diameter data from three other papers. In the research by Zhao et al., the films have a thickness of 30 nm (red circles); in the work of Vermang et al. also, the films have a thickness of 30 nm (green triangles); and finally, the results of Balderrain et al. show 400 cycles of ALD films with approximately 44 nm thickness (blue triangles). The trend remains clear: larger diameters have lower density, and smaller diameters have higher density. Simultaneously, SEM images reveal an escalating incidence of surface delamination, indicating that the reduction in the blister diameter is intricately linked to the stresses induced by recrystallization.

Figures 3(a) and 3(b) illustrate the ellipsometric parameters, Psi (Ψ) and delta (Δ), measured at an incidence angle of 75° for all annealed Al2O3 films. Initially, there is a similarity in the shape of measurements between the as-deposited samples and those annealed at 750 °C [Figs. 3(a) and 3(b)]. However, a significant shift in optical behavior becomes evident as we reach the annealing temperature of 850 °C, aligning with the emergence of blisters and delamination in the Al2O3 films.

FIG. 3.

Optical characterization of annealed Al2O3 films. (a) and (b) Ellipsometric parameters Psi (Ψ) and delta (Δ) measured at an incidence angle of 75° for films annealed at different temperatures. (c) Calculated refractive index showing a systematic increase with rising annealing temperatures in the N2 atmosphere.

FIG. 3.

Optical characterization of annealed Al2O3 films. (a) and (b) Ellipsometric parameters Psi (Ψ) and delta (Δ) measured at an incidence angle of 75° for films annealed at different temperatures. (c) Calculated refractive index showing a systematic increase with rising annealing temperatures in the N2 atmosphere.

Close modal

To determine the refractive index, a three-layer ellipsometric model was employed. This model encompasses the Si substrate, the 300 nm thermal SiO2 layer, and the Al2O3 layer of interest. Utilizing the dispersion equation of the Cauchy model, a standard choice for highly transparent materials like Al2O3, we obtained accurate fittings. Detailed information on thickness and the associated mean square error (MSE) resulting from fitting the ellipsometric measurements is provided in Table I. The solid lines in Figs. 3(a) and 3(b) represent the model’s fittings. It is important to note that for samples with blister formation and delamination within the Al2O3 layer (occurring between 850 and 1050 °C), the ellipsometric measurements deviated significantly from the model predictions, which led to a marked increase in error. Figure 3(c) illustrates the calculated refractive index, where a discernible and systematic increase in the refractive index is observed as the annealing temperature increases from the as-deposited to the annealed at 750 °C samples.

TABLE I.

Calculated thickness and mean square error (MSE) values of the ellipsometric model for Al2O3 films as-deposited and annealed in an N2 atmosphere at different temperatures.

SampleSiO2 thickness (nm)Al2O3 thickness (nm)MSE
As-deposited 308 1086 12.36 
450 °C 308 1080 11.81 
550 °C 309 1079 11.27 
650 °C 308 1076 11.43 
750 °C 308 1074 18.96 
850 °C — — — 
950 °C — — — 
1050 °C — — — 
SampleSiO2 thickness (nm)Al2O3 thickness (nm)MSE
As-deposited 308 1086 12.36 
450 °C 308 1080 11.81 
550 °C 309 1079 11.27 
650 °C 308 1076 11.43 
750 °C 308 1074 18.96 
850 °C — — — 
950 °C — — — 
1050 °C — — — 

In Fig. 4, the wettability characteristics of the annealed Al2O3 films are analyzed by the water contact angle (WCA). The observed behavior reveals that as the annealing temperature increases, the WCA also increases, transitioning from approximately 97° to 106°. This shift toward larger contact angles suggests a more hydrophobic surface. However, a noteworthy anomaly occurs in the Al2O3 film annealed at 1050 °C. Here, the contact angle experiences a decline. This observation may be attributed to the exceptional size of delamination and blisters at this temperature. In essence, the dimensions of these structural features become so substantial that they influence the behavior of water droplets. The droplet may be partially immersed by these structures, impacting the observed contact angle.

FIG. 4.

Water contact angle analysis on as-deposited and annealed Al2O3 ALD films; the high values >95° indicate a hydrophobic behavior.

FIG. 4.

Water contact angle analysis on as-deposited and annealed Al2O3 ALD films; the high values >95° indicate a hydrophobic behavior.

Close modal

Figure 5 depicts the x-ray diffractograms of the annealed Al2O3 films. The as-deposited Al2O3 film initially exhibits an amorphous structure without any discernible crystalline order. As the annealing temperature elevates, the crystallization of alumina is initiated, notably at annealing temperatures surpassing 750 °C.

FIG. 5.

X-ray diffractograms revealing structural evolution in annealed Al2O3 films. These diffractograms depict the transformation of Al2O3 films as a function of annealing temperature.

FIG. 5.

X-ray diffractograms revealing structural evolution in annealed Al2O3 films. These diffractograms depict the transformation of Al2O3 films as a function of annealing temperature.

Close modal
In the diffractograms within the temperature range of 850–1050 °C, the monoclinic θ-Al2O3 phase becomes identifiable, corroborated by reference to the crystallographic card ICSD-82504. An intense peak at the 67° angle is observed among the diffraction peaks, corresponding to the (512) plane. This particular peak was used to determine the crystallite size with the Scherrer equation, as stated as follows:
D = k λ β Cos θ ,
(2)
where k is the dimensional factor with a value of 0.94, β is the peak width, λ is the wavelength of the radiation, θ is the diffraction angle, and D is the crystallite size. The calculated crystallite sizes of the Al2O3 films at annealing temperatures of 850, 950, and 1050 °C were 12, 11.75, and 12.31 nm, respectively.

AES spectra are illustrated in Fig. 6. The spectra provide a detailed view of two distinct regions: the outer region highlighted in blue, and the inner region located within the blisters marked in red. In the outer zone, discernible spectral features include the OKLL signal, appearing around 500 eV, and the AlKLL signal at 1377 eV.19 These observations align with the expected for Al2O3, confirming the presence of aluminum oxide in this region. As we shift our focus to the inner zone within the blisters, while the OKLL peak persists, the SiKLL signal at 1602 eV appeared, which is consistent with the anticipated signals for SiO2 (the substrate material).

FIG. 6.

Auger electron spectra (AES) comparing outer Al2O3 (blue) and inner blister SiO2 regions (red).

FIG. 6.

Auger electron spectra (AES) comparing outer Al2O3 (blue) and inner blister SiO2 regions (red).

Close modal

1. Cathodoluminescence—Outside of blisters

Figure 7(a) presents the cathodoluminescence spectra for the annealed Al2O3 films. Initially, in the as-deposited Al2O3 film, the dominant emission peak centers at approximately 2.8 eV, with a secondary shoulder at 3.4 eV. As we increase the annealing temperature up to 750 °C, the shape of the cathodoluminescence spectra remains relatively stable. However, beyond this temperature, in Al2O3 films annealed at 850, 950, and 1050 °C, a significant shift occurs in the dominant emission, now centered around 3.4 eV. In addition, a shoulder appears above 4 eV. Simultaneously, there is a rapid and substantial intensification of the cathodoluminescence within this temperature range. This intensification is most pronounced in the Al2O3 film annealed at 1050 °C, where the emission intensity increases to 60 times the baseline level.

FIG. 7.

(a) Cathodoluminescence spectra outside the blister regions of annealed Al2O3 films. (b)–(d) SEM micrographs depicting specific regions of interest within Al2O3 films subjected to thermal treatments at 1050, 950, and 850 °C, respectively.

FIG. 7.

(a) Cathodoluminescence spectra outside the blister regions of annealed Al2O3 films. (b)–(d) SEM micrographs depicting specific regions of interest within Al2O3 films subjected to thermal treatments at 1050, 950, and 850 °C, respectively.

Close modal

These emissions have been associated with various defects in Al2O3, including oxygen vacancies that lead to the formation of what are known as F centers and F+ centers. These defects are characterized by the trapping of one or two electrons, respectively.16,20 It is worth noting that alternative investigations have proposed slightly different energy values for these emissions, 2.8 and 3.7 eV.13 

Another phenomenon is the agglomeration of F centers, leading to the formation of F2 centers. In this scenario, a vacancy is accompanied by three unpaired electrons.16,21 When excited, this defect generates two distinct emission peaks with energies of 4.1 and 2.4 eV. Furthermore, in the presence of F+ centers, the Al2O3 lattice can give rise to F2+ centers through clustering. These F2+ centers introduce their own emission peaks, resonating at energies of 3.4 and 3.2 eV. Some studies by Perevalov et al.22 report even lower emission energies, approximately 1.9 eV. These emissions may be linked to defects such as F2 centers or interstitial cations, further emphasizing the rich diversity of defect-related phenomena in Al2O3.

On the other hand, in Figs. 7(b)7(d), SEM micrographs depict the delaminated regions in Al2O3 films induced by annealing at temperatures of 850, 950, and 1050 °C. The regions correspond to the zones where the cathodoluminescence was taken. The edges of the delamination exhibit increased brightness in samples subjected to higher annealing temperatures.

2. Cathodoluminescence—Observing the blister regions

Figure 8 depicts the cathodoluminescence spectra collected from the blisters of annealed Al2O3 films, starting with their formation at 550 °C. The CL spectra reveal five distinctive components approximately at 1.9, 2.2, 2.7, 3.3, and 4.1 eV. The emission centered at 1.9 eV is linked to unbound oxygen within the SiO2 lattice.23 The peaks at 2.7 and 4.3 eV are associated with defects within the SiO2 lattice, resulting from the bonding of two silicon atoms and three oxygen atoms.24 Additionally, the emission at 2.2 eV is associated with oxygen vacancies in SiO2.25–27 

FIG. 8.

(a) Cathodoluminescence spectra obtained from the blister regions of annealed Al2O3 films. (b)–(d) SEM micrographs providing lateral views of the blister formation across annealing temperatures.

FIG. 8.

(a) Cathodoluminescence spectra obtained from the blister regions of annealed Al2O3 films. (b)–(d) SEM micrographs providing lateral views of the blister formation across annealing temperatures.

Close modal

On the other hand, Figs. 8(b)8(d) complement the CL spectra by offering visual insights into the blister regions at different stages. These SEM micrographs provide a lateral view of the formation of blisters at temperatures ranging from 550 to 650 °C, their subsequent bursting between 750 and 850 °C, and finally, the combined effects of bursting and delamination at the highest annealing temperatures of 950 and 1050 °C.

This section explores the ATR-IR spectra of the annealed Al2O3 films, as depicted in Fig. 9. Initially, the as-deposited film exhibits mainly three absorption peaks centered at approximately 700 , 780, and 870 cm−1. These peaks correspond to characteristic vibration modes associated with amorphous Al2O3.28 Upon subjecting the films to a moderate annealing temperature of 450 °C, the ATR-IR spectrum displays minimal alterations. However, as we progress to an annealing temperature of 550 °C, coinciding with the formation of blisters, a notable shift in the maximum absorption is observed, now centered at 870 cm−1.

FIG. 9.

ATR-IR spectra of Al2O3 ALD films as a function of annealing temperature.

FIG. 9.

ATR-IR spectra of Al2O3 ALD films as a function of annealing temperature.

Close modal

Following annealing at 550 °C, the primary absorption peaks initially centered at 700 and 780 cm−1 shifted to 600 and 650 cm−1, respectively. Additionally, new absorption bands at 1260 and 1645 cm−1 are formed. At temperatures of 650 and 750 °C, the spectra have similar shapes, with all the indicated peaks forming them. Finally, as the heat treatment temperature escalates to 850 °C, the intensities of the peaks at 1260 and 1645 cm−1 align with the stage at which blister rupture occurs. This spectrum is maintained in the film heat-treated at 950 °C.

The morphological changes in the annealed Al2O3 ALD films observed in Fig. 2, showing blistering, were accompanied by complex structural, optical, and chemical transformations. Previous research by Vermang et al. highlights the significance of hydrogen and water desorption during annealing as essential precursors to blister formation.13 

In works where there are no chemical residues, blistering is not reported. Singh and Shivashankar’s29 research highlighted that as-grown Al2O3 films are crack-free, but postdeposition annealing induces railway tracklike cracks and film delamination due to internal stress. However, this study did not address blister formation. Edlmayr et al.30 investigated the microstructure evolution of magnetron-sputtered alumina coatings after thermal exposure. While structural changes were emphasized, blister formation was not mentioned in Al2O3 films. Larsson et al.31 work on thick Al2O3 films deposited by CVD, subjected to TEM study and thermal treatment, observed dislocations and gaps but made no reference to blister formation. Johnson32 article on noncrystalline Al2O3 from remote plasma-enhanced chemical vapor deposition highlighted Al atom bonding and a negative fixed charge at the interface with SiO2. Despite insights into Al2O3 properties, blister formation was not addressed. This literature review suggests that blister pack formation is associated with deposition residues, and in cases mentioned where the chemical remnants do not exist, blisters neither exist.

Our study reveals a critical observation, as blisters in the Al2O3 films begin to evolve at annealing temperatures exceeding 550 °C, contrasting with findings by Beldarrain et al. at 450 °C and Vermang et al. at 350 °C.13,17 The hydrogen content in the film originates from residual OH groups on the surface, which accumulate cycle by cycle due to unsaturated reactions. Then, the diffusion of hydrogen atoms and water molecules during annealing results in their agglomeration, leading to the formation of gas. The segregation of these gases within the film ultimately culminated in blister formation. Furthermore, the pressure within these entrapped gas pockets plays a critical role in film delamination.33 In Xie’s study, blister formation on an Al2O3 film grown on an Al substrate led to Al atom diffusion and surface delamination.34 

In addition, to compare the density of the blisters as a function of diameter at different annealing temperatures, Fig. 2(i) was made. In all cases, as the annealing temperature increased, the density of the blisters increased. The diameter size has a similar behavior between works no matter the thickness all follows the same trend, it increases and then decreases at higher temperatures.

The optical analysis depicted in Fig. 3 showed an increase in the refractive index of the Al2O3 films with increasing annealing temperatures. This phenomenon can be attributed to the densification of the Al2O3 films and a concurrent decrease in the hydrogen content within the film. This aligns with the findings of Ottermann and Bange,35 who investigated the correlation between the H2 content and refractive index in thin films, establishing that higher H2 content correlates with lower film density and, consequently, a reduced refractive index. On the other hand, where annealing does not lead to a significant reduction in the thickness of the Al2O3 film, it can be inferred that the densification manifested mainly laterally. This lateral densification finally culminates in the delamination of the film. We also relate the refractive index to the wettability of the films. In all of them, we find angles with values >95, making them highly hydrophobic. This behavior agrees with Al2O3 coatings on SiO2 reported by Kumar et al.36 

AES investigations confirmed changes in the chemical properties of the films, both within and outside blisters. As depicted in Fig. 6, the absence of the deposited film inside blisters, as evidenced by the Auger spectra, serves as a definitive confirmation of the film’s complete delamination upon blister rupture. In the work of Ghamnia et al.21 on highly crystalline Al2O3, AES results and luminescence results have been related. In our results, we see the changes produced by thermal treatments.

Cathodoluminescence measurements conducted outside the blister regions provide insights into how the film’s structure changes during thermal treatments. Initially, the spectrum of the as-deposited film shows strong peaks at 2.8 and 3.4 eV. Some studies have noted fainter luminescence around 2.4 eV, attributed to defects in the atomic structure (known as F2 centers) or aluminum atoms in interstitial positions. Our work’s emission at 3.7 eV (330 nm) evolves, becoming relevant at higher temperatures. In Ghamnia’s work from 2003,16 where they make CL measurements in Al2O3 films with annealing temperatures at 1000, 1500, and 1700 °C, they observe that this emission is the dominant one in all cases, with 1000 °C being where the most intensity is observed. of this band due to the presence of F+ center or oxygen vacancies trapping one electron. As we increase the temperature during thermal treatment, the spectral pattern shifts. It starts to resemble what Demol et al.37 previously observed, with the main emissions at 3.9 and 4.1 eV. Interestingly, the intensity of these spectra significantly amplifies; for the film treated at 1050 °C, the intensity increases by a factor of up to 60 times. Interestingly, the intensity of these spectra is significantly amplified; for the film treated at 1050 °C, the intensity increases up to 60 times, unlike Ghamnia’s work where at higher temperatures, the emission decreases.

Finally, we observed an increase in the CL intensity in the blister areas, at annealing temperatures starting at 550 °C, which mainly capture the signal from the substrate. Before breaking, the film in these areas is less dense, and once broken, there is no trace of Al2O3 left as was observed by AES. Furthermore, we observed that the delamination was sufficiently intense from 950 °C to be able to perform wettability measurements.

Infrared spectroscopy, as illustrated in Fig. 8, offers valuable insights into the evolving chemical composition of the Al2O3 films during the annealing process. The appearance of new absorption bands beyond 550 °C, at 1250 and 1645 cm−1, provides evidence points to an interplay between crystalline evolution and chemical transformations. It suggests that the structural changes observed through x-ray diffraction, where crystallite sizes increased with elevated annealing temperatures, are linked. The emergence of distinct absorption bands in the infrared spectra highlights the formation of chemical bonds or molecular configurations because of thermal annealing.

  • In the investigation of blister formation on Al2O3 films, it is noteworthy that our study encompasses a broad temperature range, ranging from 450 to 1050 °C. While blistering has been a subject of extensive research in the literature, our work fills a crucial gap of the optical characterization outside and inside the blisters.

  • Our SEM analysis (Fig. 2) unveiled a remarkable evolution in the surface morphology of annealed Al2O3 films. Initially, the films appeared homogeneous, but as the annealing temperature increased, we observed the emergence of blisters and, at higher temperatures, pronounced delamination. The trend of density as a function of diameter in our work and in others in the literature shows a reciprocal correlation between both values.

  • Utilizing ellipsometry (Fig. 3), we uncovered a systematic increase in the refractive index of the Al2O3 films with rising annealing temperatures. This phenomenon correlated with densification and a reduction in the hydrogen content within the films. The findings aligned with previous research, emphasizing the intricate relationship between the hydrogen content, film density, and refractive index. Importantly, in cases where the film thickness remained relatively constant, lateral densification was identified as a key contributor to film delamination.

  • AES investigations (Fig. 6) confirmed changes in the chemical properties of the films, both within and outside blisters. The absence of the deposited film inside blisters provided unequivocal evidence of complete delamination upon blister rupture.

  • The cathodoluminescence analysis (Figs. 7 and 8) revealed intriguing emission spectrum changes correlated with blister formation. The dominant emission shifted, and intensity substantially increased with higher annealing temperatures. These emissions were associated with various defects in Al2O3. These emissions were associated with defects in Al2O3 including the F+ and F2 centers.

  • Infrared spectroscopy (Fig. 9) provided valuable insights into the chemical changes occurring during annealing. New absorption bands, particularly beyond 550 °C, signaled the formation of chemical bonds or molecular configurations, aligning with structural changes observed through x-ray diffraction. These observations highlighted the intricate interplay between crystalline evolution and chemical transformations.

This work was supported by FORDECYT (Grant Nos. 272894 and 21077), CONACyT (Grant Nos. A1-S-21084, A1-S-26789, and A1-S-21323), DGAPA-UNAM (Grant Nos. IN103220, IN108821, and IN119023), and SENER-CONACyT (Grant No. 117373). The authors acknowledge CONACyT for the scholarship for postgraduate studies (No. 613752). The authors would like to acknowledge the valuable technical support of J.A. Díaz, E. Iniguez, E. Aparicio, L. Arce, E. Murillo, I. Gradilla, F. Ruiz, and Jaime Mendoza.

The authors have no conflicts to disclose.

Carolina Bohórquez: Conceptualization (equal); Investigation (equal); Writing – original draft (equal); Writing – review & editing (equal). Jorge L. Vazquez: Investigation (equal); Writing – review & editing (equal). Luis E. López: Methodology (equal); Writing – review & editing (equal). Jorge A. Jurado: Methodology (equal); Writing – review & editing (equal). David Domínguez: Investigation (equal); Writing – review & editing (equal). Oscar E. Contreras: Funding acquisition (equal); Writing – review & editing (equal). Hugo J. Tiznado: Funding acquisition (equal); Writing – review & editing (equal).

The data that support the findings of this study are available from the corresponding author upon reasonable request.

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