Method for forming gap between gallium nitride epitaxial layer and sapphire substrate for laser lift-off

Abstract

The invention provides a method for forming a gap between a gallium nitride epitaxial layer and a sapphire substrate for laser lift-off. A controllable gap structure is formed on an interface of GaN and the sapphire substrate. The gap structure can be formed by regulating epitaxial process parameters on a patterned sapphire substrate, or depositing a dielectric material layer on a planar sapphire substrate, forming a pattern structure in different states through wet etching, dry etching or a photoetching process, and then carrying out GaN epitaxial growth. In the laser lift-off process, the gap serves as a dissipation path of gas generated by epitaxial decomposition, gas impact force is reduced, laser focusing depth difference is improved, epitaxial layer damage is reduced, a sharp or conical rough structure is formed after lift-off, and the light extraction efficiency of an LED chip and the yield of the lift-off process are remarkably improved. The method has the advantages of process flexibility, high epitaxial layer quality, good stripping stability and the like, and is suitable for manufacturing high-power GaN LED chips.

Description

Technical Field

[0001] The invention belongs to the technical field of semiconductor device manufacturing, and specifically relates to a method for forming a gap between a gallium nitride epitaxial layer and a sapphire substrate in an epitaxial growth manner to optimize the laser lift-off yield. Background Art

[0002] GaN-based light-emitting diodes (LEDs), as the mainstream of blue-green LEDs in the market, are widely used in display and lighting fields. In high-power LED applications, traditional GaN-based LEDs mostly use sapphire substrates as the substrate for epitaxial growth. However, due to the low thermal conductivity (about 35W / m·K) and electrical insulation of sapphire substrates, their heat dissipation effect in high-power applications is limited.

[0003] In order to overcome the problem of insufficient heat dissipation performance of traditional GaN-based LEDs in high-power applications due to the low thermal conductivity and electrical insulation of sapphire substrates, laser lift-off (LLO) technology has been widely used. This technology peels the GaN epitaxial layer from the sapphire substrate and transfers it to a metal substrate, ceramic substrate or silicon substrate with better thermal conductivity to improve heat dissipation and enhance the performance of LED devices. However, in the past 15 years, epitaxial growth of patterned sapphire substrates (PSS) in LED chip manufacturing has been widely used. The surface of this substrate has a micron-level height difference structure (usually 1-3um). The patterned height difference forms a surface light scattering effect. While improving the brightness and light extraction efficiency of the LED, it also brings about the problem of focusing depth at the interface between the patterned substrate and the epitaxial layer, resulting in uneven laser peeling that affects the peeling yield, and gas impact during the peeling process that causes damage to the epitaxial layer and other derivative adverse effects.

[0004] In the Metal Organic Vapor Chemical Deposition (MOCVD) process, whether the GaN epitaxial layer is grown on a traditional planar sapphire substrate or a patterned sapphire substrate, by adjusting the 5:3 ratio of the epitaxial gas and the epitaxial growth mode (3D and 2D growth mode), the GaN and sapphire substrate interface can be completely bonded (such as Figure 1 However, during the LLO stripping process, the decomposition reaction of GaN at the interface will produce nitrogen, which will form a gas pressure shock at the moment of stripping, which can easily cause damage or cracking of the epitaxial layer. In addition, Figure 2As shown, compared with the planar sapphire substrate, the pattern height difference structure of the PSS substrate causes a deviation in the focusing depth of the laser energy at the GaN / PSS interface. The laser does not act on the same plane, resulting in power differences, which further affects the stripping effect of the GaN epitaxial layer.

[0005] In the prior art, for example, patent document "CN115799404A", by introducing an air cavity structure inside the epitaxial layer, the stress impact during laser stripping is reduced. However, this technology fails to effectively solve the problem of uneven stripping effect caused by the difference in laser focus depth due to the high-low structure of the patterned sapphire substrate, and the surface quality of the epitaxial layer still has defects and stripping yield problems.

[0006] Therefore, how to reduce the difference in laser energy focusing depth on patterned sapphire substrates and flat sapphire substrates (including substrates with dielectric material graphic structures), optimize the laser lift-off process to improve the lift-off yield, reduce the damage to the epitaxial layer caused by gas impact, and at the same time improve the light extraction efficiency of the LED chip after lift-off has become a technical problem that needs to be urgently solved in the field of gallium nitride laser lift-off. Summary of the invention

[0007] The purpose of the present invention is to provide a method for forming a gap between a gallium nitride epitaxial layer and a sapphire substrate, wherein a controllable gap structure is formed at the interface between the GaN epitaxial layer and the sapphire substrate. The gap structure serves as a dissipation path for the gas generated after the cracking of GaN during the laser lift-off (LLO) process, reduces the gas impact force during the laser lift-off process, avoids damage to the epitaxy, and can reduce the difference in laser focus depth, improve the laser lift-off yield, and the gap structure at the interface optimizes the surface morphology of the GaN epitaxial layer after lift-off, further coarsens the naturally generated patterning, and improves the light extraction efficiency (LEE) of the LED chip and the yield of the lift-off process.

[0008] To achieve the above object, the present invention provides the following technical solutions:

[0009] A method for forming a gap structure between a gallium nitride (GaN) epitaxial layer and a sapphire substrate comprises the following steps:

[0010] Gallium nitride (GaN) epitaxial layer growth on sapphire substrate;

[0011] By controlling process parameters or structural processing, a controllable void structure is formed at the interface between the GaN epitaxial layer and the sapphire substrate, wherein the void structure has a regular shape, uniform distribution and controllable proportion;

[0012] The void structure serves as a gas escape path during the laser lift-off (LLO) process, reduces the gas impact force during the laser lift-off process, reduces the laser focusing depth difference, and optimizes the surface morphology of the GaN epitaxial layer after lift-off, thereby improving the light extraction efficiency (LEE) of the LED chip and the yield of the lift-off process.

[0013] Optionally, the void structure is formed by epitaxially growing GaN on a patterned sapphire substrate (PSS) and by regulating epitaxial growth process parameters.

[0014] Optionally, the void structure is formed by depositing a dielectric material layer on a planar sapphire substrate, forming a patterned structure by wet etching, dry etching or photolithography, and then performing epitaxial growth of gallium nitride on the patterned structure; in the present invention, the material of the dielectric material layer includes aluminum nitride (AlN), silicon oxide (SiOx), silicon nitride (SiNx) and titanium oxide (TiOx). Wherein, x represents the stoichiometric ratio of oxygen or nitrogen to silicon or titanium, and its specific value depends on the deposition process and process parameters, such as temperature, gas flow rate, deposition rate, etc. Common ratio ranges include SiOx (1≤x≤2), SiNx (0.5≤x≤1.3) and TiOx (1≤x≤2). The thickness of the dielectric material layer is 100nm-2μm.

[0015] The morphology of the void structure includes dot-shaped, grid-shaped, hemispherical or trapezoidal.

[0016] The void structure accounts for 10%–35%, has a depth of 0.5–2 μm, and a width of 1–3 μm.

[0017] The epitaxial growth process parameters include:

[0018] Gas flow: N 2 / H 2 / NH 3 =50–55 / 0–5 / 60–70slm;

[0019] V / III ratio: 1500–2000;

[0020] Epitaxial growth temperature: 1000–1100°C;

[0021] Epitaxial growth mode: initial 3D rapid growth → later 2D smooth coverage.

[0022] The laser parameters of the laser lift-off process include:

[0023] Laser wavelength: 193nm, 248nm or 355nm;

[0024] Laser energy density: 0.3–0.5 J / cm 2 ;

[0025] Laser focusing depth: Shallow, can be precisely controlled at the interface between GaN and sapphire substrate.

[0026] A pointed or conical rough structure is formed on the surface of the stripped GaN epitaxial layer.

[0027] The void structure formed by adjusting the process conditions not only affects the effect of laser lift-off, but also affects the uniformity of the epitaxial growth quality. Therefore, it must be evenly distributed and fixed in shape to avoid different void shapes and dense or sparse distribution. The laser lift-off process can reduce microcracks and local cracking of the epitaxial layer, and improve the stability and consistency of the lift-off yield.

[0028] The core principle of the present invention is to optimize the laser lift-off process by forming a controllable gap structure between the GaN epitaxial layer and the sapphire substrate during the epitaxial growth stage.

[0029] (1) Void formation mechanism

[0030] ① Forming gaps on patterned sapphire substrate (PSS)

[0031] The surface of the patterned sapphire substrate has regular micron-scale structures (such as cones, hemispheres, etc.).

[0032] During the MOCVD epitaxial growth process, voids are formed at the interface between the GaN epitaxial layer and the PSS substrate through adjustable process parameters (such as V / III ratio, growth rate, gas flow rate, and temperature).

[0033] The formation, shape and size of the voids are determined by the conversion and adjustment of the 3D→2D growth mode. When the epitaxial layer does not completely and tightly cover the PSS graphic structure, some voids are retained.

[0034] ② Forming gaps on a flat sapphire substrate

[0035] Deposit single or stacked dielectric material layers (such as AlN, SiOx, SiNx, TiOx) on a planar sapphire substrate.

[0036] A graphic structure (such as a dot matrix, a grid, a hemisphere, or a trapezoid) is formed on the dielectric material layer by wet etching, dry etching, or photolithography.

[0037] During the MOCVD epitaxial growth process, the epitaxial layer forms regular gaps around or on top of the graphic structure.

[0038] (2) Characteristics of the gap

[0039] Gap shapes: point, grid, hemispherical, trapezoidal.

[0040] Void percentage: 10–35%.

[0041] Void dimensions: depth 0.5–2 μm, width 1–3 μm.

[0042] Distribution characteristics: The voids are uniform in shape and evenly distributed, avoiding excessive concentration or sparseness.

[0043] The presence of these gaps has the following effects:

[0044] Provides a gas escape path to reduce nitrogen shock during laser lift-off.

[0045] Reduce the difference in laser focus depth at different heights so that the laser energy acts more evenly on the peeling interface.

[0046] (3) Mechanism of action of laser lift-off process

[0047] ① Improvement of laser focus depth difference

[0048] In the presence of gaps, the laser energy can be focused in a smaller range on the interface between GaN and the sapphire substrate, reducing the focusing deviation and energy unevenness of the laser on the high-low structure.

[0049] ②Relief of gas impact force

[0050] During the laser stripping process, according to the chemical reaction formula, GaN absorbs laser energy at the interface and decomposes to produce nitrogen.

[0051] The void structure serves as a dissipation path for nitrogen gas after it is generated, allowing the gas to be quickly released after it is generated at the interface, reducing instantaneous impact force and reducing cracking or damage to the epitaxial layer during laser stripping.

[0052] ③Surface morphology optimization

[0053] The surface of the epitaxial layer after peeling can naturally form a further rough pointed or conical structure due to the void structure of the interface. This rough structure helps to extract light and improve the light extraction efficiency (LEE) of the LED chip.

[0054] Compared with the prior art, the present invention has the following beneficial effects:

[0055] (1) Improve laser lift-off yield:

[0056] By forming gaps at the GaN / PSS interface, the damage to the epitaxial layer caused by nitrogen impact during the stripping process is effectively reduced;

[0057] The laser energy acts more effectively on the GaN / PSS stripping interface, and the stripping process is more stable;

[0058] (2) Improve light extraction efficiency (LEE):

[0059] After peeling, the epitaxial layer naturally forms a pointed or conical rough surface, which effectively improves the light extraction efficiency of the LED chip;

[0060] (3) Enhanced process flexibility:

[0061] By adjusting the MOCVD epitaxial growth parameters, fine control of different void morphologies and proportions can be achieved;

[0062] The stripping effect can also be further optimized by introducing single layers or stacks of different dielectric materials to form patterns. BRIEF DESCRIPTION OF THE DRAWINGS

[0063] Figure 1 : Schematic diagram of the structure for growing GaN epitaxial layers on planar sapphire and patterned sapphire substrates.

[0064] Figure 2 : Schematic diagram of planar sapphire and PSS laser energy distribution in LLO process.

[0065] Figure 3 : Schematic diagram of the cross section of the gap formed between GaN and PSS substrate by adjusting MOCVD parameters during epitaxial growth.

[0066] Figure 4 : Schematic diagram of the structure in which the voids serve as gas escape paths in the LLO process.

[0067] Figure 5 : Schematic diagram of the formation of a pointed or conical rough surface on the GaN epitaxial layer after laser lift-off.

[0068] Figure 6 : Schematic diagram of the change of void morphology under different epitaxial growth parameters.

[0069] Figure 7 : Schematic diagram of introducing a dielectric material layer on a sapphire substrate and forming a patterned structure.

[0070] Figure 8 : Schematic diagram of the structure in which the rough surface of the GaN epitaxial layer is formed under the guidance of dielectric materials after laser lift-off.

[0071] Fig. 9 : Schematic diagram of different void proportions and morphologies.

[0072] Fig.10 : Schematic diagram of the diversity of dielectric material graphic morphologies (such as hemispherical, trapezoidal, and square).

[0073] Fig.11 : Scanning electron microscope image of patterned aluminum nitride on sapphire.

[0074] Fig.12 : Comparison of brightness and light extraction efficiency of LED chips produced by patterning dielectric materials on planar sapphire and MOCVD epitaxial growth on traditional planar sapphire. DETAILED DESCRIPTION

[0075] The following is a specific implementation of the present invention, and the technical solution of the present invention is described in detail in conjunction with the accompanying drawings so that those skilled in the art can clearly understand and implement it.

[0076] Example 1: Epitaxial growth control based on patterned sapphire substrate

[0077] 1. Substrate Preparation

[0078] A patterned sapphire substrate (PSS) 100 is selected, and its surface has a micron-scale conical cross-section pattern, the pattern size is 2 μm, and the pattern density is 300 per square millimeter.

[0079] 2. Epitaxial Growth

[0080] The GaN epitaxial layer 102 is grown on the PSS substrate using a MOCVD process.

[0081] Process parameters:

[0082] Gas flow: N 2 / H 2 / NH 3 =52 / 0 / 60slm

[0083] V / III: 1800

[0084] Growth temperature: 1050℃

[0085] Mode: 3D fast island growth → 2D smooth film growth

[0086] A regular conical void is formed at the interface between the GaN epitaxial layer and the PSS, and the void 101 accounts for about 25%. Figure 3 As shown; the gaps serve as gas escape paths as shown Figure 4 shown.

[0087] 3. Laser peeling

[0088] The peeling was performed using a 248 nm UV laser with a laser energy density of at least 0.3 J / cm 2 After laser stripping, the GaN epitaxial layer forms a sharp or conical rough surface. Figure 5 shown.

[0089] The laser acts on the interface between GaN and PSS. GaN absorbs the laser energy and decomposes. The gap can serve as a gas escape path to reduce gas impact.

[0090] 4. Peel off the rear surface

[0091] The surface of the epitaxial layer forms a PSS pattern and gaps. Figure 5 The regular pointed or conical roughness structure shown increases the surface roughness and also increases the surface area, which can improve the light extraction efficiency (LEE).

[0092] Example 2: Based on planar sapphire substrate + dielectric material pattern

[0093] 1. Substrate Preparation

[0094] like Figure 7 As shown, a planar sapphire substrate 200 is selected, and an AlN dielectric material layer is deposited on the surface with a thickness of 200 nm.

[0095] The dielectric layer is formed into a hemispherical pattern 201 by using a photolithography process, with a pattern size of 1.5 μm and a density of 400 per square millimeter.

[0096] 2. Epitaxial Growth

[0097] On the patterned substrate, GaN epitaxy 203 is grown using a MOCVD process.

[0098] Process parameters:

[0099] Gas flow: N 2 / H 3 / NH 3 =50 / 5 / 70slm (standard liters per minute)

[0100] V / III: 1600

[0101] Growth temperature: 1080℃

[0102] Mode: 3D fast island growth → 2D smooth film growth

[0103] A gap 202 is formed between the GaN and the sapphire substrate, accounting for about 20%.

[0104] Fig.11 Shown is a scanning electron microscope image of patterned aluminum nitride on sapphire, where hemispherical protrusions are clearly visible.

[0105] 3. Laser peeling

[0106] The peeling was performed using a 248 nm UV laser with a laser energy density of at least 0.3 J / cm 2 above.

[0107] 4. Peel off the rear surface

[0108] The surface of the epitaxial layer is formed by the dielectric material layer pattern and the gap. Figure 8 The regular grid-like rough structure shown increases the surface area and thus improves the light extraction efficiency.

[0109] Example 3: Control of the gap morphology based on patterned sapphire substrate

[0110] 1. Substrate Preparation

[0111] A patterned sapphire substrate was used, and the pattern morphology was a trapezoidal cross-section, with a size of 2 μm and a density of 200 per square millimeter.

[0112] 2. Epitaxial Growth

[0113] Process parameters:

[0114] Gas flow: N 2 / H 2 / NH 3 =55 / 0 / 60slm (standard liters per minute)

[0115] V / III: 1700

[0116] Growth temperature: 1020℃

[0117] Mode: 3D fast island growth → 2D smooth film growth

[0118] The void morphology can be determined by the trapezoidal cross-sectional pattern and epitaxial growth parameters, accounting for about 30%.

[0119] 3. Laser peeling

[0120] The peeling was performed using a 248 nm UV laser with a laser energy density of at least 0.3 J / cm 2 above.

[0121] 4. Peel off the rear surface

[0122] The surface of the epitaxial layer presents a further rough structure due to the original PSS pattern and different void patterns, thereby improving the light extraction efficiency.

[0123] By adjusting the epitaxial growth parameters, the void ratio and void shape can be controlled. Figure 6 shown.

[0124] Example 4: Gap ratio control based on planar sapphire substrate + dielectric material pattern

[0125] By adopting the technical solution of Example 2, a gap is left at the interface between GaN and sapphire due to the blocking of the dielectric material pattern during epitaxial growth. In addition to improving the LLO yield, based on the light extraction effect on the surface of the GaN epitaxial layer after stripping, the gap ratio and gap shape at the interface between GaN and sapphire can also be appropriately changed by adjusting the MOCVD epitaxial growth parameters. Based on this concept, other examples of embodiments are described, such as Fig. 9 In addition to adjusting the MOCVD epitaxial growth parameters to change the gap ratio and gap shape at the interface between the epitaxial layer GaN and the sapphire, the different process methods or parameters can also be changed to define the dielectric material layer into different shapes by wet etching / dry etching / simple photolithography, for example: hemispherical, trapezoidal, square; or the dielectric material itself is a composite laminated form. Other examples of embodiments are as follows, Fig.10 shown.

[0126] Example 5: Light extraction efficiency verification

[0127] The brightness and light extraction efficiency of the LED chip grown by MOCVD epitaxial growth on the flat sapphire and the conventional flat sapphire were measured. Fig.12 As shown:

[0128] The luminous intensity of Patterned AlN LED is significantly higher than that of Planar LED, indicating that the AlN graphic structure improves the light extraction efficiency.

[0129] Spectral peak (WLP):

[0130] Planar LED: 467.8nm

[0131] Patterned AlN LED: 459.3nm

[0132] This indicates that the emission wavelength of Patterned AlN LED is slightly blue.

[0133] Full Width at Half Maximum (FWHM):

[0134] Planar LED: 22.6nm

[0135] Patterned AlN LED: 21nm

[0136] The FWHM of the Patterned AlN LED is narrower, indicating that its light emission is more concentrated and the crystal quality of the epitaxial layer is higher.

[0137] parameter Planar LED Patterned AlN LED WLP (peak wavelength) 467.8nm 459.3nm FWHM (Full Width at Half Maximum) 22.6nm 21nm

[0138] Conclusion Summary:

[0139] Compared with planar LED, patterned AlN LED has the following advantages:

[0140] Higher EL intensity.

[0141] Shorter emission wavelength (more blue).

[0142] Narrower FWHM (light emission is more concentrated).

[0143] Through the different process paths and gap structure control methods of the above embodiments, the present invention achieves the following significant technical effects:

[0144] Laser lift-off yield is significantly improved:

[0145] By forming gaps at the GaN / PSS interface or between dielectric material layers, the focusing depth deviation of the laser energy is effectively reduced, thereby reducing the difficulty of stripping. The impact force generated by the decomposition of nitrogen during laser stripping is also reduced, thereby preventing the epitaxial layer from being damaged due to local high energy concentration.

[0146] Different process paths (wet etching, dry etching, and photolithography) provide a variety of void formation methods and void patterns to ensure the stability and consistency of the stripping process.

[0147] Reduced laser focus depth variation:

[0148] The introduction of a void structure at the interface allows the epitaxial GaN to grow on the interface of the PSS substrate without being completely tightly fitted, thus reducing the difference in laser focus depth caused by the height difference of the PSS substrate pattern, making the GaN epitaxial layer easier to peel off.

[0149] The focal depth range of the laser energy acting on the GaN / PSS substrate interface becomes smaller, the effect of the laser energy is more uniform and effective, and the overall stability of laser lift-off is improved.

[0150] Optimization of epitaxial layer surface quality:

[0151] The PSS substrate and epitaxial layer have a void structure, and the surface of the peeled GaN epitaxial layer naturally forms a pointed or conical rough structure with a void pattern, which effectively reduces the total reflection of the interface, provides a more optimized light extraction efficiency for the LED chip, and reduces the internal loss of light.

[0152] Light extraction efficiency (LEE) is significantly improved:

[0153] The epitaxial layer with a void structure retains the original void pattern on the surface after laser peeling, forming a surface roughening effect, which effectively increases the scattering of light and the light extraction path, and significantly improves the light extraction efficiency (LEE) of the LED chip.

[0154] The optical performance has been significantly improved, making it suitable for high-brightness, high-efficiency LED application scenarios.

[0155] Enhanced process flexibility and applicability:

[0156] The present invention provides a variety of process paths (wet etching, dry etching, photolithography, composite dielectric material stacking, etc.), which can be selected and adjusted according to different substrate types (PSS sapphire substrate or planar sapphire substrate) and application requirements.

[0157] Epitaxial growth related parameters (gas flow, temperature, 5:3 ratio, 3D / 2D growth mode) can be flexibly adjusted to ensure the best stripping effect under different process conditions.

[0158] High feasibility of industrialization:

[0159] The processes used (such as MOCVD epitaxial growth equipment, wet etching, dry etching, and photolithography) are all current semiconductor manufacturing equipment and technologies in the industry, and are easy to implement on the existing mature industrial chain.

Claims

1. A method for forming a gap structure between a gallium nitride (GaN) epitaxial layer and a sapphire substrate, characterized in that: The following steps are involved: Gallium nitride (GaN) epitaxial layer growth on sapphire substrate; By controlling process parameters or structural processing, a controllable void structure is formed at the interface between the GaN epitaxial layer and the sapphire substrate, wherein the void structure has a regular shape or different directions, a fixed distribution state and a controllable proportion; During the laser lift-off (LLO) process, the void structure serves as an escape path for the gas generated after GaN cracking, thereby reducing the gas impact force during the laser lift-off process, avoiding damage to the epitaxy, and reducing the difference in laser focusing depth, thereby improving the laser lift-off yield. The void structure of the interface optimizes the surface morphology of the GaN epitaxial layer after lift-off, further coarsening the naturally generated patterning, thereby improving the light extraction efficiency (LEE) of the LED chip and the yield of the lift-off process.

2. The method according to claim 1, characterized in that The void structure is formed by epitaxially growing gallium nitride on a patterned sapphire substrate (PSS) and by regulating epitaxial growth process parameters.

3. The method according to claim 1, characterized in that The void structure is formed by depositing a dielectric material layer on a planar sapphire substrate, forming a patterned structure by wet etching, dry etching or photolithography, and then regulating epitaxial growth process parameters to perform gallium nitride epitaxial growth on the patterned structure.

4. The method according to claim 3, characterized in that The cross-sectional shapes of the graphic structure include hemispherical, trapezoidal and square.

5. The method according to claim 1, characterized in that: The void structure accounts for 10%–35%, has a depth of 0.5–2 μm, and a width of 1–3 μm.

6. The method according to claim 1, characterized in that The epitaxial growth process parameters include: Gas flow: N2 / H2 / NH3=50–55 / 0–5 / 60–70slm (standard liters per minute); V / III ratio: 1500–2000; Epitaxial growth temperature: 1000–1100°C; Epitaxial growth mode: initial 3D island-like rapid growth → later 2D film layer flat coverage.

7. The method according to claim 1, characterized in that The laser parameters of the laser lift-off process include: Laser wavelength: ArF 193nm, KrF 248nm or He:Cd 355nm; Laser energy density: 0.3–0.5 J / cm 2 ; Laser focus depth: precisely controlled at the interface between GaN and sapphire substrate.

8. The method according to claim 1, characterized in that The surface of the stripped GaN epitaxial layer forms a pointed or conical rough structure with a pattern.

9. The method according to claim 3, characterized in that: The material selection of the dielectric material layer includes: a single material AlN, SiNx, SiOx, TiOx single layer, or a composite material SiOx / TiOx, SiNx / SiOx stacked layer, with a thickness of 100nm-2μm.

Images