Micro-LED Chip Structure and Its Fabrication Method

Abstract

The present invention discloses a Micro-LED chip structure. The chip structure includes a light-transmitting substrate, an epitaxial layer formed on the light-transmitting substrate. The epitaxial layer includes an n-type semiconductor layer, a light-emitting layer, and a p-type semiconductor layer. The light-emitting layer is formed between the n-type semiconductor layer and the p-type semiconductor layer. It further includes a reflective layer, and the light reflectivity of the reflective layer is greater than that of the epitaxial layer; the reflective layer completely covers the side surface of the epitaxial layer and partially covers the side surface of the light-transmitting substrate. This reduces the light output from the side of the chip, improves the light extraction rate of the Micro-LED, reduces energy loss, and meets the requirements for a high-brightness display screen; the side surface is coated with a reflective layer, which can also prevent metal migration in the epitaxial layer and avoid the influence of the cutting depth and cutting width in the front cutting process of the chip on the overall production yield, cracks, twin crystals, and even flip losses, thereby improving the reliability of the Micro-LED chip.

Description

Technical Field

[0001] The present invention relates to the field of semiconductor light-emitting structures, and in particular, to a Micro-LED chip structure and a manufacturing method thereof. Background Art

[0002] With the development of technology, Micro-LED (Micro Light Emitting Diode) displays have been widely used and are widely applied to fields such as backlights, VR screens, mobile phone displays, and small displays.

[0003] However, with the miniaturization of the chip size, the requirement for the light-emitting brightness of Micro-LED chips is getting higher and higher. The existing black filling glue for chip packaging has a strong light absorption ability on the side of the chip, resulting in a large loss of the brightness of Micro-LED chips. The problem of low light-emitting brightness caused by light emitting from the side of the chip needs to be solved urgently. Summary of the Invention

[0004] The purpose of the present invention is to provide a Micro-LED chip structure and a manufacturing method thereof to solve the problem of light emitting from the side of the chip and improve the light-emitting brightness of the chip.

[0005] To achieve one of the above-mentioned invention purposes, an embodiment of the present invention provides a Micro-LED chip structure. The chip structure includes a light-transmitting substrate and an epitaxial layer formed on the light-transmitting substrate. The epitaxial layer includes an n-type semiconductor layer, a light-emitting layer, and a p-type semiconductor layer. The light-emitting layer is formed between the n-type semiconductor layer and the p-type semiconductor layer.

[0006] It further includes a reflective layer, and the light reflectivity of the reflective layer is greater than that of the epitaxial layer.

[0007] The reflective layer completely covers the side surface of the epitaxial layer and partially covers the side surface of the light-transmitting substrate.

[0008] As a further improvement of an embodiment of the present invention, the thickness of the area of the side surface of the light-transmitting substrate covered by the reflective layer accounts for 40% - 60% of its total thickness.

[0009] As a further improvement of an embodiment of the present invention, the n-type semiconductor layer is disposed on the upper surface of the light-transmitting substrate. Part of the upper surface of the p-type semiconductor layer extends inward to form a groove to expose the n-type semiconductor layer, and metal electrodes are disposed on the upper surface of the p-type semiconductor layer and the exposed upper surface of the n-type semiconductor layer.

[0010] As a further improvement of an embodiment of the present invention, the reflective layer is further disposed on the upper surface of the epitaxial layer and at least exposes part of the area of the metal electrode.

[0011] As a further improvement of an embodiment of the present invention, the reflective layer is a DBR reflective layer, which is composed of alternating silicon oxide layers and titanium oxide layers, and the optical thickness of each layer of material is 1 / 4 of the central reflection wavelength.

[0012] The present invention also provides a manufacturing method for a Micro-LED chip structure, and the manufacturing method includes the steps of:

[0013] Providing a light-transmitting substrate;

[0014] Growing an n-type semiconductor layer, a light-emitting layer, and a p-type semiconductor layer on the light-transmitting substrate in sequence to form an epitaxial layer;

[0015] Cutting the upper surface of the epitaxial layer from the front to form a cutting groove, and the cutting groove extends into the light-transmitting substrate;

[0016] Forming a reflective layer in the cutting groove, and the light reflectivity of the reflective layer is greater than that of the epitaxial layer.

[0017] As a further improvement of an embodiment of the present invention, the step of cutting the upper surface of the epitaxial layer from the front to form a cutting groove that extends into the light-transmitting substrate specifically includes:

[0018] Cutting the light-transmitting substrate from the front until the cutting depth reaches 40% - 60% of the thickness of the light-transmitting substrate.

[0019] As a further improvement of an embodiment of the present invention, the step of cutting the upper surface of the epitaxial layer from the front to form a cutting groove that extends into the light-transmitting substrate specifically includes the steps of:

[0020] Etching inwardly at intervals along the upper surface area of the p-type semiconductor layer to form grooves exposing the n-type semiconductor layer;

[0021] Depositing a protective layer on the upper surface of the p-type semiconductor layer and the surface area of the grooves;

[0022] Cutting the groove area from the front to form a cutting groove in the light-transmitting substrate;

[0023] Using a hot acid etching process to etch the protective layer and the cutting-generated particles in the cutting groove;

[0024] Fabricating metal electrodes on the surfaces of the p-type semiconductor layer and the exposed n-type semiconductor layer.

[0025] As a further improvement of an embodiment of the present invention, the step of forming a reflective layer in the cutting groove specifically includes the steps of:

[0026] Forming a reflective layer above the epitaxial layer;

[0027] Etch the upper surface of the p-type semiconductor layer, the exposed surface of the n-type semiconductor layer, and the reflective layer on the metal electrode region.

[0028] As a further improvement of an embodiment of the present invention, forming a reflective layer in the cutting groove specifically includes the steps of:

[0029] Form a reflective layer above the epitaxial layer;

[0030] Etch the reflective layer in the region corresponding to the metal electrode to expose at least a partial region of the metal electrode.

[0031] As a further improvement of an embodiment of the present invention, forming a reflective layer in the cutting groove specifically includes:

[0032] Alternately deposit and grow a silicon oxide layer and a titanium oxide layer to form a DBR reflective layer, and the optical thickness of each layer of material is 1 / 4 of the central reflection wavelength.

[0033] The beneficial effects of the present invention are as follows: A reflective layer with a high reflectivity is coated on the side surface of the Micro-LED chip, and the light rays at any light-emitting angle of the chip structure are totally reflected. The reflected light rays are emitted perpendicular to the chip structure, reducing the light emission from the side of the chip, improving the light extraction efficiency of the Micro-LED, reducing energy loss, and meeting the requirements of a high-brightness display screen; coating a reflective layer on the side can also prevent metal migration in the epitaxial layer and avoid the influence of the cutting depth and cutting width in the front cutting process of the chip on the overall manufacturing yield, cracks, twins, and even flip losses, improving the reliability of the Micro-LED chip. Brief Description of the Drawings

[0034] Figure 1 It is a schematic diagram of the Micro-LED chip structure in Embodiment 1 of the present invention.

[0035] Figure 2 It is a schematic diagram of the Micro-LED chip structure in Embodiment 2 of the present invention.

[0036] Figure 3 It is a schematic diagram of the Micro-LED chip structure in Embodiment 3 of the present invention.

[0037] Figure 4 It is a schematic flow chart of the manufacturing method of the Micro-LED chip structure in an embodiment of the present invention.

[0038] Figure 5 It is a schematic flow chart of the manufacturing method of the Micro-LED chip structure in Embodiment 1 of the present invention.

[0039] Figure 6 It is a schematic flow chart of the manufacturing method of the Micro-LED chip structure in Embodiment 2 of the present invention.

[0040] Figure 7 This is a schematic flow chart of the manufacturing method of the Micro-LED chip structure in Embodiment 3 of the present invention.

[0041] Figures 8-16 This is a structural step diagram corresponding to the manufacturing method of the Micro-LED chip structure in Embodiment 1, Embodiment 2, and Embodiment 3 of the present invention. Detailed implementation manners

[0042] To make the objectives, technical solutions, and advantages of the present application clearer, the technical solutions of the present application will be clearly and completely described below in conjunction with the specific implementation manners of the present application and the corresponding drawings. Obviously, the described implementation manners are only part of the implementation manners of the present application, rather than all of them. Based on the implementation manners in the present application, all other implementation manners obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present application.

[0043] The following details the implementation manners of the present invention. The examples of the implementation manners are shown in the drawings, where the same or similar reference numerals denote the same or similar elements or elements with the same or similar functions throughout. The implementation manners described below with reference to the drawings are exemplary and are only used to explain the present invention and should not be construed as a limitation of the present invention.

[0044] For convenience of description, the terms representing relative spatial positions are used herein, such as "upper", "lower", "rear", "front", etc., to describe the relationship between one unit or feature shown in the drawings and another unit or feature. The terms of relative spatial positions may include different orientations of the device in use or operation other than the orientations shown in the drawings. For example, if the device in the drawing is flipped, the unit described as being "below" or "above" other units or features will be located "below" or "above" other units or features. Therefore, the exemplary term "below" can encompass both the lower and upper spatial orientations.

[0045] As Figures 1-3 shown, the present invention provides a Micro-LED chip structure, including a light-transmitting substrate 1, an epitaxial layer 2 formed on the light-transmitting substrate 1, and a reflective layer 3.

[0046] The light-transmitting substrate 1 has an upper surface and a lower surface opposite to the upper surface, and it can be a sapphire substrate, a glass substrate with high light transmittance, or a substrate made of other materials with high light transmittance.

[0047] The epitaxial layer 2 includes an n-type semiconductor layer 21, a light-emitting layer 22, and a p-type semiconductor layer 23, and the light-emitting layer 22 is formed between the n-type semiconductor layer 21 and the p-type semiconductor layer 23.

[0048] In some embodiments of the present invention, the n-type semiconductor layer 21 is disposed on the upper surface of the light-transmitting substrate 1, and the p-type semiconductor layer 23 is disposed away from the light-transmitting substrate 1. However, in some other embodiments, it is also possible that the n-type semiconductor layer 21 is disposed away from the light-transmitting substrate 1, and the p-type semiconductor layer 23 is disposed on the upper surface of the light-transmitting substrate 1. The present invention does not make any limitation here.

[0049] The reflective layer 3 completely covers the side surface of the epitaxial layer 2 and partially covers the side surface of the light-transmitting substrate 1, and its light reflectivity is greater than that of the epitaxial layer 2. By forming the reflective layer on the side surfaces of the epitaxial layer 2 and the light-transmitting substrate 1, total internal reflection of the light emitted from the light-emitting layer 22 can occur within the chip structure, and most of the light is finally emitted perpendicular to the chip structure, thereby solving the problem of low light-emitting brightness of the LED display screen caused by side light leakage in the existing chip structure. At the same time, the overall side surface of the epitaxial layer 2 is covered with the reflective layer 3, which can also prevent metal migration in the epitaxial layer 2 and improve the reliability of the chip.

[0050] Specifically, the thickness of the area of the side surface of the light-transmitting substrate 1 covered by the reflective layer 3 accounts for 40% - 60% of its total thickness. During the manufacturing process, to form the reflective layer 3 on the side surface of the light-transmitting substrate 1, the front of the chip needs to be cut first, and then the reflective layer 3 is formed in the chip cutting groove. While effectively solving the problem of side light emission, it can avoid the influence of the overall yield, cracks, twins, and even flipping losses between the grains of the chip structure caused by the front cutting of the Micro-LED chip.

[0051] In some embodiments of the present invention, the reflective layer 3 is a DBR reflective layer.

[0052] DBR (Distributed Bragg Reflection), also known as a distributed Bragg reflector, is a periodic structure composed of two materials with different refractive indices arranged alternately in the ABAB manner. The optical thickness of each layer of material is 1 / 4 of the central wavelength, so it is a quarter-wavelength multi-layer system. Since electromagnetic waves with frequencies falling within the energy gap range cannot penetrate, the reflectivity of the Bragg reflector can reach more than 99%.

[0053] Specifically, the reflective layer 3 is a DBR reflective layer structure composed of alternating silicon oxide layers and titanium oxide layers, and the optical thickness of each layer of material is specifically 1 / 4 of the central wavelength of the light emitted from the chip structure.

[0054] Of course, in some other embodiments of the present invention, the DBR reflective layer can also be composed of any two different materials selected from hafnium oxide, magnesium fluoride, yttrium oxide, zinc sulfide, zirconium oxide, and silicon nitride arranged alternately.

[0055] The present invention provides the following three embodiments to specifically illustrate the Micro-LED chip structure in some embodiments of the present invention.

[0056] Embodiment 1

[0057] As Figure 1 shown, the Micro-LED chip structure proposed in Embodiment 1 of the present invention includes a light-transmitting substrate 1, an epitaxial layer 2 disposed on the light-transmitting substrate 1, and a reflective layer 3.

[0058] The epitaxial layer 2 includes an n-type semiconductor layer 21, a light-emitting layer 22, and a p-type semiconductor layer 23, and the light-emitting layer 22 is disposed between the n-type semiconductor layer 21 and the p-type semiconductor layer 23.

[0059] Specifically, in this embodiment, the n-type semiconductor layer 21 is disposed on the upper surface of the light-transmitting substrate 1, and a groove is formed by inward extension of a partial region on the upper surface of the p-type semiconductor layer 23 to expose the n-type semiconductor layer 21.

[0060] Furthermore, metal electrodes 4 are disposed on the upper surface of the p-type semiconductor layer 23 and on the upper surface of the exposed n-type semiconductor layer 21 for electrical connection with an external power supply.

[0061] The reflective layer 3 is a DBR reflective layer in any of the above-mentioned embodiments, which completely covers the side surface of the epitaxial layer 2 and partially covers the side surface of the light-transmitting substrate 1 to reduce light leakage from the side of the chip and achieve a high-brightness LED display screen.

[0062] Embodiment 2

[0063] As Figure 2 shown, the Micro-LED chip structure proposed in Embodiment 2 of the present invention is different from the chip structure in Embodiment 1 in that, in this embodiment, the reflective layer 3 is further disposed on the upper surface of the epitaxial layer 2 and at least a partial region of the metal electrode 4 is exposed.

[0064] Specifically, the reflective layer 3 is further disposed on the upper surface of the p-type semiconductor layer 23 and on the upper surface of the exposed n-type semiconductor layer 21, and the metal electrode 4 is completely exposed, facilitating electrical connection between the chip structure and an external power supply through the metal electrode 4.

[0065] Similarly, the reflective layer 3 is a DBR reflective layer in any of the above-mentioned embodiments, which completely covers the side surface of the epitaxial layer 2 and partially covers the side surface of the light-transmitting substrate 1 to reduce light leakage from the side of the chip. And, since the reflective layer 3 is also disposed on the upper surface of the epitaxial layer 2, most of the light emitted from the epitaxial layer 2 can be emitted vertically towards the light-transmitting substrate 1 direction of the chip structure, further improving the luminous brightness of the LED display screen.

[0066] Example 3

[0067] As Figure 3 shown, the Micro-LED chip structure proposed in Example 3 of the present invention is different from the chip structure in Example 1. In this embodiment, a reflective layer 3 is also provided on the lower surface of the light-transmitting substrate 1.

[0068] Similarly, the reflective layer 3 is a DBR reflective layer in any of the above-mentioned embodiments, which completely covers the side surface of the epitaxial layer 2 and partially covers the side surface of the light-transmitting substrate 1 to reduce light leakage from the side of the chip. And, since a reflective layer 3 is also provided on the lower surface of the light-transmitting substrate 1, it can also greatly reduce the light emitted from the direction of the light-transmitting substrate 1, and emit most of the light emitted by the epitaxial layer 2 perpendicular to the chip structure towards the upper surface of the epitaxial layer, further improving the luminous brightness of the LED display screen.

[0069] As Figure 4 shown, the present invention provides a method for manufacturing a Micro-LED chip, including the steps:

[0070] S1: Provide a light-transmitting substrate 1.

[0071] S2: Grow an n-type semiconductor layer 21, a light-emitting layer 22, and a p-type semiconductor layer 23 on the light-transmitting substrate 1 in sequence to form an epitaxial layer 2.

[0072] Of course, in some other embodiments of the present invention, a p-type semiconductor layer 23, a light-emitting layer 22, and an n-type semiconductor layer 21 can also be grown on the upper surface of the light-transmitting substrate 1 in sequence.

[0073] S3: Front-side cut the upper surface of the epitaxial layer 2 to form a cutting groove 5, and the cutting groove 5 extends into the light-transmitting substrate 1.

[0074] In some embodiments of the present invention, a front-side laser cutting process can be used. Of course, in some other embodiments of the present invention, a mechanical cutting process can also be used, which can be selected according to the actual situation.

[0075] Specifically, when front-side cutting the light-transmitting substrate 1, the cutting depth is 40% - 60% of the thickness of the light-transmitting substrate 1.

[0076] S4: Form a reflective layer 3 in the cutting groove 5, and the light reflectivity of the reflective layer is greater than the light reflectivity of the epitaxial layer.

[0077] Specifically, evaporate the reflective layer 3 in the cutting groove 5 to cover the side surface of the epitaxial layer 2 and the side surface of the light-transmitting substrate 1 in the cutting groove 5, which can avoid the influence of the loss of the overall yield, cracks, twins, and even flipping between the grains of the chip structure caused by the front-side cutting of the Micro-LED chip.

[0078] In some embodiments of the present invention, step S4 specifically includes:

[0079] An SiO₂ layer and a TiO₂ layer are alternately deposited and grown to form a DBR reflective layer, and the optical thickness of each layer of material is 1 / 4 of the central reflection wavelength.

[0080] Of course, in some other embodiments of the present invention, the DBR reflective layer may also be alternately composed of any two different materials selected from hafnium oxide, magnesium fluoride, yttrium oxide, zinc sulfide, zirconium oxide, and silicon nitride.

[0081] Corresponding to the manufacturing method of the Micro-LED chip structure in the above-mentioned embodiment 1, as Figure 5 shown, steps S3 and S4 specifically include:

[0082] S3a: Grooves exposing the n-type semiconductor layer 21 are etched inwards at intervals on the upper surface region of the p-type semiconductor layer 23, as Figure 8 shown.

[0083] S3b: A protective layer 6 is deposited on the upper surface of the p-type semiconductor layer 23 and the surface region of the grooves, as Figure 9 shown.

[0084] In a specific embodiment of the present invention, the protective layer 6 is an SiO₂ layer. Of course, other oxide materials can also be selected as the protective layer, and the present invention does not limit this here.

[0085] S3c: The groove region is cut from the front, and a cutting groove 5 is formed in the light-transmitting substrate 1, as Figure 10 shown.

[0086] After laser cutting, there will be some residual cutting-generated particles in the cutting groove.

[0087] S3d: The protective layer 6 and the cutting-generated particles in the cutting groove 5 are etched using a hot acid etching process, as Figure 11 shown.

[0088] S3e: Metal electrodes 4 are fabricated on the surfaces of the p-type semiconductor layer 23 and the exposed n-type semiconductor layer 21, as Figure 12 shown.

[0089] Specifically, a metal layer is deposited on the upper surface of the p-type semiconductor layer 23 and a partial region of the surface of the n-type semiconductor layer 21 exposed in the groove to form the metal electrodes 4 by using electron beam evaporation, magnetron sputtering, electroplating, or electroless plating processes. Here, the material of the metal electrodes 4 can be one or more metals selected from Cr, Ni, Ti, Au, Pt, Cu, and Ag.

[0090] S4a1: A reflective layer 3 is formed above the epitaxial layer 2, as Figure 13as shown

[0091] Specifically, a reflective layer 3 is deposited over the entire chip structure. The reflective layer 3 covers the outer surface of the epitaxial layer 2 and the surface of the cutting groove 5. Therefore, in the vertical direction of the chip structure, the thickness of the reflective layer 3 in the cutting groove 5 is 40% - 60% of the thickness of the light-transmitting substrate 1, which can avoid the influence of the cutting depth and width caused by the front cutting process on the overall yield, cracks, twins, and even flip losses.

[0092] S4b1: Etch the upper surface of the p-type semiconductor layer 23, the exposed surface of the n-type semiconductor layer 21, and the reflective layer 3 on the metal electrode 4 region, as Figure 14 shown

[0093] Of course, the method for fabricating the chip of the present invention further includes the step: dicing.

[0094] Specifically, back cleavage separation is performed at the cutting groove 5 to fabricate a Micro-LED chip having the structure in Embodiment 1.

[0095] Corresponding to the method for fabricating the Micro-LED chip structure in the above Embodiment 2, as Figure 6 shown, different from the method for fabricating the chip structure in Embodiment 1, step S4 specifically includes:

[0096] S4a2: Form a reflective layer 3 above the epitaxial layer 2, as Figure 13 shown

[0097] S4b2: Etch the reflective layer 3 in the region corresponding to the metal electrode 4 to expose at least a partial region of the metal electrode 4, as Figure 15 shown

[0098] Finally, back cleavage separation is performed at the cutting groove 5 to fabricate a Micro-LED chip having the structure in Embodiment 2.

[0099] Corresponding to the method for fabricating the Micro-LED chip structure in the above Embodiment 3, as Figure 7 shown, on the basis of the method for fabricating the chip structure in Embodiment 1, it further includes the step:

[0100] S4c1: Form a reflective layer 3 on the lower surface of the light-transmitting substrate 1, as Figure 16 shown

[0101] Of course, this step of the process can also start before forming the reflective layer 3 above the epitaxial layer 2, and the present invention does not limit this.

[0102] Finally, back cleavage separation is performed at the cutting groove 5 to fabricate a Micro-LED chip having the structure in Embodiment 3.

[0103] In summary, by coating a reflective layer with a high reflectivity on the side surface of the Micro-LED chip, the present invention totally reflects the light rays at any light-emitting angle of the chip structure. The reflected light rays are emitted perpendicular to the chip structure, reducing the light emission from the side of the chip, improving the light extraction efficiency of the Micro-LED, reducing energy loss, and meeting the requirements of a high-brightness display screen. Coating the reflective layer on the side can also prevent metal migration in the epitaxial layer and avoid the influence of the cutting depth and width during the front cutting process of the chip on the overall production yield, cracks, twins, and even flip losses, thereby improving the reliability of the Micro-LED chip.

[0104] It should be understood that although this specification is described according to the embodiments, not every embodiment only contains an independent technical solution. This narrative manner of the specification is only for clarity. Those skilled in the art should regard the specification as a whole, and the technical solutions in each embodiment can also be appropriately combined to form other embodiments that can be understood by those skilled in the art.

[0105] The series of detailed descriptions listed above are only specific descriptions of the feasible embodiments of the present invention, and they are not intended to limit the protection scope of the present invention. Any equivalent embodiments or changes made without departing from the spirit of the art of the present invention should be included in the protection scope of the present invention.

Claims

1. A manufacturing method of a Micro-LED chip structure, characterized in that, The manufacturing method includes the steps of: providing a light-transmitting substrate; successively growing an n-type semiconductor layer, a light-emitting layer, and a p-type semiconductor layer on the light-transmitting substrate to form an epitaxial layer; front-side cutting the upper surface of the epitaxial layer to form a cutting groove, the cutting groove extending into the light-transmitting substrate; front-side cutting the light-transmitting substrate until the cutting depth is 40% - 60% of the thickness of the light-transmitting substrate; forming a reflective layer in the cutting groove, the light reflectivity of the reflective layer being greater than that of the epitaxial layer; front-side cutting the upper surface of the epitaxial layer to form a cutting groove, the cutting groove extending into the light-transmitting substrate, specifically including the steps of: etching inwards at intervals along the upper surface area of the p-type semiconductor layer to form grooves exposing the n-type semiconductor layer; depositing a protective layer on the upper surface of the p-type semiconductor layer and the surface area of the grooves; front-side cutting the groove area to form a cutting groove in the light-transmitting substrate; corroding the protective layer and the cutting-generated particles in the cutting groove by using a hot acid etching process.

2. The manufacturing method of the Micro-LED chip structure according to claim 1, characterized in that including the step of fabricating metal electrodes on the surfaces of the p-type semiconductor layer and the exposed n-type semiconductor layer.

3. The manufacturing method of the Micro-LED chip structure according to claim 2, characterized in that, The forming of the reflective layer in the cutting groove specifically includes the steps of: forming a reflective layer above the epitaxial layer; etching the reflective layer on the upper surface of the p-type semiconductor layer, the surface of the exposed n-type semiconductor layer, and the metal electrode area.

4. The manufacturing method of the Micro-LED chip structure according to claim 2, wherein, The forming of the reflective layer in the cutting groove specifically includes the steps of: forming a reflective layer above the epitaxial layer; etching the reflective layer in the area corresponding to the metal electrode to expose at least a partial area of the metal electrode.

5. The manufacturing method of the Micro-LED chip structure according to claim 1, characterized in that, The forming of the reflective layer in the cutting groove specifically includes: alternately depositing and growing a silicon oxide layer and a titanium oxide layer to form a DBR reflective layer, the optical thickness of each layer of material being 1 / 4 of the central reflection wavelength.

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