CN111025442A

CN111025442A - Bragg reflector made of novel material

Info

Publication number: CN111025442A
Application number: CN201911190680.7A
Authority: CN
Inventors: 刘建辉; 王天鹤; 刘舒扬; 贾晓东
Original assignee: Tianjin Jinhang Institute of Technical Physics
Current assignee: Tianjin Jinhang Institute of Technical Physics
Priority date: 2019-11-28
Filing date: 2019-11-28
Publication date: 2020-04-17

Abstract

The invention belongs to the technical field of Bragg reflectors, and particularly relates to a Bragg reflector made of a novel material. The Bragg reflector comprises an upper reflecting layer and a lower reflecting layer; a light transmitting layer is arranged between the upper reflecting layer and the lower reflecting layer; the upper reflecting layer is prepared by alternately preparing a plurality of layers of high-reflectivity substances and a plurality of layers of low-reflectivity substances, so that Bragg reflectors with large reflectivity differences are formed and overlapped for many times. The existing distributed bragg reflector is generally manufactured by selecting the most common materials, and no research is carried out on novel materials, so that the existing distributed bragg reflector has the common defects of more stacked layers, relatively smaller high and low reflectivity and the like. The invention is to optimize the design material of the Bragg reflector, provide a novel material manufacturing process, reduce the number of stacked layers and improve the difference of high and low reflectivities.

Description

Bragg reflector made of novel material

Technical Field

The invention belongs to the technical field of Bragg reflectors, and particularly relates to a Bragg reflector made of a novel material.

Background

The bragg mirror (also called distributed bragg reflector) technology is derived from multilayer film optical fabrication technology and plays an increasingly important role in the research in the field of modern optoelectronics. A bragg mirror is a mirror structure that is an adjustable multilayer structure composed of two optical materials. Simply, pairs of two or more semiconductor or dielectric materials are grown in a staggered manner.

The bragg reflector is made by selecting two materials with large refractive index and small refractive index as film layers which are grown in a staggered mode respectively and are transparent to incident light. For semiconductor materials, they are also required to be lattice matched to the substrate and between them to reduce stress. The most common is the quarter-wave mirror, where each layer has a thickness corresponding to a quarter of the wavelength, and so in practice the 1/4 λ film system is used mostly, λ being the incident wavelength. Namely, low refractive index films and high refractive index films are alternately grown on a substrate, the optical thickness of each film is 1/4 lambda, and dozens of pairs of films are usually required to obtain the reflectivity higher than 95%. The larger the difference between the refractive indices of the two materials, the wider the reflection bandwidth of the bragg mirror, and the fewer the number of logarithms that need to be grown to achieve a certain reflectivity. If the mirror is used for larger angles of incidence, the relative required layer thickness is larger.

The materials from which bragg mirror technology is typically fabricated range from semiconductors to dielectrics, even including portions of metals. The semiconductor material includes InP, InGaAsP, GaAs, AlAs, etc., the dielectric material includes Si, SiO2, Al2O3, TiO2, etc., and the metal material is typically Au, Ag, etc. In the above materials, the dielectric material growth bragg reflector is generally in a polycrystalline or amorphous form, and does not require lattice matching. The most common of the bragg mirror film layer material media are combinations of SiO2 and Si, Si3N4, TiO2, and the like. However, despite the higher refractive index of Si, it has a greater absorption at long wavelengths (>900nm) and so TiO2 is sometimes used instead.

Specifically, the above-described technique is specifically explained as follows:

as shown in fig. 1, when light passes through a thin film, the reflected light is substantially formed by overlapping the upper surface reflected light and the lower surface reflected light. When the phase difference of the two beams is 180 degrees, the amplitude of the composite beam is the difference of the amplitudes of the two beams, namely destructive interference; when the phase difference is 0 ° or an integral multiple of 360 °, the amplitude is the sum of both, i.e., the so-called interference phase is long. Along with the change of the thickness of the film, the intensity of the reflected light shows periodic change, which is the visual embodiment of the Bragg principle. On the other hand, if the thickness is set to a quarter of the corresponding light wavelength, the reflected light is significantly enhanced compared to the transmitted light by means of the interference phase growth, at which the phase difference is 0 °. The simplest reflection increasing film is formed.

If the stack of high and low refractive index materials is increased and the above interference constructive process is repeated, the film reflectivity will increase continuously until it is nearly 100%. As shown in the figure.

The data show that the overall reflectivity approaches 100% with increasing numbers of mirror layers. Another important feature of the dbr is that high reflectivity is only present in a specific wavelength band, and the width of its high reflectivity region needs to be calculated by means of a transmission matrix. As can be seen from fig. 2, the dbr can be viewed as periodic pairs of high and low refractive index layers, plus a single high reflectivity layer.

Foreign research on distributed bragg reflectors includes: a staggered growth bragg mirror design based on SIO2 and amorphous silicon, represented by IMEC corporation, belgium. The fabrication materials mentioned in this scheme are mainly the combination of SIO2 and engineered amorphous silicon, and the combination of SIO2 and SIGe, while amorphous silicon is still chosen for the high reflectivity material.

An example of a distributed bragg stack is the combination of Si02 and engineered amorphous silicon with a center wavelength around 700nm, ranging from 540nm to 1000 nm. A second example is a combination of Si02 and SiGe, with a center wavelength of 1500nm and a bandwidth of 1000nm, for example from 1000nm to 2000 nm. The result of using the bragg stack as a mirror layer is an additional phase shift during reflection of the light. This phase shift shifts the center wavelength, but this shift can be simply determined using, for example, simulation tools. The solution comes from the patent of its application entitled integrated circuit for spectral imaging system, patent application No. 201080054366.7.

The bragg reflector of IMEC corporation mainly selects a combination of SIO2 and engineered amorphous silicon in terms of material selection, and still selects amorphous silicon in terms of high-reflectivity material, and also includes a combination of SIO2 and SIGe, thereby causing the effects of a large number of stacked layers, small difference in high and low reflectivity, and complicated processing and manufacturing.

Disclosure of Invention

Technical problem to be solved

The technical problem to be solved by the invention is as follows: the existing distributed bragg reflector is generally made of the most common materials, which causes common defects such as more stacked layers, relatively small high and low reflectivity, and the like.

(II) technical scheme

In order to solve the technical problem, the invention provides a Bragg reflector made of a novel material, wherein the Bragg reflector comprises an upper reflecting layer and a lower reflecting layer; a light transmitting layer is arranged between the upper reflecting layer and the lower reflecting layer;

the upper reflecting layer is prepared by alternately preparing a plurality of layers of high-reflectivity substances and a plurality of layers of low-reflectivity substances, so that Bragg reflectors with large reflectivity differences are formed and overlapped for many times.

Wherein the reflectivity of the Bragg reflector reaches more than 99%.

Wherein the high-reflectivity material is Si₃N₄。

Wherein the low-reflectivity substance is SiO₂。

In the Bragg reflector, the lower reflecting layer has the same structure and material as the upper reflecting layer, is positioned between the light-transmitting layer and the photosensitive pixel, and also has high reflection effect.

Wherein, the light-transmitting layer material is prepared from SiO2 material.

Wherein the light-transmitting layer thickness is changed to be step-growth.

Wherein the pass light layer thickness variation is electrokinetically driven by the MOEMS.

Wherein the photosensitive pixels are image sensors.

Wherein the thicknesses of the multiple layers of high-reflectivity substances are all equal;

the thicknesses of the multiple layers of low-reflectivity materials are all equal.

(III) advantageous effects

Compared with the prior art, the invention innovatively provides the SiO₂And Si₃N₄The Bragg reflector is manufactured by alternately growing two materials with different refractive indexes on a Si substrate. Using air dielectric parameters, 5-and 7-layer structures were simulated, with 5 layers having a highest reflectivity of 59% and 7 layers having a highest reflectivity of 75%.

Simulator using model of micro FP cavity light splitting film for SiO₂/Si₃N₄The spectral filter device for material growth is designed, and a single-spectrum FP cavity filter at 750nm and a 16-spectrum spectral filter covering near infrared 400 nm-1000 nm are respectively designed. The material not only meets higher reflectivity, but also can expand the spectrum transmission range, the number of layers and the thickness of the subsequent hyperspectral chip structure, the final spectrum range, the resolution ratio and the like can be influenced by different materials, and the material selected by the invention has the advantages of less stacking times, small error and simple processing and manufacturing.

Drawings

Fig. 1 is a schematic diagram of bragg principle followed by light reflection and superposition in a single-layer film.

Fig. 2 is a schematic diagram of bragg principle followed by light reflection and superposition in a multilayer thin film.

FIG. 3 is a schematic diagram of a film system structure of a single band FP cavity filter with a center wavelength of 750 nm.

Fig. 4 is a graph showing the simulated transmittance results of a single-spectral-band FP cavity filter.

Fig. 5 is a graph showing the simulated transmittance results of a 16 band FP cavity filter.

Fig. 6 is a schematic view of a 7-layer film bragg mirror and a 5-layer film bragg mirror.

Fig. 7 is a flow chart of FP cavity filter development.

FIG. 8 is a schematic diagram of a FP-cavity single-chip film system with a center wavelength of 750 nm.

FIG. 9 is a graph showing the simulated transmittance results of FP-cavity single chip.

Detailed Description

In order to make the objects, contents, and advantages of the present invention clearer, the following detailed description of the embodiments of the present invention will be made in conjunction with the accompanying drawings and examples.

The existing distributed bragg reflector is generally manufactured by selecting the most common materials, and no research is carried out on novel materials, so that the existing distributed bragg reflector has the common defects of more stacked layers, relatively smaller high and low reflectivity and the like. The invention is to optimize the design material of the Bragg reflector, provide a novel material manufacturing process, reduce the number of stacked layers and improve the difference of high and low reflectivities. Wherein the content of the first and second substances,

the micro spectral filter adopts a distributed Bragg mirror structure, and the distributed Bragg mirror is a high-reflection film system formed by alternately arranging high-refractive-index materials and low-refractive-index materials. By distributed, it is meant, on the one hand, that the reflectivity is high for light in only one particular wavelength band, while the reflectivity is low for light of other wavelengths; another aspect refers to reflectance layer-by-layer reflective superposition. Since the micro spectral filter requires a process compatible with CMOS imaging sensor fabrication, it uses CMOS process line materials. Regarding the material selection of the bragg mirror, the crystal quality of the active region of the sensor is ensured, the thermal mismatch and the stress are reduced as much as possible, the material matched with the lattice constant of the material of the active region is selected, and the compatibility with a CMOS process line is also met (the filter is ensured to be integrally grown on the sensor).

In order to effectively solve the problems of multilayer stacking and small difference of high and low reflection rate existing in the Bragg mirror originally, Si is selected in the design₃N₄As a highly reflective stack material, SiO₂And Si₃N₄Alternate growth was thus completed to complete the sample fabrication. SiO2₂The chemical property of the material is very stable, the material is a high-quality and stable electric insulating material, and the material can serve as a high-quality chemical barrier layer to protect an active area of the image sensor from external contamination, and simultaneously allows a high-temperature process; si₃N₄It is a superhard material, resistant to wear, resistant to oxidation at high temperatures, and resistant to cold and heat shocks. Si₃N₄The film has high dielectric constant, oxidation resistance, excellent mechanical property and good stability, and can be widely applied to microelectronic process and micro-electronicsIn the machining process.

After selection, SiO was found₂/Si₃N₄The high-low refractive index material has a wider high-reflection area, can achieve higher reflectivity with fewer layers, is simple to prepare and low in cost, and is used as a cavity mirror to form an FP (Fabry-Perot) cavity with narrow filter line width, so the SiO2/Si3N4 high-low alternating refractive index dielectric film is a material extremely suitable for manufacturing a micro spectral filter.

Specifically, in order to solve the problems of the prior art, the present invention provides a bragg reflector made of a novel material, as shown in fig. 3 to 6, wherein the bragg reflector includes an upper reflective layer and a lower reflective layer; a light transmitting layer is arranged between the upper reflecting layer and the lower reflecting layer;

the upper reflecting layer adopts multiple layers of high-reflectivity substances (Si)₃N₄) And a multi-layered low-reflectivity Substance (SiO)₂) And alternately preparing to form Bragg reflectors with large reflectivity difference, and overlapping for many times. The reflectivity of the Bragg reflector reaches more than 99%.

Wherein the high-reflectivity material is Si₃N₄。

Wherein the low-reflectivity substance is SiO₂。

Wherein, the light-transmitting layer material is prepared from SiO2 material.

Wherein the light-transmitting layer thickness change is step growth or is electrically driven by MOEMS.

Wherein the photosensitive pixels are image sensors.

Wherein the thicknesses of the multiple layers of high-reflectivity substances are all equal; the thicknesses of the multiple layers of low-reflectivity materials are all equal.

Example 1

This embodiment provides a manufacturing process of the bragg reflector, and as shown in fig. 7 to 9, first, a physical model of a spectroscopic thin film based on an FP cavity is constructed, a material compatible with a CMOS process is selected, and an FP cavity filter thin film is grown; designing a film system structure of the FP cavity filter through a simulator, wherein the designed filtering center wavelength is 750nm as shown in FIG. 8; from the simulation results, it can be seen in FIG. 9 that the center wavelength is 750nm and the filter bandwidth is 10 nm.

A Bragg multilayer film process is developed on a quartz substrate, the growth of a distributed Bragg FP (Fabry-Perot) cavity filter film is realized on the quartz substrate by utilizing a semiconductor process, and then the optical parameters of the FP cavity filter film are measured.

And when the optical parameters of the FP cavity filter meet the requirements, growing the FP cavity filter film on the wafer of the image sensor, cutting and packaging the grown wafer, and testing the optical parameters. And finally, completing the chip integration of the single-spectrum imaging system.

The above description is only a preferred embodiment of the present invention, and it should be noted that, for those skilled in the art, several modifications and variations can be made without departing from the technical principle of the present invention, and these modifications and variations should also be regarded as the protection scope of the present invention.

Claims

1. A Bragg reflector made of a novel material is characterized in that the Bragg reflector comprises an upper reflecting layer and a lower reflecting layer; a light transmitting layer is arranged between the upper reflecting layer and the lower reflecting layer;

2. A Bragg reflector made of a novel material according to claim 1, wherein the reflectivity of the Bragg reflector is more than 99%.

3. A Bragg reflector according to claim 1, wherein the high reflectivity material is Si₃N₄。

4. A Bragg reflector according to claim 3, wherein the low-reflectivity material is SiO₂。

5. A Bragg reflector made of the novel material according to claim 4, wherein the Bragg reflector has the same structure and material as the upper reflecting layer, is positioned between the light-transmitting layer and the photosensitive pixel, and has the same high reflection effect.

6. A Bragg reflector according to claim 5, wherein the light-transmitting layer is made of SiO 2.

7. A Bragg reflector according to claim 6, wherein the thickness variation of the light-transmitting layer is a step growth.

8. A Bragg reflector mirror made of the novel material as claimed in claim 6, wherein the thickness variation of the light-transmitting layer is electrically driven by MOEMS.

9. A Bragg reflector mirror in accordance with claim 5, wherein the light-sensitive pixel is an image sensor.

10. A Bragg reflector made of the novel material according to claim 5, wherein the thicknesses of the multiple layers of high-reflectivity material are all equal;