JP4237665B2 - Optical waveguide sensor and manufacturing method thereof - Google Patents

Description

  The present invention relates to an optical waveguide sensor that performs spectroscopic analysis using light that oozes out when light is totally reflected at interfaces having different refractive indexes, and a method for manufacturing the same.

  Light propagating through a structure such as a thin film composed of crystals or the like proceeds while repeating total reflection at the interface with the outside of the structure. When performing total reflection at the interface, the propagating light oozes out to the outside with a small refractive index within a range of a distance of about a wavelength. This exudation is called an evanescent wave and is absorbed by the substance in contact with the structure. Therefore, detection and identification of the substance in contact with the structure can be performed from the change in the intensity of light propagating through the structure. It becomes possible. The analysis method using the principle of the evanescent wave described above is called total reflection absorption spectroscopy (ATR: Attenuated Total Reflection method), and is used for chemical composition analysis of solutions and surface adsorbates (see Non-Patent Document 1). ).

Mainly, in the ATR method, both ends of a waveguide layer made of a semiconductor crystal having a higher refractive index than air are processed obliquely, light is introduced from the obliquely processed ends, and the surface of the waveguide layer is touched. It is a common method to analyze the substances present.
As an application of the ATR method to a sensor, a planar optical waveguide sensor is proposed that uses an evanescent wave to detect a substance that forms an optical waveguide layer on a substrate, transmits light, and contacts the upper surface of the optical waveguide layer. (See Patent Document 1).

  FIG. 7 is a configuration diagram illustrating a basic configuration example of a planar waveguide sensor. The planar waveguide sensor includes a substrate 701 made of glass, quartz, or the like, and an optical waveguide layer formed on the substrate 701. 702, gratings 703 and 704, a laser beam source 705, and a light detection unit 706. The optical waveguide layer 702 is made of a material having a higher refractive index than that of the substrate 701, such as silicon nitride and aluminum oxide.

  The sensor configured as shown in FIG. 7 introduces the light emitted from the laser beam source 705 into the grating 703 by making it incident on the grating 703 at a certain angle, and is a sample disposed in contact with the surface of the substrate 701. Light that oozes out to the side 707 is absorbed by the sample 707. The light reflected and propagated in the optical waveguide layer 702 is emitted from the optical waveguide layer 702 by the grating 704. The intensity of the emitted light is measured by the light detection unit 706. The amount of light absorption by the sample 707 can be determined from the intensity of the emitted light measured by the light detection unit 706, and the concentration of the sample 707 can be measured from the amount of light absorption. The sensor shown in FIG. 7 is used for detecting glucose in a sample solution.

The applicant has not yet found prior art documents related to the present invention by the time of filing other than the prior art documents specified by the prior art document information described in this specification.
JP 9-061346 A Practical spectroscopy series (4) Medical application of spectroscopy, published September 30, 1999, IPC Corporation

  However, in the conventional sensor shown in FIG. 7, in order to improve the detection sensitivity, it is necessary to make the region where evanescent light exudes wider (longer) so that the sample can absorb more evanescent light. is there. For this reason, when light is propagated through the planar optical waveguide, it is necessary to have a very large shape in order to improve sensitivity, and it has been difficult to reduce the size of the sensor shown in FIG. Further, the method of using a grating for coupling light to the optical waveguide layer has problems that it is difficult to align the optical axis and input / output efficiency is poor.

  The present invention has been made to solve the above-described problems, and an object of the present invention is to make it possible to construct a smaller analysis system based on total reflection absorption spectroscopy.

An optical waveguide sensor according to the present invention comprises a lower clad layer and a material containing silicon, and is formed on the lower clad layer so that the width in the plane direction of the lower clad layer is in the normal direction of the plane of the lower clad layer. A light incident end of a waveguide composed of a core larger than the height, a lower clad layer and a core, a light exit end of the waveguide, and a light incident end arranged continuously from the light incident end to reduce the diameter of incident light. A first mode field conversion unit; and at least a second mode field conversion unit that is arranged continuously at the light emission end and increases the diameter of the light emitted from the light emission end , and the waveguide is a single mode waveguide , and the constructed detection region part of the core, the first and second mode field conversion unit is constituted by a spot-size converter core covering the core, and an upper cladding layer covering the spot size conversion core In the area covered by the spot size conversion core, the core is formed narrower closer to the end portion, the refractive index of the spot size conversion core is obtained by the smaller than the core upper cladding layer is larger than so.
This sensor utilizes the fact that the light oozing out from the core is absorbed by the analysis object arranged on the core in the detection region.

In the optical waveguide sensor, the core bending radius is not so large if the height of the core is greater than 1/5 of the width of the core.
Further, a silicon oxide film formed on the surface of the core of the detection region may be provided.

The method for manufacturing an optical waveguide sensor according to the present invention is made of a material containing silicon on the lower cladding layer, and the width in the planar direction of the lower cladding layer is high in the normal direction of the plane of the lower cladding layer in the detection region. Halfbeak size rot, a step of state at a predetermined region continuous with the light input and output end is a core which is tapered in width closer to the end portion is narrowed is formed, the spot size conversion covering a core of a predetermined area By forming the core and forming the upper clad layer that covers the spot size conversion core, a predetermined region continuous to the light incident / exit end of the waveguide composed of the lower clad layer and the core is formed. At least a step of forming a mode field conversion unit.

In the above manufacturing method, a step of forming a spot size conversion core made of silicon oxynitride so as to selectively cover a portion that becomes the mode field conversion portion of the core, and a portion that becomes the mode field conversion portion of the core forming an upper cladding made of silicon oxide to selectively cover the spot size conversion core may be provided with a degree Engineering to state the mode field conversion portion is formed.

  In the above manufacturing method, a film made of a material containing silicon provided on the lower cladding layer may be processed by selective etching using a mask made of silicon oxide to form a core. . A film made of a material containing silicon provided on the lower clad layer is processed by selective etching using a mask made of silicon oxide to form a core, and on the upper surface of the core. A silicon oxide film thinner than the wavelength of light propagating through the core made of the mask may be formed. Alternatively, a silicon oxide film thinner than the wavelength of light propagating through the core may be formed on the surface of the core by a thermal oxidation method.

  As described above, in the present invention, a wide core made of a material containing silicon is formed on the lower clad layer, and a single-mode waveguide is constituted by the wide shaped core, and detection is performed on a part of these. By configuring the region and placing the analysis object on the detection region, analysis by the spectroscopic method for detecting infrared absorption or the like in the analysis object was made possible. Therefore, according to the present invention, the size of the analysis system can be reduced by using a single mode waveguide instead of the crystal prism in the conventional ATR method.

  In addition, since the detection region can be configured by a waveguide whose waveguide direction can be changed with a small bending radius, a long waveguide capable of improving sensitivity can be arranged in the small detection region. In addition, since the core is wide, the amount of light that oozes out toward the top surface of the core increases. Thus, according to the present invention, it is possible to obtain an excellent effect that a smaller analysis system can be constructed without lowering the sensitivity.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
FIG. 1 is a plan view showing a configuration example of an optical waveguide sensor according to an embodiment of the present invention.
The configuration of this sensor will be described. First, a V-shaped groove 101 to which the end portion of the incident-side optical fiber 200 is fixed, and a V-shaped groove 102 to which the end portion of the output-side optical fiber 300 is fixed are provided. ing. The grooves 101 and 102 are formed in the substrate portion 111 of a generally available SOI substrate.

  Following these grooves 101 and 102, a mode field size conversion region (mode field conversion unit) 120 is arranged. In the mode field size conversion region 120, a spot size conversion core 103 arranged on the incident side and an emission side are arranged. The spot size conversion core 104 is provided. The spot size conversion cores 103 and 104 are made of, for example, silicon oxynitride, and the cross-sectional shape is a rectangle having a side of about 3 to 7 μm. An optical fiber core 201 of the optical fiber 200 is connected to the spot size conversion core 103, and an optical fiber core 301 of the optical fiber 300 is connected to the spot size conversion core 104.

  A detection region 130 is provided following the mode field size conversion region 120, and a waveguide including the silicon thin wire core 105 is formed in the detection region 130. A thin silicon oxide film 106 having a thickness of about 20 to 30 nm is formed on the upper surface of the silicon fine wire core 105. The silicon oxide film 106 may be formed on the side surface of the silicon fine wire core 105. If the silicon oxide film 106 is thinner than the wavelength of light guided through the waveguide made of the silicon thin wire core 105, analysis using evanescent light is possible. Note that the silicon oxide film 106 is not necessarily required. The core may be made of silicon nitride.

  The silicon fine wire core 105 has a cross-sectional dimension of 500 nm in width and 150 nm in height. As shown in the cross-sectional view of FIG. 2, the silicon fine wire core 105 is disposed on the buried oxide layer 112 having a thickness of about 3 to 10 μm of the SOI substrate in the detection region 130. 2 is a cross-sectional view taken along the line AA ′ of FIG. 1 and 2, the side portions of the silicon fine wire core 105 are exposed, but a clad made of, for example, silicon oxide may be formed so as to fill the sides of the silicon fine wire core 105.

  By connecting a light source to the optical fiber 200 on the incident side of the optical waveguide sensor shown in FIG. 1 and connecting a spectroscope to the optical fiber 300 on the output side, a spectroscopic measurement system can be configured. In FIG. 1, the light incident end and the light exit end are provided, but a closed circuit in which these are the same may be used.

  Further, both ends of the silicon fine wire core 105 have a tapered shape (tapered shape) whose width becomes narrower toward the tip, and as shown in the cross-sectional views of FIG. 1 and FIG. Covered by the cores 103 and 104, this area becomes the mode field size conversion area 120. The tapered portion of the silicon fine wire core 105 has a tip width of about 100 nm. The spot size conversion cores 103 and 104 are about half the height and width of the optical fiber cores 201 and 301 to approximately the same size.

  The mode field size conversion region 120 is covered with an upper cladding layer 107 made of silicon oxide. The upper cladding layer 107 has a film thickness of 7 to 15 μm. Further, in the region where the end portions of the optical fibers 200 and 300 are fixed, as shown in the cross-sectional view of FIG. 4, for example, an ultraviolet curable adhesive is provided in the V-shaped grooves 101 and 102 provided in the substrate portion 111. It is fixed by. 3 shows the BB ′ section of FIG. 1, and FIG. 4 shows the CC ′ section of FIG. 1.

  In the mode field size conversion region 120, “refractive index of the silicon fine wire core 105> refractive index of the spot size conversion cores 103 and 104> refractive index of the upper cladding layer 107”. With the mode field size conversion region 120 configured in this manner, it is possible to convert the spot size of light such as infrared light guided through the optical fiber 200 to be coupled to one end of the silicon fine wire core 105 with reduced loss. It is said.

  For example, as the light passing through the detection region propagates through the tapered portion of the silicon fine wire core 105 in the direction of the optical fiber 300, the core width is gradually narrowed and the light confinement is weakened, and the mode field size tends to expand to the surroundings. . However, since the spot size conversion core 104 having a higher refractive index than the buried oxide layer 112 is present adjacent to the silicon fine wire core 105 at this time, the optical power distribution is from the silicon fine wire core 105 to the spot size conversion core 104. Gradually move.

Further, when light incident from the optical fiber 200 enters the spot size conversion core 103, the silicon fine wire in the detection region 130 passes through the tapered portion of the silicon fine wire core 105 as the incident light travels in the direction of the detection region 130. The light distribution moves to the core 105.
As described above, by connecting the silicon thin wire core 105 to the spot size conversion cores 103 and 104 by the tapered portion, it is possible to realize highly efficient mode field size conversion.

Therefore, if the optical fibers 200 and 300 are connected to the end portions of the spot size conversion cores 103 and 104, light can be efficiently incident on the optical waveguide formed of the silicon fine wire core 105 constituting the detection region 130. In addition, light can be efficiently coupled from the optical waveguide in the detection region 130 to the optical fiber.
The buried oxide layer 112 serving as the lower clad may be thinner than 3 μm. However, since the light leakage to the substrate part 111 side increases and the light propagation loss increases, it is better not to make it too thin. Good.

  In the configuration described above, when the sample to be analyzed is in contact with the upper surface of the silicon fine wire core 105 exposed in the detection region 130 in a state where light such as infrared light is guided in the waveguide made of the silicon fine wire core 105, Since the light oozing out from the silicon fine wire core 105 is absorbed according to the characteristics of the sample, the intensity of the guided light is reduced according to the intensity of the absorption. Therefore, for example, if the intensity of light guided through a waveguide composed of the silicon thin wire core 105 is measured with respect to a certain wavelength band, an absorption spectrum by the sample to be analyzed can be obtained.

  As described above, since the silicon fine wire core 105 has a cross-sectional dimension of about 500 × 150 nm, light having a wavelength of 1.2 to 1.7 μm propagates in a single mode through the waveguide. In other words, the silicon thin wire core 105 only needs to be formed in such a dimension that the waveguide formed therefrom becomes a single mode. Note that the silicon core 105 only needs to be flat and may be, for example, 400 nm wide and 200 nm high. In addition, by setting the core dimensions to a higher (thick) state such as a width of 500 nm and a height of 300 nm, it is possible to propagate light up to a wavelength of 3 μm in a single mode.

  In addition, this waveguide can change the waveguide direction with a very small curvature, ie, a minimum bending radius of about 100 μm or less. Accordingly, as shown in the plan view of FIG. 1, the silicon fine wire core 105 can be reciprocated at a narrow interval, and the region in contact with the sample can be made longer in the narrow detection region 130. It becomes. For example, with the configuration shown in FIG. 1, it is possible to arrange a sensor head made of a waveguide having a length of 100 mm in a sensor area of 5 mm square, and a longer contact distance with the sample to be analyzed. A small and highly sensitive sensor can be obtained.

In addition, in the optical waveguide sensor shown in FIG. 1, since the cross section of the silicon fine wire core 105 has a flat shape, a highly sensitive analysis is possible as described below.
When the cross section of the silicon thin wire core 105 is a flat quadrangle (h <w) having a width w wider than the height h, the confinement of light in the vertical direction is weakened, and the light travels in the waveguide. The amount of light that oozes out on the upper surface of the silicon fine wire core 105 increases. As a result, the amount of absorption by the sample to be analyzed in contact with the upper surface of the silicon fine wire core 105 can be increased, and the measurement sensitivity can be improved.

  FIG. 5 shows an example of urea analysis (detection) using the optical waveguide sensor. In FIG. 5, the result of the optical waveguide sensor composed of a silicon fine wire core having a width of 400 nm and a height of 200 nm is indicated by a black circle, and the result of the optical waveguide sensor comprising a silicon fine wire core having a width of 500 nm and a height of 150 nm is indicated by a white circle. In FIG. 5, the horizontal axis indicates the thickness of urea attached to the detection region.

As is apparent from FIG. 5, the absorption loss increases with the thickness of the adhering urea. The increase in absorption loss is greater for the white circles. The black circle indicates that the increase in loss is saturated when the urea thickness is approximately 200 nm, and the white circle indicates that the increase in loss is saturated when the urea thickness is approximately 500 nm.
As a result, the optical waveguide sensor composed of a silicon fine wire core with a width of 500 nm and a height of 150 nm with a large flatness of the core has a greater intensity and distance of light propagating through the silicon fine wire core in the detection region. It is larger than that. As is clear from this, it is understood that the silicon thin wire core should have a higher flatness within a range where a single mode can be obtained.

Further, from the above results, the distance that the light propagating through the silicon fine wire core oozes out to the upper part of the silicon fine wire core is about several hundred nm, and the silicon oxide film 106 is formed on the upper surface of the silicon fine wire core 105. Even if it does, it turns out that there is not much influence on detection.
The result of FIG. 5 shows that the greater the flatness (w / h), the greater the amount of light that oozes out to the upper part of the core, and the improvement in sensitivity as a sensor can be expected.

  However, if the height h of the core is reduced to be too flat, the confinement of light into the waveguide becomes weak, the core cannot be bent sharply, and the sensor head cannot be reduced in size. For example, if the height h of the core is smaller than 1/5 of the width w, the bending radius of the core needs to be 150 μm or more, making it impossible to reciprocate the core more frequently at a narrow interval, thereby reducing the size. Will interfere. Therefore, the core cross section should be in a state where the height h is greater than one fifth of the width w (w / 5 <h). In terms of sensitivity, it is better that h <w / 2.

Since silicon has a property of repelling water, if the measurement sample contains water, the sample is difficult to adhere to the silicon fine wire core. On the other hand, by providing the silicon oxide film 106 on the surface of the silicon fine wire core 105, the adhesion of the water-soluble sample can be improved.
In addition, by providing a functional film that selectively adsorbs and concentrates a substance to be detected in the detection region 130 constituted by the silicon fine wire core 105, it is possible to further improve the detection sensitivity of the substance. .

Next, a method for manufacturing the above-described optical waveguide sensor will be briefly described.
First, an SOI (Silicon on Insulator) substrate is prepared. The SOI substrate is, for example,
A silicon layer 113 made of single crystal silicon is provided on a buried oxide layer 112 having a thickness of about 3 to 10 μm.

When the SOI substrate as described above is prepared, a silicon oxide layer 601 is formed on the silicon layer 113 as shown in FIG. The silicon oxide layer 601 can be formed by, for example, a known plasma CVD method. Here, the silicon oxide layer 601 is formed in such a thickness that the film thickness of about 20 nm remains in the selective etching process of the silicon layer 113 described below.
Next, as shown in FIG. 6B, a resist pattern 602 is formed by a known lithography technique. The shape of the resist pattern 602 is the same as that of the silicon fine wire core 105 shown in FIG.

Next, the silicon oxide layer 601 is selectively etched using the resist pattern 602 as a mask, and then the resist pattern 602 is removed, so that the silicon oxide pattern is formed on the silicon layer 113 as shown in FIG. 603 is formed.
Next, for example, the silicon layer 113 is selectively etched by dry etching using the silicon oxide pattern 603 as a mask, and as shown in FIG. 6D, the silicon oxide film 106 is formed together with the silicon fine wire core 105. And By the etching for forming the silicon fine wire core 105, the silicon oxide pattern 603 is also etched and thinned to some extent, and the thin silicon oxide film 106 on the upper surface of the silicon fine wire core 105 is formed.

Next, a silicon oxynitride film covering the silicon fine wire core 105 in the mode field size conversion region 120 is formed by selective deposition using a stencil mask by ECR plasma CVD, for example. For example, a SiON film having a refractive index larger than silicon oxide (1.45) and smaller than silicon nitride (2.0) may be deposited by an ECR plasma method using SiH ₄ , O ₂ , and N ₂ gas. it can. The ECR plasma method is suitable for forming a SiON film because the deposition rate is as high as about 150 nm / min. After the SiON film is deposited, it is finely processed by a known lithography technique and etching technique to form a spot size conversion core 103 as shown in FIG. Although not shown, the spot size conversion core 104 is also formed at the same time.

  Next, in the mode field size conversion region 120, silicon oxide is selectively deposited so as to cover the spot size conversion cores 103 and 104, and the upper cladding layer 107 is formed as shown in FIG. And The silicon oxide to be the upper cladding layer 107 is also deposited by selective deposition using a stencil mask by ECR plasma CVD. Thereafter, a mask pattern that covers the detection region 130 and the mode field size conversion region 120 is formed, and etching is performed using the mask pattern as a mask to form a vertical incident end surface and an output end surface of the mode field size conversion region 120, Further, the buried oxide layer 113 in the fiber fixing region is selectively removed. In this etching, a dry etching technique having vertical anisotropy such as reactive ion etching may be used.

  Finally, V-shaped grooves 101 and 102 are formed in a predetermined region of the exposed substrate portion 111 in the fiber fixing region. These grooves can be formed, for example, by repeating selective etching with a plurality of mask patterns in which the interval between the openings is gradually widened. Further, if (111) plane single crystal silicon is used as the substrate portion 111, a V-shaped groove can be automatically formed by wet etching with an alkaline solution such as potassium hydroxide.

  As described above, by using the silicon oxide pattern 603 as an etching mask for the silicon layer 113 and adjusting the thickness so that it remains about 20 nm after etching, the silicon oxide film 106 for improving the adhesion of the measurement target substance. Can be formed simultaneously with the silicon fine wire core 105, which is efficient. Note that the silicon oxide pattern for the mask may be removed with hydrofluoric acid after the etching process, and a new silicon oxide film may be formed on the surface of the silicon fine wire core 105 by thermal oxidation or the like.

Further, by using SiON by plasma CVD for the spot size conversion core 103, the refractive index can be changed widely only by changing the ratio of N ₂ gas and O ₂ gas to be added, so that the detection region 130 is configured. Even if the size of the core material or the mode field conversion portion changes, it can be handled without changing the manufacturing method.
If a substrate in which silicon nitride is formed on silicon oxide serving as an underclad instead of an SOI substrate is prepared, an optical waveguide sensor having silicon nitride as a core can be manufactured by the same method as described above.

It is a top view which shows the structural example of the optical waveguide type sensor in embodiment of this invention. It is sectional drawing which shows typically the cross section of the AA 'line of FIG. It is sectional drawing which shows typically the cross section of the BB 'line | wire of FIG. It is sectional drawing which shows typically the cross section of CC 'line | wire of FIG. It is a correlation diagram showing the relationship between urea pressure and absorption loss in the detection of urea using an optical waveguide sensor. It is process drawing which shows the example of a manufacturing method of an optical waveguide type sensor. It is a block diagram which shows the structure of the conventional planar waveguide type sensor.

Explanation of symbols

DESCRIPTION OF SYMBOLS 101,102 ... Groove, 103,104 ... Spot size conversion core, 105 ... Silicon fine wire core, 106 ... Silicon oxide film, 107 ... Upper clad layer, 111 ... Substrate part, 112 ... Embedded oxide layer, 120 ... Mode field size conversion Area, 130 ... detection area, 200, 300 ... optical fiber, 201, 301 ... optical fiber core.

Claims (8)

A lower cladding layer;
A core made of a material including silicon, formed on the lower cladding layer, and having a width in the planar direction of the lower cladding layer larger than a height in a normal direction of the plane of the lower cladding layer;
A light incident end of a waveguide composed of the lower cladding layer and the core;
A light exit end of the waveguide ;
A first mode field conversion unit arranged continuously at the light incident end to reduce the diameter of incident light;
A second mode field converter that is arranged continuously at the light exit end and increases the diameter of the light emitted from the light exit end ;
The waveguide is a single mode waveguide,
A detection region is formed by a part of the core ,
The first and second mode field conversion units are:
A spot size conversion core covering the core;
Upper clad layer covering the spot size conversion core
And consists of
In the region covered with the spot size conversion core, the core is formed narrower as it is closer to the end,
An optical waveguide sensor characterized in that a refractive index of the spot size conversion core is smaller than that of the core and larger than that of the upper cladding layer . The optical waveguide sensor according to claim 1, wherein
The optical waveguide sensor, wherein the height of the core is greater than 1/5 of the width of the core. The optical waveguide sensor according to claim 1 or 2,
An optical waveguide sensor comprising: a silicon oxide film having a thickness smaller than a wavelength of light propagating through the core formed on the surface of the core in the detection region. On the lower clad layer made of a material containing silicon, in the detection region rot magnitude than the normal direction of the height of the plane of the width of the plane direction of the lower clad layer is the lower cladding layer, successively to light input and output end And a step of forming a tapered core in which the width becomes narrower as it is closer to the end in the predetermined region ,
A step of forming a spot size conversion core covering the core of the predetermined region; and
By forming an upper cladding layer that covers the spot size conversion core , a mode field conversion unit is formed in the predetermined region that is continuous with the light incident / exit end of the waveguide constituted by the lower cladding layer and the core. A method for manufacturing an optical waveguide sensor, comprising: In the manufacturing method of the optical waveguide type sensor according to claim 4,
A step of forming the spot size conversion core made of silicon oxynitride so as to selectively cover a portion to be the mode field conversion portion of the core;
Forming the upper cladding comprising the said spot size conversion Core portion serving as the mode field conversion portion of the core of silicon oxide to selectively cover, as engineering into a state in which the mode field conversion portion are formed And a method of manufacturing an optical waveguide sensor. In the manufacturing method of the optical waveguide type sensor according to claim 4,
Including a step of processing a film made of a material containing silicon provided on the lower clad layer by selective etching using a mask made of silicon oxide to form the core. A method for manufacturing an optical waveguide sensor. In the manufacturing method of the optical waveguide type sensor according to claim 6,
A film made of a material containing silicon provided on the lower cladding layer is processed by selective etching using a mask made of silicon oxide to form the core, and the upper surface of the core And a step of forming a silicon oxide film that is thinner than the wavelength of light propagating through the core made of the mask. In the manufacturing method of the optical waveguide type sensor according to claim 4,
A method of manufacturing an optical waveguide sensor, comprising: forming a silicon oxide film thinner than a wavelength of light propagating through the core by a thermal oxidation method on the surface of the core.

Images