CN109870769B - Method for preparing silicon dioxide optical micro-disc cavity by dry etching - Google Patents

Abstract

The present invention provides a method for preparing a silicon dioxide optical micro-disk cavity by dry etching, the method comprising the following steps: preparing a silicon dioxide layer on the surface of a silicon substrate by a thermal oxidation method, depositing an isolation layer on the surface of the silicon dioxide layer, and coating a photoresist layer on the surface of the isolation layer; transferring a mask pattern to the photoresist layer by exposure and development, etching the silicon dioxide layer with the photoresist layer as a template, and etching the silicon substrate with _XeF2 after degumming to obtain the silicon dioxide optical micro-disk cavity. The method is fully compatible with existing semiconductor processes, and the prepared silicon dioxide optical micro-disk cavity has a large size and an ultra-high quality factor.

Description

A method for preparing silicon dioxide optical microdisk cavity by dry etching

Technical Field

The invention belongs to the field of optical materials, and relates to a method for preparing a silicon dioxide optical micro-disk cavity, and in particular to a method for preparing a silicon dioxide optical micro-disk cavity by dry etching.

Background Art

According to different shapes, silica whispering gallery mode optical microcavities can be divided into three types: microdisk cavity, microring core microcavity, and microsphere cavity. Due to the extremely low loss of silica in the communication band, silica whispering gallery mode microcavities have the advantages of high quality factor and small mode volume. These advantages make silica whispering gallery mode microcavities an excellent platform for basic scientific research. In addition, silica whispering gallery mode microcavities also have good application prospects in the fields of sensor detection and optical communication.

In 2003, the Vahala group invented a method for preparing micro-ring core microcavities, which raised the quality factor to the order of 10 ^8. The preparation method is as follows: Photolithography: A silicon wafer is selected, and a silicon dioxide layer of the required thickness is grown on it by thermal oxidation. Spin-coating photoresist is used for photolithography to obtain a circular photoresist pattern; HF (hydrofluoric acid) etching: Using photoresist as a mask, hydrofluoric acid etches silicon dioxide, and after debonding, a silicon dioxide disk is obtained; XeF ₂ (xenon difluoride) etching: Xenon difluoride etches silicon to form a silicon column, and silicon dioxide is suspended on the silicon column to form a silicon dioxide microdisk cavity; Laser reflux: Using a carbon dioxide laser to irradiate the silicon dioxide microdisk cavity, the silicon dioxide melts and shrinks inward due to heat, and forms a micro-ring core microcavity structure under the action of surface tension. Due to the atomic-level roughness of the surface, the quality factor of the micro-ring core microcavity can reach more than 10 ^8. Similar microsphere cavities also have the same preparation scheme. This type of solution has very serious disadvantages because carbon dioxide laser reflux is uncontrollable and the size of the prepared microcavity cannot be precisely controlled.

In 2012, the Vahala group optimized the process and raised the quality factor of the microdisk cavity to a record high of more than 10 ^8. The characteristics of their method are: (1) the thickness of the silicon dioxide is large and the diameter is also very large, so the optical mode is less exposed to the air medium, thereby reducing the loss and improving the quality factor; (2) by increasing the etching time of hydrofluoric acid, the multi-angle phenomenon of the silicon dioxide side wall during etching is eliminated, the roughness of the side wall is reduced, and the quality factor is improved. It should be noted that since hydrofluoric acid etches silicon dioxide is isotropic, after extending the etching time, the lateral etching of silicon dioxide is very serious, making the diameter of the silicon dioxide microdisk smaller than the size of the image designed on the mask, which is not conducive to the precise control of the size of the microdisk cavity.

In 2015, Professor Jiang Xiaoshun's group at Nanjing University used reactive ion etching to replace traditional hydrofluoric acid etching to prepare high-quality factor silica microdisk cavities. Since plasma etching has very good directionality, the size of the prepared silica microdisk is more consistent with the pattern designed by the mask, and the precise control of the size is also beneficial to research in other fields. It should be noted that this method could only be applied to the preparation of small diameter (80μm) silica microdisk cavities at that time.

For larger-sized micro-disk cavities, there are two difficulties in their preparation: (1) It is difficult to ensure the smoothness of the pattern around the large-diameter photoresist pattern produced by photolithography. Once there are defects around the photoresist, the defects on the photoresist will be transferred to the underlying silicon dioxide layer during the subsequent dry etching. These defects will greatly affect the quality factor of the micro-disk cavity. (2) For samples with greater thickness, the photoresist will denature due to the long-term plasma bombardment of the photoresist. Traditional methods are difficult to completely remove it, and the residual photoresist will seriously restrict the quality factor of the micro-disk cavity.

Summary of the invention

In view of the problems existing in the prior art, the present invention provides a method for preparing a silicon dioxide optical micro-disk cavity by dry etching. The method is fully compatible with existing semiconductor processes, and the prepared silicon dioxide optical micro-disk cavity has a large size and an ultra-high quality factor.

To achieve the above object, the present invention adopts the following technical solutions:

The present invention provides a method for preparing a silicon dioxide optical micro-disk cavity by dry etching, the method comprising the following steps:

A silicon dioxide layer is prepared on the surface of a silicon substrate by a thermal oxidation method, an isolation layer is deposited on the surface of the silicon dioxide layer and annealed, and a photoresist layer is coated on the surface of the isolation layer;

The mask pattern is transferred to the photoresist layer through exposure and development, and the silicon dioxide layer is etched using the photoresist layer as a template. After de-bonding, the silicon substrate is etched using _XeF2 to obtain the silicon dioxide optical micro-disk cavity.

As a preferred technical solution of the present invention, the thickness of the silicon dioxide layer is 1 to 5 μm, such as 1 μm, 1.5 μm, 2 μm, 2.5 μm, 3 μm, 3.5 μm, 4 μm, 4.5 μm or 5 μm, but is not limited to the listed values, and other unlisted values within the numerical range are also applicable.

As a preferred technical solution of the present invention, the isolation layer is an amorphous silicon layer or a silicon nitride layer.

As a preferred technical solution of the present invention, the thickness of the isolation layer is 50 to 200 nm, such as 50 nm, 60 nm, 70 nm, 80 nm, 90 nm, 100 nm, 110 nm, 120 nm, 130 nm, 140 nm, 150 nm, 160 nm, 170 nm, 180 nm, 190 nm or 200 nm, but is not limited to the listed values, and other unlisted values within the numerical range are also applicable.

As a preferred technical solution of the present invention, the method for depositing the isolation layer is plasma gas enhanced chemical vapor deposition.

As a preferred technical solution of the present invention, the temperature for annealing the amorphous silicon layer is 600-800°C, such as 600°C, 620°C, 650°C, 680°C, 700°C, 720°C, 750°C, 780°C or 800°C, but is not limited to the listed values, and other unlisted values within the numerical range are also applicable.

Preferably, the annealing time for the amorphous silicon layer is 4 to 6 hours, such as 4h, 4.2h, 4.5h, 4.8h, 5h, 5.2h, 5.5h, 5.8h or 6h, but is not limited to the listed values, and other unlisted values within the numerical range are also applicable.

Preferably, the temperature for annealing the silicon nitride layer is 1000-1200°C, such as 1000°C, 1020°C, 1050°C, 1080°C, 1100°C, 1120°C, 1150°C, 1180°C or 1200°C, but is not limited to the listed values, and other unlisted values within the numerical range are also applicable.

Preferably, the annealing time for the silicon nitride layer is 2 to 3 hours, such as 2h, 2.1h, 2.2h, 2.3h, 2.4h, 2.5h, 2.6h, 2.7h, 2.8h, 2.9h or 3h, but is not limited to the listed values, and other unlisted values within the numerical range are also applicable.

In the present invention, since the film obtained by plasma enhanced chemical vapor deposition is of low quality, has many defects, and has large internal stress, it will eventually affect the underlying silicon dioxide layer during inductively coupled plasma etching, thereby reducing the quality factor of the sample. Therefore, before photolithography, the silicon wafer needs to be annealed at high temperature.

As a preferred technical solution of the present invention, the thickness of the photoresist layer is 4 to 6 μm, such as 4 μm, 4.2 μm, 4.5 μm, 4.8 μm, 5 μm, 5.2 μm, 5.5 μm, 5.8 μm or 6 μm, but is not limited to the listed values, and other unlisted values within the numerical range are also applicable.

As a preferred technical solution of the present invention, the method for etching the silicon dioxide layer is inductively coupled plasma etching.

As a preferred technical solution of the present invention, the gases used in the inductively coupled plasma etching are CF ₄ and SF ₆ .

In the present invention, the amorphous silicon layer or silicon nitride layer deposited by plasma chemical vapor is always retained in the photolithography-etching process, and is only removed together in the last step of etching the silicon substrate with xenon difluoride to form silicon pillars, thereby always maintaining the isolation state of silicon dioxide and photoresist, and the removal of amorphous silicon and silicon nitride does not add additional steps and will not damage the underlying silicon dioxide film.

In the present invention, in the process of photolithography of photoresist, since amorphous silicon and silicon nitride are opaque to ultraviolet light, light will not reach the silicon dioxide layer below, thus avoiding the generation of thin film interference phenomenon, and no standing wave fringes will appear at the bottom of the photoresist, thereby improving the roughness of the photoresist. In the process of photolithography and etching of the silicon dioxide layer, due to the presence of the amorphous silicon or silicon nitride layer, silicon dioxide and the photoresist are not in direct contact. Theoretically, the photoresist has zero residue on the silicon dioxide, thus ensuring that the sample will not be affected by the photoresist, thereby ensuring the quality factor. At the same time, the amorphous silicon or silicon nitride layer can be removed in the subsequent xenon difluoride etching, and the amorphous silicon or silicon nitride layer is removed without introducing unnecessary steps, which will not affect the quality of the silicon dioxide layer.

As a preferred technical solution of the present invention, the method for preparing a silicon dioxide optical micro-disk cavity by dry etching comprises the following steps:

A 1-5 μm thick silicon dioxide layer is prepared on the surface of a silicon substrate by a thermal oxidation method, a 50-200 nm thick isolation layer is deposited on the surface of the silicon dioxide layer by plasma gas enhanced chemical vapor deposition, the isolation layer is an amorphous silicon layer or a silicon nitride layer, the isolation layer is annealed, and after annealing, a 4-6 μm thick photoresist layer is coated on the surface of the isolation layer, the amorphous silicon layer is annealed at a temperature of 600-800° C. for 4-6 hours; the silicon nitride layer is annealed at a temperature of 1000-1200° C. for 2-3 hours;

The mask pattern is transferred to the photoresist layer through exposure and development. The photoresist is used as a template and the silicon dioxide layer is etched by using _CF4 and _SF6 inductively coupled plasma. After de-bonding, the silicon substrate is etched by using XeF2 to obtain the silicon dioxide optical micro-disk cavity.

Compared with the prior art solutions, the present invention has at least the following beneficial effects:

(1) The present invention provides a method for preparing a silicon dioxide optical micro-disk cavity by dry etching. The size of the silicon dioxide optical micro-disk cavity prepared by the method can reach 1000 to 3000 μm, which is more than 10 times higher than that of the prior art.

(2) The present invention provides a method for preparing a silicon dioxide optical micro-disk cavity by dry etching. The quality factor of the silicon dioxide optical micro-disk cavity prepared by the method can reach 6.83×10 ⁷ , which is 2 to 3 times higher than that of the prior art.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG1 is a schematic flow diagram of a method for preparing a silicon dioxide optical micro-disk cavity by dry etching provided by the present invention;

FIG2a is a top view of a silicon dioxide optical micro-disk cavity prepared by the present invention;

FIG2b is a side view of a silica optical microdisk cavity prepared by the present invention;

FIG3a is a schematic structural diagram of a product intermediate after a silicon dioxide layer is prepared on a silicon substrate surface by a thermal oxidation method in the present invention;

FIG3 b is a schematic structural diagram of an intermediate product of a plasma enhanced chemical vapor deposition amorphous silicon layer or silicon nitride layer in the present invention;

FIG3c is a schematic diagram of the structure of a product intermediate after coating with photoresist in the present invention;

FIG3 d is a schematic diagram of a photoresist layer exposure step in the present invention;

FIG3e is a schematic structural diagram of a product intermediate after the photoresist layer is developed in the present invention;

FIG3f is a schematic structural diagram of a product intermediate after inductively coupled plasma etching of a silicon dioxide layer in the present invention;

FIG3g is a schematic diagram of the structure of the product intermediate after degumming in the present invention;

FIG3h is a schematic diagram of a product after xenon difluoride etching a silicon substrate according to the present invention;

FIG4 is a graph showing the effects of undeposited film, deposited amorphous silicon film, and deposited silicon nitride film on the quality factor of a silicon dioxide optical micro-disk cavity;

FIG5 is a schematic diagram of coupling between a tapered optical fiber and a sample in a quality factor testing method of the present invention;

FIG. 6 is a schematic diagram of a quality factor testing device in the present invention.

The present invention is further described in detail below. However, the following examples are only simplified examples of the present invention and do not represent or limit the scope of protection of the present invention. The scope of protection of the present invention shall be subject to the claims.

DETAILED DESCRIPTION

To better illustrate the present invention and facilitate understanding of the technical solution of the present invention, typical but non-limiting embodiments of the present invention are as follows:

Example 1

This embodiment provides a method for preparing a silicon dioxide optical micro-disk cavity by dry etching, comprising the following steps:

A 4 μm thick silicon dioxide layer is prepared on the surface of a silicon substrate by a thermal oxidation method, the silicon dioxide layer is cleaned, hexamethyldisilazane is coated on the surface of the silicon dioxide layer, a 50 nm thick amorphous silicon layer is deposited on the surface of the silicon dioxide layer by plasma gas enhanced chemical vapor deposition, the obtained isolation layer is annealed, and a 4 μm thick photoresist layer is coated on the surface of the isolation layer after annealing, and the temperature of the annealing treatment of the amorphous silicon layer is 600° C. and the time is 6 hours;

The mask pattern is transferred to the photoresist layer through exposure and development. The photoresist layer is used as a template to etch the silicon dioxide layer using _CF4 and _SF6 inductively coupled plasma. After debonding, the silicon substrate is etched using _XeF2 to obtain a silicon dioxide optical micro-disk cavity with a radius of 1000 μm.

Example 2

This embodiment provides a method for preparing a silicon dioxide optical micro-disk cavity by dry etching. The method has the same conditions as in Embodiment 1 except that the isolation layer is a silicon nitride layer. The silicon nitride layer is annealed at a temperature of 1000° C. for 3 hours.

Example 3

This embodiment provides a method for preparing a silicon dioxide optical micro-disk cavity by dry etching, comprising the following steps:

A 4 μm thick silicon dioxide layer is prepared on the surface of a silicon substrate by a thermal oxidation method, the silicon dioxide layer is cleaned, hexamethyldisilazane is coated on the surface of the silicon dioxide layer, a 200 nm thick amorphous silicon layer is deposited on the surface of the silicon dioxide layer by plasma gas enhanced chemical vapor deposition, the obtained isolation layer is annealed, and a 6 μm thick photoresist layer is coated on the surface of the isolation layer after annealing, and the temperature of the annealing treatment of the amorphous silicon layer is 800° C. and the time is 4 hours;

The mask pattern is transferred to the photoresist layer through exposure and development. The photoresist layer is used as a template to etch the silicon dioxide layer using _CF4 and _SF6 inductively coupled plasma. After debonding, the silicon substrate is etched using _XeF2 to obtain a silicon dioxide optical micro-disk cavity with a radius of 3000 μm.

Example 4

This embodiment provides a method for preparing a silicon dioxide optical micro-disk cavity by dry etching. The method has the same conditions as those in Embodiment 3 except that the isolation layer is a silicon nitride layer. The silicon nitride layer is annealed at a temperature of 1200° C. for 2 hours.

Comparative Example 1

This comparative example provides a method for preparing a silicon dioxide optical micro-disk cavity by dry etching. Except that the amorphous silicon layer is not deposited, other conditions of the method are the same as those of Example 1.

Comparative Example 2

This comparative example provides a method for preparing a silicon dioxide optical micro-disk cavity by dry etching. Except that the amorphous silicon layer is not deposited, other conditions of the method are the same as those of Example 3.

The photoresist used in Examples 1-6 and Comparative Examples 1 and 2 in the specific implementation manner of the present invention is photoresist AZ6130.

The quality factors of the silica optical micro-disk cavities prepared in Examples 1-4 and Comparative Examples 1 and 2 were tested. The test results are shown in Table 1.

Methods for testing quality factor:

First, prepare a tapered optical fiber, strip off the optical fiber cladding, fix both ends of the optical fiber on a stepper motor, and then use a hydrogen flame to heat the optical fiber. During the heating process, the optical fiber is continuously stretched until the diameter of the optical fiber is stretched to about 1 to 3 μm.

Connect the tapered fiber to the optical path, place the sample on a three-dimensional translation stage, and adjust the position of the fiber and the sample until the evanescent field of the fiber is coupled into the sample. The schematic diagram of the tapered fiber and sample coupling is shown in Figure 5. The corresponding optical mode can be observed on the oscilloscope, and the optical quality factor can be obtained by mathematical fitting later. The structure of the test device is shown in Figure 6.

Table 1

Quality Factor Example 1 4.15×10 ⁷ Example 2 6.83×10 ⁷ Comparative Example 1 2.36×10 ⁷ Example 3 5.61×10 ⁷ Example 4 4.23×10 ⁷ Comparative Example 2 2.89×10 ⁷

From the test results in Table 1, it can be seen that the quality factor of the silica optical micro-disk cavity with a diameter of 1000 μm provided by Examples 1 and 2 of the present invention is 2 to 3 times the quality factor of the silica optical micro-disk cavity prepared by the preparation method without depositing an isolation layer in Comparative Example 1; similarly, the quality factor of the silica optical micro-disk cavity with a diameter of 3000 μm provided by Examples 3 and 4 is about 2 times the quality factor of the silica optical micro-disk cavity prepared by the preparation method without depositing an isolation layer in Comparative Example 2.

The applicant declares that the present invention illustrates the detailed structural features of the present invention through the above-mentioned embodiments, but the present invention is not limited to the above-mentioned detailed structural features, that is, it does not mean that the present invention must rely on the above-mentioned detailed structural features to be implemented. Those skilled in the art should understand that any improvement of the present invention, equivalent replacement of the components selected by the present invention, addition of auxiliary components, selection of specific methods, etc., all fall within the protection scope and disclosure scope of the present invention.

The preferred embodiments of the present invention are described in detail above. However, the present invention is not limited to the specific details in the above embodiments. Within the technical concept of the present invention, a variety of simple modifications can be made to the technical solution of the present invention, and these simple modifications all belong to the protection scope of the present invention.

It should also be noted that the various specific technical features described in the above specific embodiments can be combined in any suitable manner without contradiction. In order to avoid unnecessary repetition, the present invention will not further describe various possible combinations.

In addition, various embodiments of the present invention may be arbitrarily combined, and as long as they do not violate the concept of the present invention, they should also be regarded as the contents disclosed by the present invention.

Claims (7)

1. A method for preparing a silicon dioxide optical microdisk cavity by dry etching, characterized in that the method comprises the following steps: A silicon dioxide layer is prepared on the surface of a silicon substrate by a thermal oxidation method, an isolation layer is deposited on the surface of the silicon dioxide layer and annealed, and a photoresist layer is coated on the surface of the isolation layer; The isolation layer is an amorphous silicon layer or a silicon nitride layer, and the method for depositing the isolation layer is plasma gas enhanced chemical vapor deposition; Transferring the mask pattern to the photoresist layer through exposure and development, etching the silicon dioxide layer using the photoresist layer as a template, and etching the silicon substrate using _XeF2 after de-bonding to obtain the silicon dioxide optical micro-disk cavity; The method for etching the silicon dioxide layer is inductively coupled plasma etching; The thickness of the silicon dioxide layer is 1-5 μm, the thickness of the isolation layer is 50-200 nm, and the thickness of the photoresist layer is 4-6 μm; The size of the silica optical micro-disk cavity is 1000-3000 μm, and the quality factor of the silica optical micro-disk cavity is 6.83×10 ⁷ at most. 2 . The method according to claim 1 , wherein the temperature of annealing the amorphous silicon layer is 600-800° C. 3. The method according to claim 1, characterized in that the annealing time of the amorphous silicon layer is 4 to 6 hours. 4 . The method according to claim 1 , wherein the temperature of annealing the silicon nitride layer is 1000-1200° C. 5. The method according to claim 1, characterized in that the annealing time of the silicon nitride layer is 2 to 3 hours. 6. The method according to any one of claims 1, characterized in that the gases used in the inductively coupled plasma etching are _CF4 and _SF6 . 7. The method according to any one of claims 1 to 6, characterized in that the method comprises the following steps: A 1-5 μm thick silicon dioxide layer is prepared on the surface of a silicon substrate by a thermal oxidation method, a 50-200 nm thick isolation layer is deposited on the surface of the silicon dioxide layer by plasma gas enhanced chemical vapor deposition, the isolation layer is an amorphous silicon layer or a silicon nitride layer, the isolation layer is annealed, and after annealing, a 4-6 μm thick photoresist layer is coated on the surface of the isolation layer, the amorphous silicon layer is annealed at a temperature of 600-800° C. for 4-6 hours; the silicon nitride layer is annealed at a temperature of 1000-1200° C. for 2-3 hours; The mask pattern is transferred to the photoresist layer through exposure and development. The photoresist layer is used as a template to etch the silicon dioxide layer using _CF4 and _SF6 inductively coupled plasma. After debonding, the silicon substrate is etched using _XeF2 to obtain the silicon dioxide optical micro-disk cavity.