CN113247859A

CN113247859A - Method for preparing crack type nano gap structure based on femtosecond laser

Info

Publication number: CN113247859A
Application number: CN202110524022.8A
Authority: CN
Inventors: 姜澜; 周世鹏; 李晓炜; 高帅; 黄骥
Original assignee: Beijing Institute of Technology BIT
Current assignee: Beijing Institute of Technology BIT
Priority date: 2021-05-13
Filing date: 2021-05-13
Publication date: 2021-08-13
Anticipated expiration: 2041-05-13
Also published as: CN113247859B

Abstract

The invention relates to a femtosecond laser-based crack type nano gap structure preparation method, and belongs to the technical field of laser application. According to the invention, a traditional Gaussian femtosecond laser beam is shaped into a double-peak or multi-peak beam by an airspace shaping technology, and an amorphous silicon etching mask with a plurality of stress concentration structures is induced on a silicon surface based on the phenomena of amorphization of a femtosecond laser induced material and femtosecond laser induced shock waves. Such an etch mask with stress concentrating structures is critical for the formation of nano-crack propagation during wet etching. In the subsequent wet etching process, under the influence of solution undercutting, the etching mask with the stress concentration structure releases the stress due to being etched into a suspension state, and meanwhile, nano crack propagation is generated at the stress concentration part under the action of the surface tension of the solution, so that a nano gap structure with an extremely small size (below 10 nm) is formed.

Description

Method for preparing crack type nano gap structure based on femtosecond laser

Technical Field

The invention relates to a femtosecond laser-based crack type nano gap structure preparation method, and belongs to the technical field of laser application.

Background

The unique ability of the nano-gap structure to manipulate substances (photons, electrons, chemical molecules, biomass, etc.) in extremely narrow spaces makes it have unique and important application values in the fields of near-field optics, microelectronic circuits, biological or chemical sensing, etc. Especially, when the structure size of the nanometer gap reaches below 10nm, the structure size is equivalent to the size of a single molecule, and the method is expected to be applied to the preparation of molecular electrodes. At present, although the relatively mature preparation methods of the nano-structure, such as electron beam lithography, focused ion beam direct writing and the like, can realize the reliable preparation of the nano-gap structure, the processing cost is high, and the preparation of the structure below 10nm is still difficult. In order to further reduce the size of the nanometer gap, scholars at home and abroad develop a plurality of composite processing technologies, such as: mechanically controlled fracture joints, electrochemical deposition, flexible structure self-assembly, and the like. The mechanically controllable fracture joint is characterized in that a mechanical device applies acting force to a nanometer electrode pair which is prepared in advance and provided with a stress notch, so that the nanometer electrode pair is bent and deformed, finally, the joint is fractured to form a nanometer gap, and the size can be controlled to be several nanometers. However, this method requires the use of electron beam lithography to pre-process the pre-structure (which is costly) and is inefficient; electrochemical deposition can reduce the size of the gap by further growing materials such as metal on the gap structure with larger size, but an electrode structure with larger gap size still needs to be prepared in advance, and certain chemical pollution exists; the self-assembly mode has low processing cost, and the flexible structure which is originally far away in interval is deformed and close to form a nano gap by applying external stimulation (photo-thermal effect, solution surface tension and the like) to the prepared flexible nano column array structure (PDMS, PMMA and other materials). But the process is relatively complex and cumbersome. Therefore, it is very challenging to find an economical, efficient and reliable nano gap preparation method.

The strong nonlinearity of femtosecond lasers with materials, the non-equilibrium interaction process brings about many interesting phenomena (shock waves, plasma eruptions, crystal transitions, etc.). In the case of a single crystal silicon material, a crystalline state of the material is changed from crystalline silicon to amorphous silicon in a region irradiated by femtosecond laser. Amorphous silicon has a much different chemical activity to wet etching solutions than crystalline silicon. By utilizing the etching rate difference, researchers prepare various micro/nano structures (microwires, nanorings, nanodiscs and the like) on the silicon surface through femtosecond laser direct writing modification assisted chemical etching. However, in the femtosecond laser direct writing mode, due to the diffraction limit limitation, the size of the structure processed on the silicon surface is limited to the sub-wavelength level, and the preparation of small-size (tens of nanometers) nanometer gaps cannot be realized. And because the used light beam is the traditional Gaussian light beam, the prepared structure is relatively simple, and the complex function is difficult to realize. The femtosecond laser spatial shaping technology can realize the arbitrary modulation of the intensity distribution of the laser field. By utilizing the shaped light field, the maskless preparation of a complex structure, the parallel processing of a large-area array and even the breakthrough of the optical diffraction limit can be realized, and the result which is difficult to realize by the traditional Gaussian laser is obtained. The technology of space shaping femtosecond laser processing and chemical etching is combined, and a new idea for preparing the nano gap junction is provided for people.

Disclosure of Invention

The invention aims to solve the problems of high cost, low efficiency, complex process and the like of the traditional nano gap processing method and provides a method for preparing a crack type nano gap structure based on femtosecond laser.

The purpose of the invention is realized by the following technical scheme:

a method for preparing a crack type nanometer gap structure based on femtosecond laser comprises the steps of loading a shaping phase diagram on a liquid crystal spatial light modulator, and shaping conventional Gaussian femtosecond laser into a multi-peak beam. The laser energy is adjusted so that each adjacent laser intensity peak of the multi-peak beam induces an overlap of the formed amorphous silicon masks, thereby forming stress concentration notches at the mask interfaces. The stress induced by the shock wave induced by the femtosecond laser reaches a maximum at the interface gap. And then, releasing the stress of the amorphous silicon by using a wet etching method, wherein the amorphous silicon is broken at the gap due to the release of the stress, so that a nano gap structure is formed.

The method specifically comprises the following steps:

loading a phase diagram designed in advance on a liquid crystal spatial light modulator, and shaping incident linearly polarized Gaussian femtosecond laser into a double-peak beam or a multi-peak beam;

secondly, irradiating the monocrystalline silicon by using the shaped femtosecond laser to form an amorphous silicon etching mask with a stress concentration structure on the silicon surface;

cleaning the laser-treated sample by using a dilute hydrofluoric acid (HF) solution to remove a natural oxide layer, so as to avoid the influence of the natural oxide layer on the etching process;

step five, carrying out wet etching on the sample with the oxide layer removed in an alkaline etchant to obtain the required crack type nano gap structure

The beam shaping phase used on the spatial light modulator is expressed as:

wherein the content of the first and second substances,

is the phase of the corresponding point in polar coordinate space, theta is the polar angle,

n is the number of intensity peaks, which may be even.

A conventional circular gaussian femtosecond laser can only form a single amorphous silicon etch mask per pulse. The number of intensity peaks of the shaped multi-peak light beam can be flexibly controlled by using the air light modulator, so that the number of stress concentration gaps can be controlled according to actual requirements, and the number of nano gap structures generated after etching can be regulated and controlled. The relationship between the number of cracks formed m and the number of laser intensity peaks n is as follows:

in order to amorphize the femtosecond laser irradiation region material, the single pulse energy of the femtosecond laser used in the experiment was smaller than the ablation threshold of the single crystal silicon.

Because different crystal faces of monocrystalline silicon have different etching rates, in order to enable the highest etching rate difference between the surface layer amorphous silicon and the crystalline silicon, the upper surface of a processed monocrystalline silicon sample must be a 100 crystal face.

The alkaline etching solution is formed by mixing 25 mass percent of potassium hydroxide solution and isopropanol solution.

In order to obtain extremely small-sized cracks, the etching time of a monocrystalline silicon sample in an alkaline etchant is 30s, and the crack gap is increased due to the excessively long etching time.

Advantageous effects

1. The invention combines the amorphization of the space shaping femtosecond laser induced material with the chemical wet etching process, realizes the preparation of the silicon surface with high efficiency, no mask and controllable nanometer crack structure with extremely small size (below 10 nm), and is an economic, high-efficiency and reliable processing method. The silicon material is convenient to be compatible with the existing large-scale integrated circuit preparation process, and complex, expensive and time-consuming processes such as electron beam lithography and the like are not needed.

2. The generation principle of the nano gap in the invention is based on the crack propagation of the amorphous silicon mask at the stress concentration part in the wet etching process, and the crack propagation allows the preparation of the nano gap with the size of less than 10nm under the condition of using near infrared femtosecond laser (with the wavelength of 800nm), thereby greatly breaking through the limitation of optical diffraction limit (sub-wavelength). High-energy beams with high cost, strict requirements on processing environment, such as electron beams, ultraviolet light and the like are not needed.

3. The invention can adjust the mask size, the overlapping degree and the like by changing the shaping pulse shape, the NA value of the focusing objective lens, the laser energy, the pulse number, the chemistry and other processing parameters, thereby flexibly adjusting the quantity, the distribution and the length-width ratio of the nanometer gaps.

Drawings

Fig. 1 shows a schematic diagram of a processing light path device for realizing the shaping of a double-peak or multi-peak light beam based on a femtosecond laser space shaping technology, and a numerical simulation diagram of laser intensity distribution after shaping.

Fig. 2 shows a schematic process diagram of the formation of a crack-type nano gap electrode structure after chemical wet etching of the precursor structure with stress concentration.

FIG. 3 shows SEM characterization results of a crack type nano gap structure with a size of 9nm prepared by the method.

Wherein, the device comprises a 1-reflection type liquid crystal spatial light modulator, a 2-lens 1, 3-lens 2, 4-focusing objective lens and a 5-processing sample.

Detailed Description

The present invention will be described in further detail with reference to the accompanying drawings.

Referring to fig. 1, a method for preparing a crack type nano gap based on femtosecond laser. First, a linearly polarized femtosecond laser (center wavelength 800nm) is irradiated to a liquid crystal Spatial Light Modulator (SLM) at a small angle. In this example, the optical field intensity profile of the incident laser is transformed from a gaussian to a bimodal beam with the phase expressed as follows:

the spatially shaped femtosecond laser passes through two equal focal lengths (f)₁＝f₂500mm) optical lens to avoid diffraction distortion of the phase of the laser beam due to long-distance propagation. In the 4f system, the distance from the liquid crystal surface of the SLM to the lens 1 is ensured to be equal to the focal length of the lens, the distance between the lens 1 and the lens 2 is the sum of the focal lengths of the two lenses, and the distance between the lens 2 and the focusing objective lens is the focal length of the lens 2.

And then focused through a 50X (NA ═ 0.8) objective lens. Based on the principle of laser diffraction propagation, numerical calculation is performed on the intensity distribution of the focused shaped light field, and the result is shown in fig. 1 and 2.

The femtosecond laser single pulse energy is controlled by adjusting the energy density attenuation sheet, and the laser energy is controlled below the ablation threshold of the monocrystalline silicon material, so that an amorphous silicon etching mask is formed on the silicon surface in an induced mode. The sample selected in this case was single crystal silicon (100) and the laser single pulse energy was 15 nJ.

In this case, to ensure single pulse processing, we set the laser pulse repetition rate to 10Hz, i.e. 10 pulses per second are generated. And meanwhile, the opening time of the mechanical shutter is controlled to be 0.1s, so that only one laser pulse irradiates a sample at one time.

After laser irradiation, an amorphous silicon etching mask is formed on the surface of the monocrystalline silicon in an induced mode. The shaped bimodal beam would have two circular amorphous masks in close proximity where the laser surface is induced. Meanwhile, due to the expansion of the laser heat affected zone, the formed masks have a certain degree of overlap in the middle, so that stress concentration is formed at the overlapping part of the masks.

And then, etching the sample subjected to laser irradiation in an alkaline etching solution for 30s, wherein the etching time of the sample in an alkaline etching agent is not easy to overlong, and the increase of the etching time can cause the increase of crack gaps.

Finally obtaining the nano gap structure. In this example, the etching solution is composed of a 25% by mass potassium hydroxide solution and an isopropyl alcohol solution mixed in a volume ratio of 3: 1.

Fig. 2 shows a schematic diagram of a crack-type nanogap. After the irradiation of the space shaping femtosecond laser, the surface of the monocrystalline silicon is provided with an amorphous silicon etching mask with a stress concentration structure. In the wet etching process, the lower part of the stress concentration part of the mask is etched into a suspended state, and the internal stress generated by laser impact is released. Meanwhile, the etching mask generates cracks at the joints under the action of surface tension of the solution, and the size of the cracks can be adjusted by controlling the size of the mask using different focusing objectives.

Fig. 3 shows the structure of a crack type nano-gap, the gap size being about 9 nm. The experimental result proves the feasibility of forming the nano gap structure by the femtosecond laser impact stress release caused by wet etching and the crack of the surface amorphous silicon mask caused by the surface tension of the solution.

The above detailed description is intended to illustrate the objects, aspects and advantages of the present invention, and it should be understood that the above detailed description is only exemplary of the present invention and is not intended to limit the scope of the present invention, and any modifications, equivalents, improvements and the like made within the spirit and principle of the present invention should be included in the scope of the present invention.

Claims

1. A method for preparing a crack type nanometer gap structure based on femtosecond laser is characterized in that: loading a shaping phase diagram on a liquid crystal spatial light modulator, and shaping a conventional Gaussian femtosecond laser into a double-peak or multi-peak beam; adjusting laser energy to enable amorphous silicon masks formed by induction of every adjacent laser intensity peak of the multi-peak light beam to be overlapped, and accordingly forming stress concentration gaps at the mask junctions; stress introduced by shock waves generated by femtosecond laser induction can reach a maximum value at the notch; then, releasing the internal stress of the material by using a wet etching method, wherein the amorphous silicon is broken at the notch due to the release of the stress, so that a nano gap structure is formed; the method specifically comprises the following steps:

secondly, irradiating the monocrystalline silicon by using the shaped femtosecond laser to form an amorphous silicon etching mask with a plurality of stress concentration structures on the silicon surface;

cleaning the laser-treated sample by using a dilute hydrofluoric acid solution to remove the natural oxide layer, so as to avoid the influence of the natural oxide layer on the etching process;

and fifthly, performing wet etching on the sample with the oxide layer removed in an alkaline etchant for 30s, and inducing the amorphous silicon to generate cracks by utilizing stress release to obtain the nano gap structure with the extremely small size.

2. The femtosecond laser-based crack-type nano gap structure preparation method as set forth in claim 1, wherein: step one, the phase diagram is as follows:

wherein the content of the first and second substances,

the phase of the corresponding point in polar coordinate space is shown, theta is the polar angle, k is 0,1,2 … … n/2-1, n is the number of intensity peaks, and only an even number can be taken.

3. The femtosecond laser-based crack-type nano gap structure preparation method as set forth in claim 1, wherein: the number of nanocracks m depends on the number of multimodal beam intensity peaks n, and has the following relationship:

4. the femtosecond laser-based crack-type nano gap structure preparation method as set forth in claim 1, wherein: in order to amorphize the femtosecond laser irradiation region material, the single pulse energy of the femtosecond laser used in the experiment was smaller than the ablation threshold of the single crystal silicon.

5. The femtosecond laser-based crack-type nano gap structure preparation method as set forth in claim 1, wherein: and (3) the etching rates of different crystal faces of the monocrystalline silicon are different, and in order to enable the highest etching rate difference between the amorphous silicon and the crystalline silicon, the upper surface of the monocrystalline silicon sample in the second step must be a 100 crystal face.

6. The femtosecond laser-based crack-type nano gap structure preparation method as set forth in claim 1, wherein: and step five, mixing the etching solution with 25 mass percent of potassium hydroxide solution and isopropanol solution.

7. The femtosecond laser-based crack-type nano gap structure preparation method as set forth in claim 1, wherein: in order to obtain extremely small-sized cracks, the etching time of a monocrystalline silicon sample in an alkaline etchant is 30s, and the crack gap is increased due to the excessively long etching time.