CN114453770A - Method for double-pulse femtosecond laser slicing of SiC substrate - Google Patents

Abstract

The invention discloses a double-pulse femtosecond laser slicing and peeling method for SiC substrates. After the low-energy femtosecond laser double-pulse cutting, a "fast heat and cold-stripping" method is used to generate enough internal stress to further damage the laser cutting After the amorphous structure, the magnitude of the mechanical stress required for external exfoliation is reduced. The invention greatly reduces the thickness of the damage layer caused by the laser slicing process through the double-pulse laser slicing method, and avoids a large amount of waste of materials; at the same time, the "fast heating and cooling" method is used to greatly reduce the need for peeling after slicing. The magnitude of the external pulling force allows it to be easily peeled off; the method of the present invention has little material loss, short processing time, low cost and little pollution to the environment.

Description

A method of double-pulse femtosecond laser slicing of SiC substrate

technical field

The invention belongs to the technical field of semiconductors, and relates to the cutting of silicon carbide, in particular to a method for slicing a SiC substrate by double-pulse femtosecond laser.

Background technique

With the development of science and technology, people's requirements for electronic devices are getting higher and higher, and high temperature, high frequency, radiation resistance and high power have become the most basic needs. The third-generation semiconductor materials have the characteristics of wider band gap, higher thermal conductivity, higher radiation resistance, and larger electron saturation drift rate, and have important application value in the fields of optoelectronics and microelectronics. SiC is a typical representative of the third-generation semiconductor, but its hardness is very high, with a Mohs hardness of 9.5, second only to diamond (10), and it is not easy to cut. The existing SiC cutting methods mainly include free mortar cutting, diamond wire saw cutting and femtosecond laser cutting.

The main drawbacks of traditional free mortar cutting and diamond wire saw cutting are large material loss and long processing time. Due to the limitation of the size of the wire saw, this method can only cut wafers with larger thicknesses. In addition, the surface roughness and warpage of the diced wafer are large, and accompanied by a large number of surface and subsurface damages (brittle spalling, cracks, amorphization, phase transformation, dislocation proliferation, etc.), the thickness of the damaged layer exceeds 100μm. In order to remove these damages, subsequent processes such as thinning, rough grinding, fine grinding, and CMP are introduced, which not only increases costs, but also pollutes the environment. The loss of materials exceeds an astonishing 50%.

Patent document CN110549016A discloses a single-pulse femtosecond laser cutting method of silicon carbide. Through the multi-photon excitation principle of femtosecond laser, the SiC crystal structure at the laser focusing plane is destroyed, thereby achieving a Mohs hardness of 9.5. Precise cutting of SiC crystals. The method has the advantages of accurate cutting, material saving, simple process, no pollution, good repeatability and the like. However, the diced SiC wafers cannot be easily peeled off, and a large external mechanical stress is still required to dissociate them. Moreover, the thickness of the laser sliced damage layer after cutting is close to 100 μm, which is still relatively large. In addition, the laser single pulse energy required by this method is relatively high, and there is a greater cost pressure.

SUMMARY OF THE INVENTION

The purpose of the present invention is to solve the problems of large material loss, long processing time, high cost and serious environmental pollution in the traditional slicing method; Due to the defects such as too high and too large slice damage thickness, a double-pulse femtosecond laser slicing and peeling method of SiC substrate combined with the "heating-cooling" process is proposed. Cold-stripping" method to generate enough internal stress to further destroy the amorphous structure after laser cutting and reduce the magnitude of the mechanical stress required for external stripping.

In order to achieve the above object, the present invention adopts the following technical solutions:

A method for slicing a SiC substrate with double-pulse femtosecond laser, comprising the following steps:

1) Clean the silicon carbide wafer and fix it on the processing equipment;

2) Set the parameters of the femtosecond laser;

3) Set the optical path of the double-pulse laser, first manufacture the optical path with the optical path difference, and then gather them together to make the double-pulse optical path, and focus on the inside of the silicon carbide wafer;

4) Mobile processing equipment, using dual-pulse femtosecond laser to precisely cut silicon carbide wafers;

5) Step 4) The cut silicon carbide wafer is heated in a protective atmosphere, taken out and cooled rapidly, and the silicon carbide wafer is dissociated by using a tension device.

In the present invention, the present invention utilizes a femtosecond laser with a specific energy, combined with a beam splitter and a reflector, to first manufacture two optical paths with a certain optical path difference, and then converge the two optical paths with a picosecond optical path difference. Together, a double-pulse optical path is made, which is finally focused inside the silicon carbide wafer for a single scan. Since the "blackened" structure caused by the first pulse will absorb the energy of the second pulse, the thickness of the damage layer after double-pulse laser cutting is much smaller than that of single-pulse laser cutting. After the double-pulse laser cutting process is completed, the silicon carbide wafer is subjected to high-temperature treatment in an argon-filled environment, and then quickly placed in water to cool it, so that sufficient internal stress is generated during the rapid heating and cooling process to further damage the damage. layer structure to reduce the magnitude of the external stress required for final peeling.

When the femtosecond laser interacts with matter, the valence electrons absorb the energy of multiple photons and are in an excited state, resulting in a high-density plasma. When the plasma concentration reaches the "critical density", the crystal material will absorb a large amount of laser energy. , damage occurs, and the chemical bonds of SiC are broken. The advantage of femtosecond lasers is that the time for electron absorption photons to be excited is in the femtosecond range (during the pulse action), and the time for electro-phonon coupling is on the order of picoseconds, so the laser energy absorbed by the electrons has not yet been transmitted to the ions , the laser action time is over. In the whole process, although the temperature of electrons is high, the temperature of ions is still very low. The whole process can be considered as a non-thermal processing process, and thermal diffusion, thermal melting and ablation will not occur.

For the double-pulse femtosecond laser, the first pulse generates a multi-photon excitation process, while the photons of the second pulse use the electrons excited by the first pulse as free electrons to generate avalanche ionization, which is more efficient to generate free electrons. Therefore, the double-pulse cutting efficiency is much higher than that of the single-pulse, since the energy of the photons of the second pulse is absorbed by the "free electrons" caused by the first pulse, the "blackened" structure damaged by the cutting will not propagate along the laser. The direction is overextended, limiting the thickness of the damaged layer.

With the movement of the processing table in the horizontal plane, the "blackened" damage area after laser slicing is covered with the entire silicon carbide plane, and the laser slicing process ends at this time. After the end of the process, the SiC wafer after cutting is subjected to rapid high-temperature heating treatment in an argon atmosphere, and then quickly taken out and put into water for quenching. Blackening" amorphous structure. Finally, using a pulling device, the wafer can be dissociated with less force.

As a preferred solution of the present invention, in step 1), the cleaning of the silicon carbide wafer is to ultrasonically clean the silicon carbide wafer in alcohol.

As a preferred solution of the present invention, in step 2), the parameters of the femtosecond laser are: the wavelength is 780 nm, the pulse width is 125-135 fs, the pulse energy is 25-35 μJ, and the repetition frequency is 9.5-10.5 kHz.

As a preferred solution of the present invention, the parameters of the femtosecond laser are: the wavelength is 750-800 nm, the pulse width is 130 fs, the pulse energy is 30 μJ, and the repetition frequency is 10 kHz.

As a preferred solution of the present invention, in step 3), the optical path is: using a half-wave plate and a polarization beam splitter to convert the linearly polarized laser pulses into s and p polarized pulses with the same energy; The pulse optical path introduces different routes, and by controlling the distance traveled by the two optical paths, an optical path difference of 1-10 ps is formed; finally, the half-wave plate and the polarization beam splitter are used to combine them, and finally a single double-pulse laser beam is formed. A convex lens that focuses it on the processing table to the required depth for slicing the SiC wafer.

As a preferred solution of the present invention, the optical path difference is 5ps.

As a preferred solution of the present invention, in step 4), the moving speed of the processing equipment is 1.5-2.5 mm/s.

As a preferred solution of the present invention, in step 4), the moving speed of the processing equipment is 2 mm/s.

As a preferred solution of the present invention, in step 5), the heat treatment is as follows: heat preservation at 480-550° C. for 4-6 hours; water cooling after taking out.

As a preferred solution of the present invention, in step 5), the protective atmosphere is an argon atmosphere.

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

The invention greatly reduces the thickness of the damage layer caused by the laser slicing process through the double-pulse laser slicing method, and avoids a large amount of waste of materials; at the same time, the "fast heating and cooling" method is used to greatly reduce the need for peeling after slicing. The magnitude of the external pulling force allows it to be easily peeled off; the method of the present invention has little material loss, short processing time, low cost and little pollution to the environment.

Description of drawings

Fig. 1 is a SiC wafer residue obtained by the method of the present invention.

FIG. 2 is an enlarged view of a cross-section of a SiC wafer peeled off by the method of the present invention.

Figure 3 shows the Raman spectra of SiC wafers before and after double-pulse femtosecond laser cutting.

FIG. 4 is a schematic diagram of the double-pulse laser light path of the present invention.

Detailed ways

The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention.

Referring to FIG. 4 , the double-pulse laser optical circuit device adopted in the present invention is converted into s and p-polarized pulses with the same energy through the polarization beam splitter after the pulses sent out by the laser transmitter enter the half-wave plate. After reflection by one mirror, it is vertically incident on the half-wave plate (ie, the λ/2 plate in Figure 4) and another polarization beam splitter; the p-polarized pulse is reflected by the other two mirrors and then enters the half-wave plate, which is the same as the s The polarized pulses are combined at the polarizing beam splitter to form a single double-pulsed laser beam.

Example

This embodiment provides a method for double-pulse femtosecond laser slicing of a SiC substrate, including the following steps:

1) Put the SiC wafer in alcohol for ultrasonic cleaning, take out the surface stains, then dry it, and finally fix it on the laser processing table;

2) Adjust the parameters of the femtosecond laser, adjust the parameters to a wavelength of 780 nm, a pulse width of 130 fs, a pulse energy of 30 μJ, a repetition frequency of 10 kHz, and the moving speed of the laser processing table is set to 2 mm/s;

3) Using a double-pulse laser optical circuit device, a double-pulse laser is produced; first, a half-wave plate and a polarization beam splitter are used to convert the linearly polarized laser pulses into s and p-polarized pulses with the same energy; the two pulse optical paths are introduced into different By precisely controlling the distance traveled by the two optical paths to form an optical path difference of 5ps, and finally using a half-wave plate and a polarizing beam splitter to combine them, a single double-pulse laser beam is finally formed. the depth required for the slicing of the SiC wafer which is focused on the processing table;

4) By moving the processing table, the nonlinear absorption of the laser converts the silicon carbide into amorphous silicon and graphite to achieve the destruction of the crystal structure. Precise cutting;

5) After completing step 4), the SiC wafer is placed in a muffle furnace filled with argon atmosphere, rapidly heated to 500° C., maintained for 5 hours, and then taken out and quickly put into water to cool. Finally, with the help of a pulling device, the wafer can be peeled off with less force.

By the method of the present invention, the SiC wafer residue obtained after cutting is shown in FIG. 1 , and the enlarged view of the exfoliated section of the SiC wafer is shown in FIG. 2 .

Figure 3 shows the Raman spectra of SiC wafers before and after double-pulse femtosecond laser cutting. Before cutting, the wafer has very sharp SiC Raman peaks. After cutting, the SiC peaks basically disappear and are replaced by broadband Raman signals of Si and C.

It can be seen that the present invention greatly reduces the thickness of the damaged layer caused by the laser slicing process through the double-pulse laser slicing method, and avoids a lot of waste of materials; at the same time, using the "fast heating and cooling" method, greatly reduces the peeling after slicing. The required external pulling force can be easily peeled off; the method of the present invention has little material loss, short processing time, low cost and little pollution to the environment.

The above are only preferred embodiments of the present invention, and are not intended to limit the present invention in any form or substance. It should be pointed out that for those skilled in the art, without departing from the method of the present invention, the Several improvements and supplements can be made, and these improvements and supplements should also be regarded as the protection scope of the present invention. All those skilled in the art, without departing from the spirit and scope of the present invention, can make use of the above-disclosed technical content to make some changes, modifications and equivalent changes of evolution, all belong to the present invention. Equivalent embodiments; at the same time, any modification, modification and evolution of any equivalent changes made to the above-mentioned embodiments according to the essential technology of the present invention still fall within the scope of the technical solutions of the present invention.

Claims (10)

1. A method for slicing a SiC substrate by using a double-pulse femtosecond laser is characterized by comprising the following steps of:

1) cleaning the silicon carbide wafer, and fixing the silicon carbide wafer on processing equipment;

2) setting parameters of a femtosecond laser;

3) setting the light path of the double-pulse laser, firstly manufacturing the light path with optical path difference, then converging the light path to manufacture the double-pulse light path, and focusing the light path in the silicon carbide wafer;

4) moving the processing equipment, and accurately cutting the silicon carbide wafer by using double-pulse femtosecond laser;

5) and 4) heating the cut silicon carbide wafer in a protective atmosphere, taking out the silicon carbide wafer, quickly cooling the silicon carbide wafer, and dissociating the silicon carbide wafer by using a pulling device.

2. The method for double-pulse femtosecond laser dicing of the SiC substrate according to claim 1, wherein in the step 1), the silicon carbide wafer is cleaned by ultrasonic cleaning of the silicon carbide wafer in alcohol.

3. The method for double-pulse femtosecond laser dicing of the SiC substrate according to claim 1, wherein in the step 2), the parameters of the femtosecond laser are as follows: the wavelength is 780nm, the pulse width is 125-135fs, the pulse energy is 25-35 muJ, and the repetition frequency is 9.5-10.5 kHz.

4. The method for double-pulse femtosecond laser dicing of the SiC substrate according to claim 3, wherein the parameters of the femtosecond laser are as follows: wavelength of 750-800nm, pulse width of 130fs, pulse energy of 30 muJ, and repetition frequency of 10 kHz.

5. The method for double-pulse femtosecond laser dicing of the SiC substrate according to claim 1, wherein in the step 3), the optical path is as follows: converting the linear polarization laser pulse into s-polarized pulse and p-polarized pulse with the same energy by using a half-wave plate and a polarization beam splitter; introducing s and p polarization pulse optical paths into different routes, and forming an optical path difference of 1-10ps by controlling the distance traveled by the two optical paths; and finally, converging the laser beams by using a half-wave plate and a polarization beam splitter to finally form a single-strand double-pulse laser beam, and focusing the single-strand double-pulse laser beam on the SiC wafer on the processing table to the depth required by the slicing of the SiC wafer through a convex lens.

6. The method of claim 5, wherein the optical path difference is 5 ps.

7. The method for double-pulse femtosecond laser dicing of the SiC substrate according to claim 1, wherein in the step 4), the moving speed of the processing equipment is 1.5-2.5 mm/s.

8. The method for double-pulse femtosecond laser dicing of the SiC substrate according to claim 7, wherein in the step 4), the moving speed of the processing equipment is 2 mm/s.

9. The method for double-pulse femtosecond laser dicing of the SiC substrate according to claim 1, wherein in the step 5), the heating treatment is as follows: preserving the heat for 4-6h at 480-550 ℃; taking out and then cooling by water.

10. The method for double-pulse femtosecond laser dicing of the SiC substrate according to claim 1, wherein in the step 5), the protective atmosphere is an argon atmosphere.

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