CN117359104A - Chemical mechanical polishing method for silicon carbide crystal based on laser technology under liquid phase - Google Patents

Abstract

The invention relates to a chemical mechanical polishing method of silicon carbide crystals based on a laser technology under a liquid phase, which comprises the following steps: a laser scanning auxiliary procedure under liquid phase, namely placing the Si C crystal to be processed in a container containing a liquid medium, soaking the Si C crystal to be processed in the liquid medium, and carrying out laser scanning processing on the surface of the Si C crystal to be processed by the laser beam of a nanosecond laser to ablate and oxidatively modify the Si C crystal to be processed; scanning the laser beam along an X-direction in a serpentine path; rotating the container, the laser beam scanning in a serpentine path along the Y-direction; the X direction and the Y direction are intersected and parallel to a plane where the surface of the Si C crystal to be processed is located; and a chemical mechanical polishing procedure, namely removing the surface material of the Si C crystal to be processed after laser treatment. According to the invention, the laser scanning is adopted to ablate and oxidize the Si C crystal, and then the CMP is adopted to remove the material, so that the high-efficiency and low-damage ultra-precise polishing of the Si C crystal can be realized.

Description

Chemical mechanical polishing method for silicon carbide crystal based on laser technology under liquid phase

Technical Field

The invention relates to the technical field of silicon carbide crystal preparation, in particular to a chemical mechanical polishing method of silicon carbide crystals based on a laser technology under a liquid phase.

Background

With the rapid development of industries such as power supply, controller and wireless communication, the requirements on performance indexes and reliability of power semiconductor devices are increasingly increased. The performance of the power semiconductor device based on the silicon material is close to the physical limit of the power semiconductor device, and the requirements of the industry on the aspects of working voltage, current bearing capacity, working frequency, heat dissipation capacity, reliability and the like of the device cannot be met, so that the development and application of the third generation wide forbidden band (Eg >2.3 eV) power semiconductor material represented by SiC are greatly emphasized, and the power semiconductor device is widely applied to the military/civil technical fields of Integrated Circuit (IC) manufacturing, precise optics, new generation radars, electronic countermeasure, aerospace communication, automobiles and the like. The technical route in the key field of national development strategy planning is explicitly indicated: there is a strong push and emphasis on breaking through the fabrication and applications of third generation semiconductor substrates and devices of SiC and gallium nitride (GaN).

As a semiconductor material which is developed more mature at present, siC crystals have been widely used in integrated circuit (Integrated circuit, IC) devices, high-density information storage devices, and microelectromechanical systems (Micro-Electro-Mechanical System, MEMS). The production process of the SiC crystal comprises the processes of pulling a single crystal, grinding an outer circle, slicing, grinding, polishing and the like, wherein the polishing technology is used as one of key technologies for improving the surface quality of materials in the modern manufacturing industry, and can remarkably reduce the surface roughness of the materials and obtain a smooth and flat workpiece surface. The quality and precision of the surface finish of SiC crystals directly determine the performance of the fabricated device, and therefore, extremely high demands are placed on its finish surface defects, surface roughness, (subsurface) damage, residual stress, lattice integrity, and surface type precision. However, siC crystals themselves have a series of characteristics that are unfavorable for smooth ideal surface processing at the atomic level, such as high hardness (> 28 GPa), large brittleness, strong anisotropy, and strong chemical inertness (no reaction with acid and base), making them a well-known crystal material that is extremely difficult to process. Therefore, at present, improving the surface quality and the processing efficiency of ultra-precise polishing of SiC crystals, and efficiently obtaining an ultra-smooth and undamaged atomic-level surface are still key bottleneck problems to be solved in the manufacture of SiC crystal elements, and meanwhile, the problem is also key to realize the large-scale industrialized application of third-generation semiconductor materials.

Chemical mechanical polishing (Chemical Mechanical Polishing, CMP) is one of the best ways to achieve global planarization of material surfaces, and can better meet the requirements of the semiconductor industry on high polishing precision of device surfaces. The technology firstly modifies the crystal surface through chemical reaction, and mainly aims to reduce the hardness of the surface layer material, then removes the modified layer through the mechanical action between abrasive particles and the modified layer, and finally obtains the crystal surface with high quality through continuous alternation of the chemical and mechanical actions. However, in SiC-CMP, due to the intrinsic hard and brittle characteristics of the material and very stable physicochemical properties, there are still problems of great processing difficulty and low processing efficiency, and the material removal rate (Material Removal Rate, MRR) is typically several tens to several hundreds of nanometers per hour. Therefore, how to simultaneously improve the surface processing quality and the processing efficiency of SiC crystals and efficiently obtain ultra-smooth and undamaged atomic-level surface quality is a key problem to be solved in SiC crystal processing. At present, the problems of scratch and corrosion damage caused by high polishing pressure and chemical corrosion, low material removal rate MRR and the like are still key bottleneck problems for restricting the SiC element in relevant important engineering applications such as chips, radars and the like in China. The ablation and the strong oxidation modification of the SiC crystal by the laser technology provide a beneficial thought for solving the problem of extremely low processing efficiency in ultra-precise polishing of the SiC crystal. And the problems of recast layer, ablation debris redeposition, cracks and the like in the ultra-fast pulse laser micro-machining process can be solved by carrying out laser machining under the condition of liquid medium. Therefore, the novel method and the material removal mechanism for the high-efficiency ultra-precise polishing processing of the SiC crystal element are explored and researched to meet the requirements of national related key engineering on the high processing efficiency and the high surface quality of the SiC crystal, so that the method has important theoretical significance and engineering application value, and has important reference significance for guiding the processing of hard and brittle materials such as GaN, sapphire and the like.

A new method for pretreatment of CMP by femtosecond laser irradiation is proposed in the prior art, but the MRR of SiC-CMP obtained by the method is as low as 27nm/min (about 1.62 mu m/h), and the improvement of the MRR in the CMP process is limited. In addition, a method for polishing SiC ceramics by underwater femtosecond laser is also proposed in the prior art, and the average surface roughness is only 0.72 μm although the average removal depth of the material surface reaches 32.19 μm. It can thus be seen that it is difficult to achieve the desired goals of both "efficient removal" and "high surface quality" when using currently available CMP techniques with laser irradiation as a pretreatment. In summary, the prior art has the problem that the "high-efficiency removal" and the "high surface quality" cannot be satisfied simultaneously.

Disclosure of Invention

Therefore, the invention aims to solve the technical problem that the high-efficiency removal and the high surface quality cannot be simultaneously satisfied in the prior art.

In order to solve the technical problems, the invention provides a silicon carbide crystal chemical mechanical polishing method based on a laser technology under a liquid phase, which comprises the following steps:

a laser scanning auxiliary procedure under liquid phase, namely placing the SiC crystal to be processed in a container containing a liquid medium, soaking the SiC crystal to be processed in the liquid medium, and carrying out laser scanning processing on the surface of the SiC crystal to be processed by the laser beam of a nanosecond laser to ablate and oxidatively modify the SiC crystal to be processed; scanning the laser beam along an X-direction in a serpentine path; rotating the container, the laser beam scanning in a serpentine path along the Y-direction; the X direction and the Y direction intersect and are parallel to a plane where the surface of the SiC crystal to be processed is located;

and a chemical mechanical polishing procedure, namely removing the surface material of the SiC crystal to be processed after laser treatment.

In one embodiment of the invention, the X direction is perpendicular to the Y direction.

In one embodiment of the invention, the liquid level of the liquid medium is 1.5-2.5 mm above the upper surface of the SiC crystal to be processed.

In one embodiment of the invention, the liquid medium is deionized water.

In one embodiment of the invention, the nanosecond laser employs a Gaussian-distributed ultraviolet nanosecond laser source.

In one embodiment of the invention, the angle at which the laser beam of the nanosecond laser is incident on the surface of the SiC crystal to be processed is 90 °.

In one embodiment of the invention, the angle of incidence of the laser beam of the nanosecond laser is adjusted by an adjustment device comprising a rotary power section, a rotary platform connected to the power section, and a support connected to the rotary platform, on which the container is placed.

In one embodiment of the invention, the adjustment means is located on one side of the container and the lugs comprise risers arranged in an L-shape and a cross plate connected to the rotating platform and to the container.

In one embodiment of the invention, the polishing solution in the chemical mechanical polishing process is 1wt.% diamond+1 wt.% h2o2+98wt.% deionized water.

In one embodiment of the invention, the chemical mechanical polishing process employs low pressure chemical mechanical polishing.

Compared with the prior art, the technical scheme of the invention has the following advantages:

according to the chemical mechanical polishing method for the silicon carbide crystal based on the laser technology under the liquid phase, the SiC crystal is firstly ablated and oxidatively modified by adopting laser scanning, and then the material is removed by adopting CMP, so that the high-efficiency and low-damage ultra-precise polishing of the SiC crystal can be realized.

Drawings

In order that the invention may be more readily understood, a more particular description of the invention will be rendered by reference to specific embodiments thereof that are illustrated in the appended drawings, in which

FIG. 1 is a flow chart of a method for chemical mechanical polishing of silicon carbide crystals based on laser technology in the liquid phase in a preferred embodiment of the present invention;

FIG. 2 is a schematic view of the apparatus used in the chemical mechanical polishing method of silicon carbide crystals based on the laser technique under liquid phase shown in FIG. 1;

FIG. 3 is a schematic diagram of a path of the laser scan of FIG. 1 twice;

FIG. 4 is a schematic diagram of a path of the laser scan of FIG. 1 twice;

FIG. 5 is an AFM image of surface topography, wherein (a) is example 1; (b) is example 2; (c) is example 3; (d) is example 4.

Detailed Description

The invention will be further described in connection with the accompanying drawings and specific examples which are set forth so that those skilled in the art will better understand the invention and will be able to practice it, but the examples are not intended to be limiting of the invention.

Referring to fig. 1 to 3, the invention provides a chemical mechanical polishing method for silicon carbide crystals based on a laser technology under a liquid phase, comprising:

laser scanning assisted process in liquid phase SiC crystals to be processed are placed in a container containing a liquid medium, which is immersed in liquid medium 600, which in some embodiments is deionized water. The laser beam 700 of the nanosecond laser is irradiated on the surface of the SiC crystal to be processed for laser scanning processing, so as to ablate and oxidatively modify the SiC crystal to be processed. In some embodiments, the nanosecond laser employs a Gaussian-distributed ultraviolet nanosecond laser source. Scanning the laser beam along an X-direction in a serpentine path; rotating the container, the laser beam scanning in a serpentine path along the Y-direction; the X direction and the Y direction intersect and are parallel to a plane where the surface of the SiC crystal to be processed is located;

and a chemical mechanical polishing procedure, namely removing the surface material of the SiC crystal to be processed after laser treatment. In some embodiments, the polishing solution in the chemical mechanical polishing process is 1wt.% diamond+1 wt.% h2o2+98wt.% deionized water. In some embodiments, the chemical mechanical polishing process employs low pressure chemical mechanical polishing.

To illustrate the effects of the present application, the following three comparative examples (examples 1 to 3 hereinafter, example 1 is that SiC crystals were not subjected to laser treatment, CMP was directly performed, example 2 is that SiC crystals were subjected to air unidirectional single laser scanning, and then CMP was performed, example 3 is that SiC crystals were subjected to liquid phase unidirectional single laser scanning, and then CMP was performed), and the examples of the present application (example 4 hereinafter, that is, siC crystals were subjected to liquid phase crossing direction twice laser scanning, and then CMP test was performed) were performed. Examples 1 to 4 all carried out experiments using 10 mm. Times.10 mm. Times.0.5 mm SiC crystals to be processed, the polishing times were all 10min. The polishing solution consists of an oxidant H2O2, abrasive grain diamond and deionized water.

The mass of SiC crystals used in all examples was weighed before and after CMP processing, all crystals required 15min of ultrasonic cleaning and blow-drying before weighing, and each piece required 5 times of weighing before and after CMP to average. The MRR of the SiC crystal in the CMP test is calculated according to the weighing result before and after CMP and the polishing time (10 min).

Each SiC crystal was positioned with the notch of the crystal, and the roughness Ra of 9 10 μm×10 μm regions shown in fig. 4 was measured, with the middle region being the center of the crystal, and the midpoints of the remaining regions being distributed on two rings trisecting the crystal diameter, and each four regions in turn trisecting the ring in which each was located. And taking an average value of the finally obtained 9 roughness data to obtain the surface roughness Ra of the sample.

As shown in table 1 and fig. 5, it can be seen from the comparison of example 2 and example 1 that CMP after laser polishing in air significantly improves the MRR of SiC crystal, but the surface roughness significantly increases, which makes it difficult to meet the industry demand. As can be seen from a comparison of example 3 and example 2, the single laser processing in the liquid phase followed by CMP, although the MRR is somewhat as followsBut the surface quality is greatly improved. As can be seen from the results of example 4, performing laser processing twice in the liquid phase followed by CMP, the MRR was slightly increased (about 5%) compared with the MRR results of example 3, and the surface quality was very close (R _a Differing by 0.009 nm).

In conclusion, the invention shows that the SiC crystal CMP process based on the laser technology assistance under the liquid phase greatly improves the MRR of the CMP technology, and has obvious advantages; at the same time, the surface quality after processing is very similar to that of the CMP technology.

TABLE 1 Material removal Rate and surface quality index for examples 1-4

Specifically, the laser-assisted CMP process provided by the invention has the advantages that firstly, the ablation and oxidation modification effects of the nanosecond laser under the liquid phase are utilized to improve the MRR in the CMP process, and then, the low-pressure CMP is utilized to remove the surface material of the SiC crystal after laser treatment, so that the surface roughness and damage of the SiC crystal are reduced. Therefore, the embodiment can obtain the SiC crystal without or with low surface damage and global planarization, realize high-efficiency material removal of the ultra-precise polished SiC crystal, provide technical support for ultra-smooth surface processing of the third-generation hard and brittle semiconductor material represented by SiC, solve the bottleneck problem that the MRR is lower when the CMP technology is used for ultra-precisely polishing the SiC crystal in the CMP technology assisted by other technologies, and further expand the application prospect of the laser-assisted CMP technology.

In summary, the method is specially used for ultra-precise polishing of SiC crystals, firstly, laser scanning is adopted to ablate and oxidize and modify the SiC crystals, and then CMP is adopted to remove materials, so that high-efficiency and low-damage ultra-precise polishing of the SiC crystals can be realized.

Further, the X direction is perpendicular to the Y direction. Specifically, the X direction is perpendicular to the Y direction so that two laser scans more uniformly process the surface of the SiC crystal to be processed. The surface ablation and oxidation modification effects of the SiC crystal to be processed are better.

Further, the liquid level of the liquid medium is 1.5-2.5 mm higher than the upper surface of the SiC crystal to be processed. Specifically, the remelting layer and ablation fragments formed by ablating the surface of the SiC crystal to be processed can be conveniently ensured to leave the surface of the SiC crystal by laser scanning, the remelting layer and the ablation fragments are directly generated in a liquid medium and are dispersed by the liquid medium, and on the other hand, the energy of a laser beam can be ensured to be absorbed by the liquid medium, so that excessive ablating of the surface of the SiC crystal to be processed is prevented, surface damage is caused, and the surface roughness of the SiC crystal is reduced.

Further, the angle at which the laser beam of the nanosecond laser is incident on the surface of the SiC crystal to be processed is 90 °.

Further, the incident angle of the laser beam of the nanosecond laser is adjusted by an adjusting device including a rotation power part 100 (e.g., a motor), a rotation platform 200 connected to the rotation power part 100, and a holder 300 connected to the rotation platform 200, and the container 400 is placed on the holder 300. The adjustment device is located on one side of the container, and the lugs comprise risers and cross plates arranged in an L shape, the risers are connected with the rotary platform 200, and the cross plates are connected with the container 400. The container 400 (i.e., the surface of the SiC crystal 500 to be processed placed in the container 400) is brought into an angle with the laser beam by rotation of the rotary power portion 100 until the angle is 90 °.

In some embodiments, the laser scanning assisted process in the liquid phase is accomplished using a laser processing system as shown in fig. 1. The laser processing system adopts an ultraviolet nanosecond laser source with 355nm wavelength and Gaussian distribution, and the pulse repetition frequency is adjustable within the range of 10-100 kHz. The initial diameter of the output laser beam is about 7.0mm, and the laser beam is finally focused on the surface of the SiC crystal to be processed through an F-theta lens with a focal length of 160 mm. A galvanometer scanning system controlled by a computer operates on an XOY plane. The SiC crystal to be processed was placed in a container with an alignment accuracy of 10 μm. Deionized water is poured into the container, and the SiC crystal to be processed is immersed by the deionized water. The level of deionized water was about 2mm from the surface of the SiC crystal to be processed. The laser scanning interval h was set to 10 μm, the pulse width was set to 10. Mu.s, and the scanning speed was set to 100mm/s. The laser processing method is as shown in fig. 2, in which the SiC crystal to be processed is first processed in a serpentine path to complete the first laser scanning processing, then the SiC crystal to be processed is rotated by 90 ° and then the second laser scanning processing is performed on the SiC crystal to be processed in the same manner. And after the laser treatment of the processed SiC crystal is finished, performing a chemical mechanical polishing procedure. The chemical mechanical polishing process is completed by using a UNIPO-1200S automatic pressure grinding polisher CMP equipment. The equipment polishing process parameters are set as follows: polishing pressure of 1psi, revolution of upper disc motor of 60rpm, revolution of lower disc motor of 100rpm, polishing time of 10min, polishing liquid of 1wt.% diamond+1 wt.% H2O2+98wt.% deionized water. In addition, the flow rate of the polishing liquid was set to 25ml/min, and precisely controlled by a peristaltic pump. All CMP were performed at room temperature (25 ℃) for a duration of 10min. In both the preparation of the polishing liquid and the subsequent CMP, the polishing liquid was continuously stirred with a magnetic stirrer, ensuring uniform dispersion of abrasive particles in the polishing liquid.

It is apparent that the above examples are given by way of illustration only and are not limiting of the embodiments. Other variations and modifications of the present invention will be apparent to those of ordinary skill in the art in light of the foregoing description. It is not necessary here nor is it exhaustive of all embodiments. And obvious changes and modifications which are extended therefrom are still within the scope of the invention.

Claims (10)

1. A chemical mechanical polishing method for silicon carbide crystals based on a laser technology under a liquid phase is characterized by comprising the following steps of: comprising the following steps:

a laser scanning auxiliary procedure under liquid phase, namely placing the SiC crystal to be processed in a container containing a liquid medium, wherein the SiC crystal to be processed is soaked by the liquid medium, and laser beams of a nanosecond laser are irradiated on the surface of the SiC crystal to be processed for laser scanning processing so as to ablate and oxidatively modify the SiC crystal to be processed; the laser beam scans along an X direction in a serpentine path; rotating the container, the laser beam scanning in a serpentine path along the Y-direction; the X direction and the Y direction intersect and are parallel to a plane where the surface of the SiC crystal to be processed is located;

and a chemical mechanical polishing procedure, namely removing the surface material of the SiC crystal to be processed after laser treatment.

2. The method for chemical mechanical polishing of silicon carbide crystals based on laser technology under liquid phase according to claim 1, wherein: the X direction is perpendicular to the Y direction.

3. The method for chemical mechanical polishing of silicon carbide crystals based on laser technology under liquid phase according to claim 1, wherein: the liquid level of the liquid medium is 1.5-2.5 mm higher than the upper surface of the SiC crystal to be processed.

4. The method for chemical mechanical polishing of silicon carbide crystals based on laser technology under liquid phase according to claim 1, wherein: the liquid medium is deionized water.

5. The method for chemical mechanical polishing of silicon carbide crystals based on laser technology under liquid phase according to claim 1, wherein: the nanosecond laser adopts a Gaussian-distributed ultraviolet nanosecond laser source.

6. The method for chemical mechanical polishing of silicon carbide crystals based on laser technology under liquid phase according to claim 1, wherein: the angle at which the laser beam of the nanosecond laser is incident on the surface of the SiC crystal to be processed is 90 °.

7. The method for chemical mechanical polishing a silicon carbide crystal based on laser technology in liquid phase according to claim 6, wherein: the incidence angle of the laser beam of the nanosecond laser is adjusted through an adjusting device, the adjusting device comprises a rotary power part, a rotary platform connected with the power part and a support connected with the rotary platform, and the container is placed on the support.

8. The method for chemical mechanical polishing a silicon carbide crystal based on laser technology in liquid phase according to claim 7, wherein: the adjusting device is located on one side of the container, the support comprises a vertical plate and a transverse plate which are distributed in an L shape, the vertical plate is connected with the rotary platform, and the transverse plate is connected with the container.

9. The method for chemical mechanical polishing of silicon carbide crystals based on laser technology under liquid phase according to claim 1, wherein: the polishing solution in the chemical mechanical polishing procedure is 1wt.% diamond+1 wt.% H2O2+98wt.% deionized water.

10. The method for chemical mechanical polishing of silicon carbide crystals based on laser technology under liquid phase according to claim 1, wherein: the chemical mechanical polishing process employs low pressure chemical mechanical polishing.