Wafer surface quality control method based on laser-induced phase change

Abstract

The invention relates to a wafer surface quality control method based on laser-induced phase change, which comprises the following steps of: firstly, scanning and irradiating the surface of a wafer by adopting ultrafast laser, setting proper laser parameters and laser scanning and irradiation longitudinal overlapping ratio and transverse overlapping ratio, and inducing the surface of the wafer to generate phase change (amorphous state or metastable phase) while inhibiting the formation of a micro groove; then the wafer subjected to phase change is immersed in a specific etching solution with the etching rate far higher than that of a base body on a phase change area, the etching time and concentration are adjusted in cooperation with ultrasonic-assisted treatment, it is ensured that the modified layer is completely removed, meanwhile, the integrity of the base body is kept, and high-precision selective machining is achieved. A repeatable surface quality control method is established, the problem of high surface roughness of wafer laser machining is solved, the surface roughness of the machined wafer is remarkably reduced, the etching rate is remarkably increased, the machining time is shortened, and the method can be suitable for various transparent crystals such as sapphire wafers and optical windows and has a wide application prospect. Good market prospects are realized.

Description

Technical Field

[0001] The present invention belongs to the technical field of wafer processing, and in particular relates to a wafer surface quality control method based on laser-induced phase change. Background Art

[0002] The statements in this section only provide background information related to the present disclosure, and these statements may constitute prior art. In the process of implementing the present invention, the inventors found that there are at least the following problems in the prior art.

[0003] Polishing is a key step in achieving a flat, damage-free surface during wafer processing, especially for hard and brittle materials such as sapphire, silicon, and silicon carbide, where surface quality directly impacts chip performance. Currently, methods for controlling wafer surface quality include mechanical polishing, chemical polishing, laser polishing, and laser-assisted chemical polishing. Mechanical polishing is inefficient and prone to subsurface damage; chemical polishing requires strong acid and high temperature conditions, resulting in low efficiency and high pollution; and conventional laser polishing (such as nanosecond laser and continuous laser) has significant thermal effects, easily generating microcracks and recast layers.

[0004] Currently, there are technologies that use ultrafast lasers for wafer processing. Ultrafast lasers have the characteristics of high peak power density, short action time, and the ability to provide extreme high temperature and high pressure conditions. They are widely used in the processing of hard and brittle materials. However, pure ultrafast laser processing controls surface morphology by adjusting laser parameters, but this can lead to high surface roughness and residual recast layers. When laser processing is used alone, the mechanism by which processing parameters affect surface morphology is unclear, resulting in unstable roughness control.

[0005] There are also chemical laser hybrid processes, such as patent application number 202111083488.5, titled "A Laser-Assisted Polishing Method for Single Crystal Silicon Wafers," which uses chemical mechanical polishing on silicon wafers irradiated by ultrafast lasers. However, these solutions are mostly limited to silicon wafer materials and only use lasers to "soften" the silicon surface to improve the efficiency of chemical mechanical polishing, resulting in poor polishing efficiency. Summary of the Invention

[0006] In view of the above problems, the present invention aims to solve some of the problems in the prior art, or at least alleviate these problems.

[0007] A wafer surface quality control method based on laser-induced phase change comprises the following steps:

[0008] Ultrafast laser scanning is used to irradiate the wafer surface: wherein specific laser parameters are set to induce a phase change on the wafer surface, forming a phase change zone;

[0009] Test whether a phase change occurs on the wafer surface: If no phase change occurs, repeat the laser scanning and irradiation operation on the wafer surface;

[0010] Selective chemical etching: The wafer that has undergone phase change is immersed in a reagent that etches the phase change area at a higher rate than the substrate, and then ultrasonically assisted cleaning and drying are performed.

[0011] Furthermore, during the ultrafast laser scanning irradiation process on the wafer surface, specific longitudinal overlap ratio and lateral overlap ratio of the laser scanning irradiation are set to suppress the formation of micro grooves.

[0012] Optionally, it is characterized in that the specific laser scanning irradiation longitudinal pulse overlap rate is 50%-90%; the specific laser scanning irradiation transverse pulse overlap rate is 60%-90%.

[0013] Preferably, the specific laser scanning irradiation longitudinal pulse overlap rate is 80%; the specific laser scanning irradiation transverse pulse overlap rate is 75%.

[0014] Furthermore, the laser induces a phase change on the wafer surface, so that the crystalline phase is transformed into an amorphous or metastable phase; the laser is a Gaussian beam or a flat-top beam, the laser pulse width is between 250fs-10ps, and the wavelength is in the range of 343nm-1064nm.

[0015] Furthermore, the reagent is an etching solution of a specific concentration; the etching time is within 24 hours, and the etching solution concentration is within the range of 30%-70%.

[0016] Optionally, the reagent is KOH solution (potassium hydroxide solution).

[0017] Optionally, Raman spectroscopy or X-ray diffractometer is used to test whether a phase change occurs on the wafer surface.

[0018] The present invention has the following beneficial effects:

[0019] The present invention selects appropriate laser parameters and appropriate longitudinal and lateral overlap rates of laser scanning irradiation, and uses a specific etching solution with a much higher etching rate for the phase change region than for the substrate. On the basis of suppressing the formation of micro-grooves on the wafer surface, not only the surface roughness of the wafer is significantly reduced, but also the etching rate can be significantly improved through selective etching, thus shortening the processing time. The invention is applicable to a variety of transparent crystals such as sapphire wafers and optical windows, and has great market prospects. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 is a process flow chart of the present invention;

[0021] Figure 2 is a schematic diagram of the wafer surface of a sapphire wafer used in an embodiment of the present invention;

[0022] Figure 3: The test images of the wafer surface before and after laser scanning by Raman spectroscopy in an embodiment of the present invention; wherein a is the test image before laser scanning, and b is the test image after laser scanning;

[0023] Figure 4 is a schematic diagram of a wafer surface after chemical etching in an embodiment of the present invention;

[0024] Figure 5 This is a test chart showing the effect of different overlaps on the surface roughness of sapphire wafers in the present invention. DETAILED DESCRIPTION

[0025] The present invention will be further described below in conjunction with the accompanying drawings. The embodiments of the present invention are only used to illustrate the present invention and are not intended to limit the present invention. Without departing from the technical concept of the present invention, various substitutions and modifications can be made based on common technical knowledge and customary means in the field, and all should be included in the scope of the present invention.

[0026] The applicant's research found that ultrashort pulses cause electrons to absorb photon energy instantaneously, forming a high-temperature electron gas, while the lattice has not yet responded (non-equilibrium state). Electrons transfer energy to the lattice through scattering, causing lattice instability or bond breakage, triggering non-thermal melting, structural reorganization, or stress phase transition. Rapid cooling can form metastable phases (such as amorphous and nanocrystals). Ultrafast energy deposition coupled with non-equilibrium dynamics breaks through traditional thermodynamic limitations and can achieve phase change regulation.

[0027] Furthermore, materials in different phase transition states exhibit different chemical etching rates. Crystalline structures have regular structures, strong atomic bonds, and dense arrangement, making them difficult for the etchant to penetrate and resulting in lower etching rates. Amorphous states have disordered atomic arrangement, numerous defects, and dangling bonds, making them more reactive to the etchant and resulting in higher etching rates. Furthermore, metastable phases (such as high-pressure phases) have higher internal energies, resulting in unstable bonding and faster etching rates. Phase transitions can also alter surface energy or oxide layer properties, further affecting etching selectivity.

[0028] In summary, the applicant has determined that the reason why the polishing efficiency of the chemical laser composite process is not ideal (including the insignificant reduction in wafer surface roughness, long processing time, and can only be used on silicon wafer materials) is mainly because the laser scanning of this technology does not set appropriate laser parameters, does not involve phase change induced etching selectivity, and no phase change zone is formed on the wafer surface. In addition, the chemical etching rate of unmodified wafers is low, resulting in low efficiency and high surface roughness.

[0029] Therefore, the applicant designed the following solution.

[0030] like Figure 1 As shown, a wafer surface quality control method based on laser-induced phase change includes the following steps:

[0031] Ultrafast laser scanning is used to irradiate the wafer surface; wherein specific laser parameters are set to cause the laser to induce a phase change on the wafer surface, forming a phase change zone;

[0032] Test whether a phase change occurs on the wafer surface; if no phase change occurs, repeat the laser scanning and irradiation operation on the wafer surface;

[0033] The wafer that has undergone phase change is immersed in a reagent whose etching rate for the phase change region is higher than that for the substrate, and is then subjected to ultrasonic-assisted cleaning and drying.

[0034] With the above scheme, it is only necessary to set appropriate laser parameters, such as wavelength, energy density, pulse width, etc., to induce a phase change on the wafer surface, thereby using ultrafast laser to induce the formation of a phase change zone (such as an amorphous or metastable phase) on the surface of the material. Since the chemical activity of the phase change zone is significantly higher than that of the original matrix (such as a crystalline state), in combination with a reagent whose etching rate of the phase change zone is higher than (especially much higher than) that of the matrix (such as HF etching rate of amorphous silicon is dozens of times faster than that of crystalline silicon), the modified layer is preferentially dissolved, thereby greatly improving the selectivity and efficiency of chemical etching through laser-induced phase change. Therefore, on the basis of significantly reducing the surface roughness of the wafer, the etching rate is also significantly improved, shortening the processing time. In combination with adjusting the etching time and concentration, and ultrasonic assisted processing, etc., it is further ensured that the integrity of the matrix is ​​retained while the modified layer is completely removed, thereby achieving high-precision selective processing. This application is universal and applicable to a variety of transparent crystals such as sapphire wafers and optical windows (extended to SiC silicon carbide, diamond, etc.).

[0035] Appropriate laser parameters, such as wavelength, energy density, and pulse width, are set to induce a phase transition on the wafer surface, particularly a transition from a crystalline phase to an amorphous or metastable phase. The laser can be a Gaussian beam or a flat-top beam. The parameters for laser-induced phase transition are a pulse width between 250 fs and 10 ps, ​​and a wavelength between 343 nm and 1064 nm.

[0036] However, the applicant discovered in actual testing that the aforementioned method easily forms microgrooves on the wafer surface. Microgrooves produced by laser processing significantly increase surface roughness, but this effect can be reduced by optimizing laser parameters and spot overlap.

[0037] Therefore, the applicant sets a specific longitudinal pulse overlap rate and transverse pulse overlap rate of the laser scanning irradiation during the ultrafast laser scanning irradiation process on the wafer surface to suppress the formation of micro grooves.

[0038] Generally speaking, the laser pulse spot overlap rate and peak power density can be adjusted as long as they can suppress the formation of micro grooves and induce material phase change. However, according to the applicant's research and experimental findings, different overlap rates have different effects on the roughness of the wafer surface. Figure 5The figure shows the effect of different overlap rates on the surface roughness of sapphire wafer. p ) constant (50%), the surface roughness increases with the transverse pulse overlap rate (δ a ) increases from 74% to 98% in a monotonically increasing trend, and increases from 546nm to 1848nm. The maximum roughness (1848nm) occurs at δ a =98% and δ p = 0, and the minimum roughness (546nm) appears at δ a =74% and δ p =83%. After 24 hours of chemical etching with 40% KOH solution, the surface roughness is significantly reduced, and the lowest surface roughness can reach 374nm.

[0039] Therefore, setting the specific longitudinal laser scanning irradiation pulse overlap ratio to 50%-90% and the specific transverse laser scanning irradiation pulse overlap ratio to 60%-90% can suppress the formation of microgrooves while inducing material phase transformation. Preferably, the specific longitudinal laser scanning irradiation pulse overlap ratio is 80% and the specific transverse laser scanning irradiation pulse overlap ratio is 75% to further reduce wafer surface roughness.

[0040] The choice of reagent is crucial. Generally, an etchant of a specific concentration is chosen, such as a strong acid, a strong base, or a mixture of alkali solutions. Preferred reagents, such as KOH (potassium hydroxide) or HF (hydrofluoric acid), etch the phase change region much faster than the substrate. This significantly increases the selective etching rate of the phase change region and shortens processing time. Experimental results show that KOH solution works best.

[0041] The present application may use testing methods such as Raman spectroscopy or X-ray diffractometer to test whether a phase change occurs on the wafer surface, so as to determine whether to immerse the wafer in an etching solution for selective etching.

[0042] The following takes the processing of sapphire wafers as an example:

[0043] (1) Take a sapphire wafer with an original surface roughness of 650nm. Figure 2 As shown. Raman spectroscopy test shows that it is α-Al2O s ,like Figure 3 As shown in (a).

[0044] (2) Laser scanning is used to irradiate the sapphire wafer surface. The beam intensity is Gaussian distribution, the laser wavelength is 1030nm, and the energy density is 16.2J / cm 2 , the pulse width is 2ps, and the laser scanning transverse pulse overlap rate (δ a ) is 74%, and the longitudinal pulse overlap rate (δ p) is 83%.

[0045] (3) After the Raman spectroscopy test, the sapphire wafer was scanned and amorphous and γ-Al2O3 appeared on the surface, such as Figure 3 (b) shown.

[0046] (4) The laser-scanned wafer was placed in a 40% KOH solution and etched with the aid of ultrasonic vibration for 4 hours.

[0047] (5) The surface of the sapphire wafer after corrosion is as follows Figure 4 As shown, the surface roughness was detected to be 374 nm, a decrease of 42.5%.

[0048] The present invention first uses ultrafast laser scanning to irradiate the wafer surface, and sets appropriate laser parameters and the longitudinal and lateral overlap rates of the laser scanning irradiation, thereby suppressing the formation of microgrooves while inducing a phase change on the wafer surface. The wafer undergoing phase change is then immersed in a specific etching solution that etches the phase change region much faster than the substrate, and the etching time and concentration are adjusted, and ultrasonic-assisted treatment is used to ensure that the modified layer is completely removed while retaining the integrity of the substrate, thereby achieving high-precision selective processing. The present invention establishes a set of repeatable surface quality control methods that not only solves the problem of high surface roughness during wafer laser processing, significantly reducing the surface roughness of the processed wafer, but also significantly improves the etching rate, shortens the processing time, and is applicable to a variety of transparent crystals such as sapphire wafers and optical windows, with excellent market prospects.

Claims

1. A wafer surface quality control method based on laser-induced phase change, characterized in that: The following steps are involved: Ultrafast laser scanning is used to irradiate the wafer surface: wherein specific laser parameters are set to induce a phase change on the wafer surface, forming a phase change zone; Test whether a phase change occurs on the wafer surface: If no phase change occurs, repeat the laser scanning and irradiation operation on the wafer surface; Selective chemical etching: The wafer that has undergone phase change is immersed in a reagent that etches the phase change area at a higher rate than the substrate, and then ultrasonically assisted cleaning and drying are performed.

2. The wafer surface quality control method based on laser-induced phase change according to claim 1, characterized in that: During the ultrafast laser scanning irradiation process on the wafer surface, specific longitudinal overlap rate and lateral overlap rate of the laser scanning irradiation are set to suppress the formation of micro grooves.

3. The wafer surface quality control method based on laser-induced phase change according to claim 2, characterized in that: The specific laser scanning irradiation longitudinal pulse overlap rate is 50%-90%; the specific laser scanning irradiation transverse pulse overlap rate is 60%-90%.

4. The wafer surface quality control method based on laser-induced phase change according to claim 3, characterized in that: The specific laser scanning irradiation longitudinal pulse overlap rate is 80%; the specific laser scanning irradiation transverse pulse overlap rate is 75%.

5. The wafer surface quality control method based on laser-induced phase change according to claim 1 or 2, characterized in that: The laser induces a phase change on the wafer surface, which converts the crystalline phase into an amorphous or metastable phase. The laser is a Gaussian beam or a flat-top beam, with a laser pulse width between 250fs-10ps and a wavelength within the range of 343nm-1064nm.

6. The wafer surface quality control method based on laser-induced phase change according to claim 1, characterized in that: The reagent is an etching solution with a specific concentration; the etching time is within 24 hours, and the etching solution concentration is within the range of 30%-70%.

7. The wafer surface quality control method based on laser-induced phase change according to claim 1, characterized in that: The reagent is KOH solution (potassium hydroxide solution).

8. The wafer surface quality control method based on laser-induced phase change according to claim 1, characterized in that: Raman spectroscopy or X-ray diffractometer is used to test whether phase change occurs on the wafer surface.

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