Calcium magnesium zirconium doped gadolinium gallium garnet (SSGG) single crystal substrate polishing method

Abstract

The invention relates to the technical field of processing methods of optoelectronic materials, and mainly relates to a grinding and polishing process of a magnetic thin film epitaxial substrate material-calcium magnesium zirconium doped gadolinium gallium garnet (Ca, Mg, Zr: GGG, hereinafter abbreviated as SGGG) single crystal substrate. Comprising the key steps of disc loading, thinning and leveling, mechanical grinding and polishing, chemical mechanical polishing (CMP) and the like, as shown in figure 1. The invention also relates to a special binder, a grinding liquid and a polishing liquid in the SGGG substrate processing process, and the ultra-smooth (the roughness is less than or equal to 0.5 nm) and ultra-flat (the flatness is less than or equal to 5 [mu] m) SGGG single crystal substrate can be obtained through the technological process, the corresponding grinding liquid and the corresponding chemical polishing liquid, so that the epitaxial growth requirement of a YIG or RIG magnetic film is met.

Description

Technical Field

[0001] The invention relates to a chemical mechanical polishing process for a magnetic thin film epitaxial substrate material in the field of optoelectronic materials--calcium magnesium zirconium doped gadolinium gallium garnet single crystal substrate. Background Art

[0002] Y3Fe5O 12 (YIG) magneto-optical film is not only the core material of the Faraday rotation component in the currently commercialized 0.8-1.6μm fiber optical isolator, but also an important material widely used in optical waveguides, integrated optics and other fields. The rapid development of optical communication devices has also put forward new requirements for magneto-optical film materials with large size, high Verdet constant and low optical loss. YIG is collectively called Re3Fe5O after rare earth ion doping or substitution. 12 (RIG). Compared with other magnetic materials, RIG material has a higher Verdet constant and lower optical loss, and has a very strong magneto-optical effect and high transmittance in the optical communication window (1550nm) band.

[0003] The performance of magneto-optical epitaxial thin films directly depends on the quality of the substrate single crystal. The difference in lattice constant and thermal expansion coefficient between the film and the substrate causes the film to crack easily, making it very difficult to prepare films with a thickness of more than 100μm. At present, the Bi∶RIG or Ce∶RIG thick film materials suitable for mainstream magneto-optical isolation devices are often prepared into 45° Faraday rotator plates, and the required thickness is often 340-370μm. It is particularly important to select substrates with similar lattice constants, similar thermal expansion coefficients, easy nucleation, high quality, and large size.

[0004] The lattice constant of the YIG film is 12.376, and the thermal expansion coefficient is 9.03×10 -6 / ℃, the more commonly used single crystal substrates are GGG substrates and doped GGG substrates (such as GSGG (scandium-doped gadolinium gallium garnet), YSGG (scandium-doped yttrium gallium garnet), etc. Since the price of raw material scandium oxide is very expensive, the cost of preparing GSGG or YSGG substrates is greatly increased. So the molecular formula is Gd 3- x Ca x Ga 5-x-2y Mg y Zr x+y O 12 (SGGG) crystals are obtained by doping with Ca 2+ Mg 2+ 、Zr 4+ Ions replace Sc 3+Replace the Gd and Ga ions in GGG, where 0.35≤x≤0.4, 0.25≤y≤0.3, to achieve the purpose of reducing the cost of crystal growth; the crystal structure is a cubic garnet structure, in which the Gd in the dodecahedron 3+ The cation Ca 2+ Substitution, Ga in the octahedron 3+ Mg 2+ and Zr 4+ The unit cell parameter range is 12.48-12.50, the melting point is 1750℃, the structural space group is Ia3d, the Mohs hardness is 7.5, and the adaptation rate with the RIG lattice constant is less than 0.05%, which is conducive to the growth of high-quality RIG epitaxial films by LPE technology.

[0005] The patent applications for polishing doped with GGG include: A processing method for ion-doped gadolinium gallium garnet wafers (application number 202111359070.2), which discloses the polishing process of plasma-doped GGG laser crystals such as Nd:GGG, Er:GGG, and Yb:GGG. The focus of the laser crystal is on requiring higher smoothness (10 / 5), parallelism (30″) and flatness (L / 10). As a substrate for magnetic thin films, SGGG emphasizes the requirement that the surface has lower roughness (≤0.5nm) and flatness (≤5μm). In addition, due to the doping of various ions in SGGG, cleavage and cracking are prone to occur. During polishing, it is necessary to solve the cleavage and cracking and achieve an ultra-smooth, low-roughness substrate surface. A lot of process exploration is required. At present, there is no systematic research on the polishing process of SGGG substrates. Summary of the invention

[0006] The problem to be solved by the present invention is to provide a polishing method for effectively solving the difficulty of easy cracking and cleavage of SGGG crystals and obtaining an ultra-smooth SGGG substrate with relatively low roughness, thereby meeting the substrate requirements for epitaxial growth of YIG or RIG magnetic films.

[0007] The technical solution adopted by the present invention to solve the technical problem is: a polishing method of an SGGG substrate with an ultra-smooth surface, which specifically comprises the following steps: (1) Plate mounting: Use adhesive to evenly stick the cut SGGG crystal blank onto a flat glass plate. (2) Thinning and leveling: On a four-axis or single-axis grinder and a cast iron grinding disc, add grinding fluid, control the flow rate, apply pressure, adjust the speed, and determine the grinding time according to the thinning thickness and surface knife marks to obtain a SGGG grinding disc with a smooth surface and no obvious knife marks or line marks, with a roughness of <200nm. (3) Mechanical grinding and polishing: On a four-axis or single-axis grinder and a polyurethane polishing disc, add grinding fluid, apply pressure, adjust the speed, and adjust the polishing time according to the surface of the grinding disc to obtain a SGGG fine grinding disc with no obvious scratches and lines on the surface, with a roughness of <2nm. (4) Chemical mechanical polishing: On a four-axis or single-axis grinder and frosted leather polishing disc, add chemical polishing liquid, apply pressure, adjust the speed, and the chemical polishing time depends on the roughness of the substrate surface. Obtain an ultra-smooth surface with no bright spots, SGGG polishing sheet, roughness ≤ 0.5nm, flatness ≤ 5μm.

[0008] As a preferred embodiment, the adhesive in step (1) is made by mixing rosin and white wax, and the weight ratio of the two is in the range of 1:1-1.5:1. The addition of rosin is to increase the melting point of white wax. Generally, the higher the rosin content, the greater the viscosity.

[0009] As a preferred embodiment, in step (2-4), pressure is applied by adding weights to the swing arm of the four-axis or single-axis machine, and weights of different weights are selected according to the pressure, and the pressure is determined by the number and size of the SGGG substrates.

[0010] As a preferred embodiment, the grinding fluid in step (2) is W 14 It is made of a suspension of silicon carbide (SiC) added to 30-60 times the weight of pure water.

[0011] As a preferred embodiment, the coarse grinding liquid in step (3) is a suspension made of W7 boron carbide (B4C) and pure water in a weight ratio of 30-50 times; the fine grinding liquid is W 3.5 A suspension made of diamond micropowder and pure water at a weight ratio of 100-1000 times.

[0012] As a preferred embodiment, the polishing liquid in step (3) is 0.25 An emulsion of SiO2 with 1-5 times the weight ratio of pure water is prepared, and the pH value of the polishing liquid is adjusted to 9-10 by phosphoric acid (H3PO4) and potassium hydroxide (KOH).

[0013] The pressure applied in the present invention has a great influence on the grinding and polishing rate and surface quality. Increasing the pressure enhances the mechanical grinding effect and accelerates the grinding rate. However, excessive pressure will lead to uneven grinding and polishing, accelerated wear of the polishing pad, increased local temperature, and easy appearance of scratches and marks, which will reduce the polishing quality. Effects of the Invention

[0014] The polishing method of the present invention adopts a grinding liquid with particle sizes ranging from coarse to fine. The coarser grinding liquid can quickly reduce the thickness and remove the cutting marks or line marks on the surface. The finer grinding liquid can effectively remove the scratches and grooves on the surface of the grinding sheet. During the chemical mechanical polishing process, the SiO2 polishing liquid will react chemically with the SGGG to generate silicate compounds on the surface, and the layer of compounds is removed by mechanical grinding. The present invention simplifies the processing procedures of the SGGG single crystal substrate, improves the processing efficiency, reduces the processing cost, and improves the processing quality of the wafer, and obtains an ultra-smooth (roughness ≤ 0.5nm) and ultra-flat (flatness ≤ 5μm) SGGG substrate substrate to meet the epitaxial growth requirements of YIG or RIG magnetic thin films. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Figure 1 It is a schematic diagram of the SGGG single crystal substrate processing process flow.

[0016] Figure 2 is Atomic force microscope (AFM) image of the SGGG substrate after thinning and flattening.

[0017] Figure 3 is Atomic force microscopy (AFM) image of SGGG substrate after mechanical grinding and polishing.

[0018] Figure 4 is Atomic force microscopy (AFM) image of SGGG substrate after chemical mechanical polishing. DETAILED DESCRIPTION

[0019] The present invention is further described below with reference to specific examples, which are not intended to limit the scope of protection of the present invention.

[0020] Embodiment 1: 1. Plate: Heat the chamfered SGGG cut blank and glass laminate to 110℃ on a heating platform at the same time, and use a special adhesive to evenly and firmly bond the blank to the laminate. 2. Leveling and thinning: Use a four-axis surface grinder and a cast iron grinding disc, using a particle size of W 14 Silicon carbide (SiC) suspension (SiC:H2O weight ratio 1:30), applied pressure 90g / cm 2 , the rotation speed is set to 90r / min, the grinding fluid flow rate is 150ml / min. Due to the high concentration of the grinding fluid, the high applied pressure, the fast rotation speed and the short grinding time, the target thickness is thinned in only 1.5h. There are no obvious knife marks on the surface of the wafer, but there are obvious scratches. . 3. Mechanical polishing: Use a four-axis grinder and polyurethane grinding disc, first use W7 boron carbide (B4C) suspension as the grinding liquid (B4C: H2O weight ratio 1:30), the flow rate is 15ml / min, and 80g / cm2 The pressure was set to 80r / min, and the surface was ground until there were a few scratches and the sand holes were evenly distributed. 3.5 Diamond suspension was used as the grinding liquid (C:H2O weight ratio 1:100), the flow rate, pressure and rotation speed were the same as W7, and there were no obvious sand holes on the surface. Due to the high pressure, there were slight scratches and marks on the surface. 4. Chemical polishing: using surface grinding machine and W 0.25 SiO2 emulsion was used as the polishing liquid (SiO2:H2O weight ratio 1:3), the pH was adjusted to 10, the flow rate was 15ml / min, and the applied pressure was 80g / cm 2 , the rotation speed is 110r / min, and the surface is ground until there is no bright road. The detected roughness reaches 0.35nm and the flatness is close to 5 microns.

[0021] According to the above experimental scheme, several technical parameters are changed in the following different implementation schemes as shown in the following table: 1. Upper plate: The blank piece should be firmly bonded and flat. 2. Leveling and thinning: The wafer surface must be relatively flat and free of cuts. 3. Mechanical polishing: The surface must be free of sand holes and scratches. 4. Chemical polishing: The surface should be free of bright spots, with a flatness of ≤5 microns and a roughness of ≤0.5 nanometers.

Claims

1. A grinding and polishing process for a calcium magnesium zirconium doped gadolinium gallium garnet (SGGG) single crystal substrate, characterized in that The main steps include: (1) Plate: Use adhesive to evenly stick the cut SGGG crystal blank onto a flat glass plate; (2) Thinning and leveling: On a four-axis or single-axis grinding machine and a cast iron grinding disc, the grinding fluid flow rate is 50-200 ml / min and the applied pressure is 50-100 g / cm 2 , speed 50~100r / min, grinding time: 0.5~4h, so that the surface of SGGG grinding sheet is smooth, without obvious knife marks and line marks, and the roughness is <200nm; (3) Mechanical grinding and polishing: on a four-axis or single-axis grinding machine and a polyurethane polishing disc, the grinding liquid flow rate is 5-15 ml / min, and the applied pressure is 50-80 g / cm 2 , speed: 80~110r / min, polishing time: 1~6h, so that the surface of the fine grinding disc has no obvious scratches and marks, and the roughness is <2nm; (4) Chemical mechanical polishing: On a four-axis or single-axis grinder and frosted leather polishing disc, the flow rate of chemical polishing liquid is 5-15ml / min, and the grinding pressure is 70-100g / cm 2 , rotation speed: 100~120r / min, polishing time: 0.5~3h, the surface of the polished sheet obtained is ultra-smooth, without bright road, roughness ≤0.5nm, flatness ≤5μm.

2. The polishing method of the SGGG substrate according to claim 1, characterized in that: In the step (1), the adhesive is prepared by mixing rosin and paraffin in a ratio of 1:1-1.5:1, and a heating platform with temperature control is used to heat the adhesive and the blank sheet so as to bond them to the glass plate.

3. The polishing method of the SGGG substrate according to claim 1, characterized in that: The grinding fluid in step (2) is a suspension of silicon carbide (SiC) with a particle size of W14.

4. The polishing method of the SGGG substrate according to claim 1, characterized in that: The grinding liquid in step (3) is composed of a suspension of W7 boron carbide (B4C) and W3.5 diamond (C) with a particle size.

5. The polishing method of the SGGG substrate according to claim 1, characterized in that: In the step (4), the polishing liquid is W 0.25 The SiO2 emulsion is adjusted to a pH of 9-10 by using phosphoric acid (H3PO4) and potassium hydroxide (KOH).

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