Method for rapidly improving bending and warping degree of large-size gallium nitride substrate through N-surface etching process

Abstract

The invention belongs to the technical field of semiconductor crystal processing, and relates to a method for rapidly improving the bending and warping degree of a large-size gallium nitride substrate through an N-surface etching process. The method comprises the following steps: carrying out coarse grinding, leveling and thinning on a GaN crystal by using a large-particle-size diamond grinding liquid; grinding the Ga surface and the N surface of the GaN substrate by using small-particle-size diamond grinding liquid to reduce the surface roughness; performing N-surface etching in an alkali solution; ga surface chemical mechanical polishing; and cleaning and drying. According to the method for optimizing the surface shape precision of the GaN substrate through N-surface etching, the surface shape error is rapidly corrected by means of the anisotropic characteristic of the GaN material, and the time needed for surface shape correction by depending on pure mechanical polishing traditionally is remarkably shortened; the etching can completely eliminate the residual subsurface damage layer after fine grinding, provides a more ideal initial surface for CMP, and is beneficial to obtaining a more perfect final surface. The wet etching process is easy to realize uniform processing of a large-size wafer, and is beneficial to expansion of the process to a larger-size GaN substrate.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of semiconductor crystal processing, and relates to a method for quickly improving the bending and warping degree of a large-size gallium nitride substrate through N-face etching process. BACKGROUND

[0002] Gallium nitride (GaN) substrate is a core material of the third generation of semiconductors, and has a surge in demand in the fields of power electronics, radio frequency devices and Micro-LED. With the development of devices towards high power and high integration, the large-scale preparation of 4-6 inch large-size GaN substrates has become a breakthrough point of the development trend of the industry. However, the large-size GaN substrate grown by hydride vapor phase epitaxy (HVPE) generally has serious surface defects due to the influence of heteroepitaxial stress accumulation and growth temperature gradient; in the epitaxial growth and device processing, high bowing and warping values are not welcome. The substrate with large bowing will cause the epitaxial film to produce undesirable stress, and even cause the film to break, reduce the crystallinity of the film, and thus reduce the device performance.

[0003] Research on traditional semiconductors shows that the shape of the substrate is crucial to product quality. Unlike silicon and sapphire wafers, the damage layer thickness and roughness of the Ga face after fine grinding of the Ga face are significantly reduced due to the different hardness of the Ga face and the N face, but the N face will be obviously convex, increasing the difficulty of polishing and causing uneven polishing effect. In addition, the random distribution of micro-area protrusions / recesses on the surface leads to poor quality of the epitaxial layer; the thermal resistance of the substrate-epitaxial interface increases by more than 30%, which restricts the heat dissipation performance of high-power devices. The current industry relies on chemical mechanical polishing for surface shape correction, but there are fundamental limitations of GaN substrates: global uniform material removal cannot correct local deformation; Ga polar face single polishing time > 10 hours, and the removal rate is negatively correlated with flatness; mechanical stress induces micro-cracks, deep scratches and other damages, reducing the strength of the substrate. Although dry etching (such as ICP-RIE) is tried to correct the surface shape accuracy, there is an inherent contradiction between the global uniformity of traditional etching process and the non-uniform deformation of GaN substrate; the thermal loading effect of plasma etching induces new thermal deformation, and the accuracy of the substrate is still uncontrollable, and local etching amount cannot be accurately controlled. Therefore, it is urgent to develop a large-size GaN substrate surface shape correction method with high accuracy, high efficiency and low damage, and to break through the existing technical bottlenecks through innovative etching strategies. SUMMARY

[0004] The present application is aimed at the existing 2-inch GaN substrate after fine grinding to reduce roughness, the GaN substrate will produce a false plane, the surface shape will change seriously, which will affect the subsequent chemical mechanical polishing and device preparation, and therefore a method for quickly optimizing the warping degree of a large-size gallium nitride substrate through N-face etching process is invented.

[0005] To achieve the above-mentioned purposes, the application provides the following technical solutions. A method for quickly improving the bending and warping degree of a large-size gallium nitride substrate by N-face etching process, comprising the following steps: (1) rough grinding, flattening and thinning: Ga and N faces of a GaN crystal are ground on a copper plate using a large-particle-size diamond grinding liquid. A 2-inch GaN crystal grown by HVPE has a thickness greater than 1 mm and a large thickness difference, so the wafer is preliminarily flattened to a reference thickness of the target thickness; (2) fine grinding to reduce roughness: Ga and N faces of the GaN substrate are ground on a tin plate using a smaller-particle-size diamond grinding liquid to remove a serious damage layer caused by rough grinding and reduce the surface roughness; (3) N-face etching: the N face ((000-1) face) of the GaN is etched in an alkali solution; (4) Ga-face chemical mechanical polishing: the Ga face of the wafer is placed on a polishing disc of a CMP device with the Ga face downward; nano-sized silicon dioxide or cerium oxide-based polishing liquid is used for polishing; (5) cleaning and drying: first, the GaN substrate is cleaned by megasonic wave cleaning using deionized water (DIW) to remove micrometer-sized particles, then organic matter and metal ion contamination are removed using a diluted acid solution and hydrogen peroxide, and a large amount of DIW is used for rinsing to completely flush away all chemical residues; finally, the wafer is high-speed spun to dry and packaged.

[0006] Preferably, in the step (1), the grinding temperature is 20-30 ℃, the pressure is 20-50 N / piece, the rotation speed is 20-40 rpm, and the liquid flow rate is 1-5 ml / min, so that the material is efficiently removed and the thickness is preliminarily controlled to 500-600 μm. According to the application, preferably, in the step (1), the large-particle-size grinding liquid is a 4-8 μm polycrystalline grinding liquid.

[0007] According to the application, preferably, in the step (1), the grinding temperature is 25±1 ℃, the pressure is 30 N / piece, the rotation speed is 30 rpm, and the liquid flow rate is 2 ml / min.

[0008] Preferably, in the step (2), the grinding temperature is 20-30 ℃, the pressure is 20-50 N / piece, the rotation speed is 20-40 rpm, and the liquid flow rate is 1-5 ml / min, so that there is no bright white scratch under a strong light and the Bow / Warp value of the substrate is convexly changed.

[0009] According to the application, preferably, in the step (2), the small-particle-size grinding liquid is a 1-3 μm polycrystalline grinding liquid.

[0010] According to the application, preferably, in the step (2), the grinding temperature is 25±1 ℃, the pressure is 20 N / piece, the rotation speed is 20 rpm, and the liquid flow rate is 2 ml / min.

[0011] Preferably, in the step (3), the wafer is immersed in a 50-90 ℃, 20-40 wt% alkali solution with N face downward; the alkali solution is KOH solution or NaOH solution; and the etching time is 20-50 min. Since the etching rate of N face is much higher than that of Ga face ((0001) face) and m face, the etching is preferentially carried out along the c-axis direction at a high speed; the surface processing mechanism is the sensitivity of etching rate to local stress state; the etching rate of convex or stress concentration area is faster, while the etching rate of concave or low stress area is slower; such differential etching can actively and quickly "peak clipping and valley filling", significantly improving the overall surface shape (reducing the Bow / Warp value); at the same time, the etching can completely remove the mechanical damage layer left by fine grinding.

[0012] According to the application, preferably, in the step (3), the substrate is subjected to ultrasonic cleaning to remove the diamond grinding fluid before etching.

[0013] According to the application, further preferably, in the step (3), the KOH etching solution is selected to be 65 ℃ and 30 wt%.

[0014] According to the application, preferably, in the step (3), the etching time is 30-40 min.

[0015] Preferably, in the step (4), the polishing temperature is 20-30 ℃, the pressure is 70-100 N / sheet, the rotation speed is 20-40 rpm, and the polishing liquid is recycled; through the synergistic effect of chemical oxidation softening and mechanical grinding to remove the softened layer, efficient and low-damage material removal is realized, and an atomic-level flat, damage-free and ultra-low roughness device-level Ga polarity face ((0001) face) is obtained.

[0016] According to the application, preferably, in the step (4), the polishing liquid designed for GaN is a basic nano-sized silica-based polishing liquid.

[0017] According to the application, preferably, in the step (4), the polishing temperature is 25±1 ℃, the pressure is 80 N / sheet, the rotation speed is 25 rpm, and the polishing liquid is recycled.

[0018] According to the application, preferably, in the step (5), the acid solution is sulfuric acid and hydrochloric acid (sulfuric acid concentration is 98%, hydrochloric acid concentration is 37%), and the hydrogen peroxide concentration is 30%, wherein the volume ratio of sulfuric acid: hydrogen peroxide: deionized water is 1:1:5, and the volume ratio of hydrochloric acid: hydrogen peroxide: deionized water is 1:1:6.

[0019] According to the application, preferably, in the step (5), the cleaning temperature is controlled at 60-80 ℃.

[0020] The application has the following beneficial effects: N-plane etching optimizes the surface accuracy of GaN substrates. This method utilizes the anisotropic properties of GaN material to quickly correct surface errors, significantly reducing the time required for traditional surface finishing relying on pure mechanical polishing. Etching completely eliminates the subsurface damage layer remaining after grinding, providing a more ideal starting surface for CMP and facilitating a more perfect final surface. Wet etching processes can easily achieve uniform processing of large-size wafers, which is beneficial for extending this process to even larger GaN substrates. Attached Figure Description

[0021] The accompanying drawings, which form part of this application, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an undue limitation of the invention. In the drawings: Figure 1 This is a warpage distribution diagram of the GaN substrate before etching in Example 1; Figure 2 This is a warpage distribution diagram of the GaN substrate after etching in Example 1; Figure 3 Images of the N-polar GaN substrate before and after etching in Example 1. Detailed Implementation

[0022] The present invention will be further described below with reference to specific embodiments, and the advantages and features of the present invention will become clearer with the description. However, the embodiments are merely exemplary and do not constitute any limitation on the scope of the present invention. Those skilled in the art should understand that modifications or substitutions can be made to the details and form of the technical solutions of the present invention without departing from the spirit and scope of the present invention, but all such modifications and substitutions fall within the protection scope of the present invention.

[0023] Example 1 A method for rapidly optimizing the bending and warpage of large-size gallium nitride substrates using an N-side etching process includes the following steps: (1) Coarse grinding, leveling, and thinning: The 2-inch GaN crystal grown by the HVPE method has a thickness greater than 1 mm, with large thickness differences. The wafer is initially leveled to achieve the target thickness. The GaN crystal is ground on a copper disk using a large-particle diamond polishing slurry on both the Ga and N sides. The polishing temperature is room temperature, the pressure is 30 N / wafer, the rotation speed is 30 rpm, and the slurry flow rate is 2 ml / min, which efficiently removes material and initially controls the thickness to 500 μm.

[0024] (2) Fine grinding to reduce roughness: Using a finer grinding slurry, the GaN substrate is ground on both the Ga and N sides on a tin tray to remove the severely damaged layer caused by coarse grinding and reduce surface roughness. The grinding temperature is room temperature, the pressure is 30 N / piece, the rotation speed is 20 rpm, and the liquid flow rate is 2 ml / min. No bright white scratches are visible under strong light, and the substrate Bow / Warp value shows a convex change, such as... Figure 1 As shown, the BOW value is 69.403 μm and the warp value is 87.998 μm.

[0025] (3) N-plane etching: Utilizing the high-speed, anisotropic etching characteristics of the N-plane ((000-1) plane) of GaN in a specific alkaline solution. The wafer is immersed in a 30 wt% alkaline solution at 65 °C with the N-plane facing down for 30 min. Since the etching rate of the N-plane is much higher than that of the Ga plane ((0001) plane) and the m-plane, etching preferentially proceeds rapidly along the c-axis. The surface shape processing mechanism is the sensitivity of the etching rate to the local stress state. The etching rate is faster in protrusions or stress concentration areas, while it is slower in depressions or low-stress areas. This differential etching can actively and quickly "shave peaks and fill valleys," significantly improving the overall surface shape (reducing the Bow / Warp value). At the same time, etching can completely remove the mechanical damage layer remaining from polishing. Figure 2 As shown, the BOW value is 4.631 μm and the warp value is 9.633 μm.

[0026] (4) Ga-face chemical mechanical polishing: The wafer is placed face down on the polishing pad of the CMP equipment. An alkaline nanoscale silica polishing slurry designed for GaN is used. The polishing temperature is room temperature, the pressure is 80 N / wafer, the rotation speed is 30 rpm, and the polishing slurry is recycled. Through the synergistic effect of chemical oxidation to soften the surface and mechanical grinding to remove the softened layer, efficient and low-damage material removal is achieved, resulting in an atomically flat, damage-free, and ultra-low roughness device-grade Ga polar surface ((0001) surface).

[0027] (5) Cleaning and drying: First, use deionized water (DIW) combined with megason cleaning to remove micron-sized particles. Then, use sulfuric acid:hydrogen peroxide:deionized water volume ratio = 1:1:5 and hydrochloric acid:hydrogen peroxide:deionized water volume ratio = 1:1:6 for ultrasonic cleaning to remove organic matter and metal ion contamination. The sulfuric acid concentration is 98%, the hydrochloric acid concentration is 37%, and the hydrogen peroxide concentration is 30%. Finally, use a large amount of DIW to thoroughly rinse away all chemical residues. Finally, spin dry at high speed and package.

[0028] Figure 3 Images of the N-polar GaN substrate before and after etching in Example 1 are shown. Before etching, the surface was slightly yellow and transparent after polishing. The test results are as follows. Figure 1As shown, the BOW value is 69.403 μm and the warp value is 87.998 μm; after etching, the surface becomes rougher and whiter. The test results are as follows. Figure 2 As shown, the BOW value is 4.631 μm and the warp value is 9.633 μm.

[0029] Example 2 A method for rapidly optimizing the bending and warpage of large-size gallium nitride substrates using an N-side etching process includes the following steps: (1) Coarse grinding, leveling, and thinning: The 2-inch GaN crystal grown by the HVPE method has a thickness greater than 1 mm, with large thickness differences. The wafer is initially leveled to achieve the target thickness. The GaN crystal is ground on a copper disk using a large-particle diamond polishing slurry on both the Ga and N sides. The polishing temperature is room temperature, the pressure is 30 N / wafer, the rotation speed is 30 rpm, and the slurry flow rate is 2 ml / min, which efficiently removes material and initially controls the thickness to 500 μm.

[0030] (2) Fine grinding to reduce roughness: Using a finer grinding slurry, the GaN substrate was ground on a tin tray on both the Ga and N sides to remove the severely damaged layer caused by coarse grinding and reduce the surface roughness. The grinding temperature was room temperature, the pressure was 30 N / piece, the rotation speed was 20 rpm, and the liquid flow rate was 2 ml / min. No bright white scratches were observed under strong light. The Bow / Warp value of the substrate showed a convex change, with a BOW value of 83.347 μm and a warp value of 60.554 μm. (3) N-face etching: Utilizing the high-speed, anisotropic etching characteristics of the N-face ((000-1) face) of GaN in a specific alkaline solution. The wafer was immersed in a 30 wt% alkaline solution at 50 °C with the N-face facing down for 40 min. Since the etching rate of the N-face is much higher than that of the Ga face ((0001) face) and the m-face, etching preferentially proceeds rapidly along the c-axis. The surface shape processing mechanism is the sensitivity of the etching rate to the local stress state. The etching rate is faster in protrusions or stress concentration areas, while it is slower in depressions or low-stress areas. This differential etching can actively and quickly "shave peaks and fill valleys", significantly improving the overall surface shape (reducing the Bow / Warp value). At the same time, etching can completely remove the mechanical damage layer remaining from the grinding. The BOW value is 5.349 μm, and the warp value is 4.980 μm.

[0031] (4) Ga-face chemical mechanical polishing: The wafer is placed face down on the polishing pad of the CMP equipment. An alkaline nanoscale silica polishing slurry designed for GaN is used. The polishing temperature is room temperature, the pressure is 80 N / wafer, the rotation speed is 30 rpm, and the polishing slurry is recycled. Through the synergistic effect of chemical oxidation to soften the surface and mechanical grinding to remove the softened layer, efficient and low-damage material removal is achieved, resulting in an atomically flat, damage-free, and ultra-low roughness device-grade Ga polar surface ((0001) surface).

[0032] (5) Cleaning and drying: First, use deionized water (DIW) combined with megason cleaning to remove micron-sized particles. Then, use sulfuric acid:hydrogen peroxide:deionized water volume ratio = 1:1:5 and hydrochloric acid:hydrogen peroxide:deionized water volume ratio = 1:1:6 for ultrasonic cleaning to remove organic matter and metal ion contamination. The sulfuric acid concentration is 98%, the hydrochloric acid concentration is 37%, and the hydrogen peroxide concentration is 30%. Finally, use a large amount of DIW to thoroughly rinse away all chemical residues. Finally, spin dry at high speed and package.

[0033] Example 3 A method for rapidly optimizing the bending and warpage of large-size gallium nitride substrates using an N-side etching process includes the following steps: (1) Coarse grinding, leveling, and thinning: The 2-inch GaN crystal grown by the HVPE method has a thickness greater than 1 mm, with large thickness differences. The wafer is initially leveled to achieve the target thickness. The GaN crystal is ground on a copper disk using a large-particle diamond polishing slurry on both the Ga and N sides. The polishing temperature is room temperature, the pressure is 30 N / wafer, the rotation speed is 30 rpm, and the slurry flow rate is 2 ml / min, which efficiently removes material and initially controls the thickness to 550 μm.

[0034] (2) Fine grinding to reduce roughness: The GaN substrate was ground on a tin tray using a finer grinding slurry to remove the severely damaged layer caused by coarse grinding and reduce the surface roughness. The grinding temperature was room temperature, the pressure was 30 N / piece, the rotation speed was 20 rpm, and the liquid flow rate was 2 ml / min. No bright white scratches were observed under strong light. The Bow / Warp value of the substrate showed a convex change, with a BOW value of 46.025 μm and a warp value of 65.163 μm. (3) N-face etching: The high-speed, anisotropic etching characteristics of the N-face ((000-1) face) of GaN in a specific alkaline solution are utilized. The wafer is immersed in a 25 wt% alkaline solution at 60 °C with the N-face facing down for 40 min. Since the etching rate of the N-face is much higher than that of the Ga face ((0001) face) and the m face, the etching will preferentially proceed rapidly along the c-axis. The surface shape processing mechanism is the sensitivity of the etching rate to the local stress state. The etching rate is faster in protrusions or stress concentration areas, while the etching rate is slower in depressions or low stress areas. This differential etching can actively and quickly "shave peaks and fill valleys", significantly improving the overall surface shape (reducing the Bow / Warp value). At the same time, etching can completely remove the mechanical damage layer left by fine grinding. The BOW value is 3.357 μm, and the warp value is 7.082 μm.

[0035] (4) Ga-face chemical mechanical polishing: The wafer is placed face down on the polishing pad of the CMP equipment. An alkaline nanoscale silica polishing slurry designed for GaN is used. The polishing temperature is room temperature, the pressure is 80 N / wafer, the rotation speed is 30 rpm, and the polishing slurry is recycled. Through the synergistic effect of chemical oxidation to soften the surface and mechanical grinding to remove the softened layer, efficient and low-damage material removal is achieved, resulting in an atomically flat, damage-free, and ultra-low roughness device-grade Ga polar surface ((0001) surface).

[0036] (5) Cleaning and drying: First, use deionized water (DIW) combined with megason cleaning to remove micron-sized particles. Then, use sulfuric acid:hydrogen peroxide:deionized water volume ratio = 1:1:5 and hydrochloric acid:hydrogen peroxide:deionized water volume ratio = 1:1:6 for ultrasonic cleaning to remove organic matter and metal ion contamination. The sulfuric acid concentration is 98%, the hydrochloric acid concentration is 37%, and the hydrogen peroxide concentration is 30%. Finally, use a large amount of DIW to thoroughly rinse away all chemical residues. Finally, spin dry at high speed and package.

Claims

1. A method for rapidly improving the bending and warpage of large-size gallium nitride substrates using an N-plane etching process, characterized in that, Includes the following steps: (1) Rough grinding, leveling and thinning: The GaN crystal was ground on a copper disk using large-particle diamond grinding fluid on the Ga and N sides respectively; (2) Fine grinding to reduce roughness: Using diamond polishing slurry with smaller particle size, the GaN substrate is polished on the Ga and N sides on the tin tray to remove the severely damaged layer caused by coarse grinding and reduce the surface roughness; (3) N-face etching: The N-face of GaN is etched in an alkaline solution; (4) Ga surface chemical mechanical polishing: Place the wafer with the Ga surface facing down on the polishing pad of the CMP equipment; polish using nano-sized silicon dioxide or cerium oxide-based polishing slurry; (5) Cleaning and drying: First, use deionized water to clean the GaN substrate with megasonic wave to remove micron-sized particles. Then, use diluted acid solution and hydrogen peroxide to remove organic matter and metal ion contamination. Finally, use a large amount of deionized water to rinse. Finally, spin dry at high speed and package.

2. The method according to claim 1, characterized in that, In step (1), the grinding temperature is 20-30 ℃, the pressure is 20-50 N / piece, the rotation speed is 20-40 rpm, and the liquid flow rate is 1-5 ml / min, and the GaN substrate thickness is initially controlled to be 500-600 μm; preferably, the large-particle-size grinding slurry in step (1) is a polycrystalline grinding slurry with a diameter of 4-8 μm; more preferably, the grinding temperature in step (1) is 25±1 ℃, the pressure is 30 N / piece, the rotation speed is 30 rpm, and the liquid flow rate is 2 ml / min.

3. The method according to claim 1, characterized in that, In step (2), the grinding temperature is 20-30 ℃, the pressure is 20-50 N / piece, the rotation speed is 20-40 rpm, the liquid flow rate is 1-5 ml / min, there are no bright white scratches under strong light, and the substrate Bow / Warp value shows a convex change; preferably, the small-particle-size grinding slurry in step (2) is a 1-3 μm polycrystalline grinding slurry; more preferably, the grinding temperature in step (2) is 25±1 ℃, the pressure is 20 N / piece, the rotation speed is 20 rpm, and the liquid flow rate is 2 ml / min.

4. The method according to claim 1, characterized in that, In step (3), the wafer is immersed face down in an alkaline solution with a concentration of 20-40 wt% at 50-90 ℃; the alkaline solution is KOH solution or NaOH solution; the etching time is 20-50 min.

5. The method according to claim 1, characterized in that, In step (3), the substrate needs to be ultrasonically cleaned to remove the diamond polishing slurry before etching. Preferably, the KOH etching solution selected in step (3) is 65°C and has a concentration of 30 wt%.

6. The method according to claim 1, characterized in that, The etching time in step (3) is 30-40 min.

7. The method according to claim 1, characterized in that, In step (4), the polishing temperature is 20-30 ℃, the pressure is 70-100 N / piece, the rotation speed is 20-40 rpm, and the polishing fluid is recycled.

8. The method according to claim 1, characterized in that, The polishing liquid used in step (4) is an alkaline nano-sized silica-based polishing liquid.

9. The method according to claim 1, characterized in that, In step (4), the polishing temperature is 25±1℃, the pressure is 80 N / piece, the rotation speed is 25 rpm, and the polishing fluid is recycled.

10. The method according to claim 1, characterized in that, In step (5), the acid solution is sulfuric acid and hydrochloric acid, with a sulfuric acid concentration of 98%, a hydrochloric acid concentration of 37%, and a hydrogen peroxide concentration of 30%. The volume ratio of sulfuric acid: hydrogen peroxide: deionized water is 1:1:5, and the volume ratio of hydrochloric acid: hydrogen peroxide: deionized water is 1:1:

6. Preferably, the cleaning temperature in step (5) is controlled at 60-80 ℃.

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