Aluminum nitride thermal field assisted chemical mechanical polishing method

Abstract

The invention discloses an aluminum nitride thermal field assisted chemical mechanical polishing method, and belongs to the field of polishing. The invention aims to solve the problems of surface scratches, pits and cracks caused by the fact that abrasive particles can directly remove the surface of aluminum nitride because the surface chemical reaction rate is extremely low and a corrosion softening layer cannot be formed in time in the existing chemical mechanical polishing. The preparation method comprises the following steps: 1, preparing a polishing solution; and 2, polishing treatment is conducted under the conditions that the polishing solution circularly flows and is heated. The method is used for aluminum nitride thermal field auxiliary chemical mechanical polishing.

Description

Technical Field

[0001] This invention belongs to the field of polishing. Background Technology

[0002] Aluminum nitride (ANT) has been widely used in advanced electronic packaging and substrate applications due to its high thermal conductivity, excellent electrical insulation properties, and coefficient of thermal expansion that matches that of semiconductor materials such as silicon and silicon carbide. In high-power-density electronic devices and high-frequency communication systems, ANT not only provides electrical insulation and structural support but also plays a crucial role in device thermal management. With the rapid development of 5G communication, radio frequency devices, and third-generation semiconductor power devices, the demand for high-performance ANT substrates and PCBs continues to grow, and their processing quality has become a key factor affecting device performance and reliability.

[0003] Currently, aluminum nitride processing mainly employs mechanical grinding, lapping, and polishing. However, due to the high brittleness and poor grain boundary bonding of aluminum nitride, mechanical processing can cause defects such as surface scratches, grain detachment, and pits. Chemical mechanical polishing (CMP) is an effective method for improving the surface quality of ceramics, but aluminum nitride has a large band gap and good chemical stability. Existing CMP processes result in extremely low surface chemical reaction rates, failing to form a timely corrosion softening layer. Consequently, the abrasive grains directly remove the aluminum nitride surface, causing surface scratches, pits, and cracks. Summary of the Invention

[0004] This invention aims to address the problem that in existing chemical mechanical polishing, the surface chemical reaction rate is extremely low, making it impossible to form a corrosion softening layer in time. As a result, the abrasive particles directly remove the aluminum nitride surface, causing surface scratches, pits, and cracks. Therefore, this invention provides a thermal field-assisted chemical mechanical polishing method for aluminum nitride.

[0005] A thermal field-assisted chemical mechanical polishing method for aluminum nitride is performed according to the following steps:

[0006] I. Preparation of polishing fluid:

[0007] Abrasive, dispersant, complexing agent and water are mixed, and then stirred and ultrasonically dispersed in sequence to obtain polishing fluid;

[0008] The pH of the polishing solution is 3-7;

[0009] II. Polishing:

[0010] The aluminum nitride to be polished is fixed onto the polishing head, and polishing is performed under the conditions of a polishing slurry circulation flow rate of 50mL / min~120mL / min and a polishing slurry temperature of 50℃~90℃, thus completing the aluminum nitride thermal field assisted chemical mechanical polishing method.

[0011] The beneficial effects of this invention are:

[0012] To achieve synergistic chemical-mechanical removal, an external energy field is needed to activate the surface activity of aluminum nitride and increase its chemical reaction rate. Therefore, this invention proposes a method for chemical-mechanical polishing of polycrystalline aluminum nitride under a constant-temperature immersion environment by controlling the temperature of the polishing slurry. Heat is transferred to the aluminum nitride surface through the polishing slurry, heating the sample surface and activating the unsaturated atoms such as Al and N on the aluminum nitride surface. This leads to the breaking of Al-N bonds, promoting the hydrolysis reaction of aluminum nitride and generating a mixed corrosion product of aluminum hydroxide and aluminum metahydride. The acid and complexing agent in the polishing slurry dissolve the corrosion products in a timely manner, continuously exposing new corrosion layers on the surface and accelerating the chemical reaction. Simultaneously, heating also increases the diffusion rate of liquid molecules in the polishing slurry, which is beneficial for surface chemical reactions. Under the mechanical action of the polishing abrasive, high-efficiency and high-quality polishing of polycrystalline aluminum nitride is achieved.

[0013] Compared to mechanical polishing of polycrystalline aluminum nitride or existing chemical mechanical polishing, this invention utilizes closed-loop isotropic control of the polishing slurry temperature and immerses the workpiece in the isotropic slurry during polishing. By leveraging the temperature's influence on interfacial reaction kinetics, it significantly promotes the formation of a reaction film on the polycrystalline aluminum nitride surface, thereby strengthening the synergistic mechanism of "chemical reaction-mechanical removal," increasing the removal rate, and avoiding surface scratches, pits, and other defects. Furthermore, this invention, with its water-based polishing slurry system and temperature control as its core, does not rely on strong corrosive solutions such as acids / alkalis, reducing chemical usage and wastewater discharge risks. It boasts advantages such as being environmentally friendly and easily applicable in engineering. Attached Figure Description

[0014] Figure 1 This is a schematic diagram of the equipment used for polishing in Example 1;

[0015] Figure 2 The surface roughness morphology of polished aluminum nitride in Example 4 and Comparative Experiment 1 are shown in the figures. (a) Comparative Experiment 1, (b) Example 4.

[0016] Figure 3 The graph shows a comparison of the removal rates of polished aluminum nitride materials in Examples 1 to 4 and Comparative Experiments 1 to 2.

[0017] Figure 4 The images show a comparison of the surface roughness of polished aluminum nitride in Examples 1 to 4 and Comparative Experiments 1 to 2. Detailed Implementation

[0018] Specific Implementation Method 1: This implementation method provides a thermal field-assisted chemical mechanical polishing method for aluminum nitride, which is carried out according to the following steps:

[0019] I. Preparation of polishing fluid:

[0020] Abrasive, dispersant, complexing agent and water are mixed, and then stirred and ultrasonically dispersed in sequence to obtain polishing fluid;

[0021] The pH of the polishing solution is 3-7;

[0022] II. Polishing:

[0023] The aluminum nitride to be polished is fixed onto the polishing head, and polishing is performed under the conditions of a polishing slurry circulation flow rate of 50mL / min~120mL / min and a polishing slurry temperature of 50℃~90℃, thus completing the aluminum nitride thermal field assisted chemical mechanical polishing method.

[0024] The main principle of this specific implementation method is to use temperature to achieve chemical corrosion of the aluminum nitride surface. Aluminum nitride hardly undergoes any surface chemical reaction at room temperature; surface reactions only occur under certain temperature conditions, as shown below:

[0025]

[0026]

[0027] The corrosion layer has very low hardness. If the pH of the polishing solution is simultaneously adjusted to acidic, the acidic conditions can further accelerate the dissolution of the surface corrosion layer, synergistically enhancing the abrasive mechanical action and achieving corrosion layer removal. The reaction in the acidic solution is shown below:

[0028]

[0029] The complexing agent further interacts with Al 3+ Combined, reducing the Al content of the polishing slurry 3+Concentration promotes the continuous occurrence of the reaction. Simultaneously, temperature further accelerates the dissolution reaction of the corrosion layer. Unlike the temperature principle in existing technologies, in this specific embodiment, temperature is a necessary condition to ensure the occurrence of the chemical reaction, further proposing a synergistic effect with the chemical action of the polishing slurry to improve the chemical reaction rate of the aluminum nitride surface. Existing polishing or corrosion technologies involving heating mainly target metals, oxides, or easily reactive materials, which can already undergo chemical reactions at room temperature. Introducing temperature is only used to accelerate the reaction kinetics or improve the performance of the polishing slurry. However, aluminum nitride has extremely high chemical stability at room temperature, almost never undergoing hydrolysis or corrosion reactions, resulting in the inability to form a low-hardness reaction layer, thus severely limiting the efficiency and surface quality of chemical mechanical polishing. In this specific embodiment, temperature is not used to simply accelerate the existing reaction, but rather to induce a hydrolysis reaction on the aluminum nitride surface, generating a low-hardness aluminum hydroxide corrosion layer. Experiments and theories both show that this reaction hardly occurs at room temperature, and a stable surface reaction layer can only be formed above a certain temperature threshold. Therefore, heating is a necessary means to achieve the technical solution in this specific embodiment, thus creating the preconditions for simultaneous acidic dissolution and abrasive removal. Furthermore, this specific embodiment does not introduce temperature in isolation, but rather designs temperature synergistically with the acidic chemical system and the complexing agent: temperature induces the formation of an aluminum hydroxide corrosion layer on the aluminum nitride surface, the acidic chemicals dissolve this corrosion layer, and the complexing agent continuously reduces the concentration of reaction products in the solution by complexing Al³⁺, thus promoting the continuous reaction from both thermodynamic and kinetic perspectives. This synergistic effect has not been disclosed in the prior art for aluminum nitride materials.

[0030] Meanwhile, polycrystalline aluminum nitride exhibits significant anisotropy. Due to the differences in hardness among different grains, mechanical polishing alone will result in significant height differences between the grains, leading to a rough surface. Chemical reactions can reduce the hardness differences among the grains. Harder aluminum nitride grains are more chemically reactive, while softer grains are less chemically reactive. By synergistically controlling the chemical rate through temperature and pH, the thickness of the reaction layer among different grains can be effectively controlled, resulting in a more consistent removal depth for different grains during polishing. This reduces the height differences between grains and ultimately lowers surface roughness.

[0031] The beneficial effects of this embodiment are:

[0032] To achieve synergistic chemical-mechanical removal, an external energy field is needed to activate the surface activity of aluminum nitride and increase its chemical reaction rate. Therefore, this embodiment proposes a method for chemical-mechanical polishing of polycrystalline aluminum nitride under a constant-temperature immersion environment by controlling the polishing slurry temperature. Heat is transferred to the aluminum nitride surface through the polishing slurry, heating the sample surface and activating the unsaturated atoms such as Al and N on the aluminum nitride surface. This leads to the breaking of Al-N bonds, promoting the hydrolysis reaction of aluminum nitride and generating a mixed corrosion product of aluminum hydroxide and aluminum metahydroxide. The acid and complexing agent in the polishing slurry dissolve the corrosion products in a timely manner, continuously exposing new corrosion layers on the surface and accelerating the chemical reaction. Simultaneously, heating also increases the diffusion rate of liquid molecules in the polishing slurry, which is beneficial for surface chemical reactions. Under the mechanical action of the polishing abrasive, high-efficiency and high-quality polishing of polycrystalline aluminum nitride is achieved.

[0033] Compared to mechanical polishing of polycrystalline aluminum nitride or existing chemical mechanical polishing, this embodiment utilizes closed-loop isotropic control of the polishing slurry temperature and immerses the workpiece in the isotropic slurry during polishing. By leveraging the temperature's regulatory effect on interfacial reaction kinetics, it significantly promotes the formation of a reaction film on the polycrystalline aluminum nitride surface, thereby strengthening the synergistic mechanism of "chemical reaction-mechanical removal," increasing the removal rate, and avoiding surface scratches, pits, and other defects. Furthermore, this embodiment, with its water-based polishing slurry system and temperature control as its core, does not rely on strong corrosive solutions such as acids / alkalis, reducing chemical usage and wastewater discharge risks. It boasts advantages such as being environmentally friendly and easily applied in engineering.

[0034] Specific Implementation Method Two: This implementation method differs from Specific Implementation Method One in that the abrasive mentioned in step one is diamond abrasive or alumina abrasive, and the particle size is 200nm~1μm. Everything else is the same as in Specific Implementation Method One.

[0035] Specific Implementation Method Three: This implementation method differs from Specific Implementation Method One or Two in that: the dispersant mentioned in step one is sodium hexametaphosphate; and the complexing agent mentioned in step one is urea. Everything else is the same as in Specific Implementation Method One or Two.

[0036] Specific Implementation Method Four: This implementation method differs from Specific Implementation Methods One to Three in that: in step one, when the pH of the polishing solution is ≥3 and <7, it is adjusted using phosphoric acid with a concentration ≥85wt%. Everything else is the same as in Specific Implementation Methods One to Three.

[0037] Specific Implementation Method Five: This implementation method differs from Specific Implementation Methods One to Four in that the concentration of abrasive in the polishing fluid described in step one is 5wt%~10wt%, the concentration of dispersant is 0.5wt%~1wt%, and the concentration of complexing agent is 0.25wt%~1wt%. Everything else is the same as in Specific Implementation Methods One to Four.

[0038] Specific Implementation Method Six: This implementation method differs from Specific Implementation Methods One to Five in that: in step one, the stirring and ultrasonic dispersion are performed sequentially. Specifically, the stirring is carried out at a speed of 600 rpm to 1000 rpm for 20 to 50 minutes, followed by ultrasonic dispersion at an ultrasonic power of 150 W to 300 W for 5 to 20 minutes. Everything else is the same as in Specific Implementation Methods One to Five.

[0039] Specific Implementation Method Seven: This implementation method differs from Specific Implementation Methods One to Six in that the aluminum nitride to be polished in step two is polycrystalline aluminum nitride. Everything else is the same as in Specific Implementation Methods One to Six.

[0040] Specific Implementation Method Eight: This implementation method differs from Specific Implementation Methods One to Seven in that: during the polishing process in step two, the polishing disc speed is 40 rpm to 90 rpm, the polishing head speed is 20 rpm to 50 rpm, and the polishing pressure is 30 kPa to 50 kPa. Everything else is the same as in Specific Implementation Methods One to Seven.

[0041] Specific Implementation Method Nine: This implementation method differs from Specific Implementation Methods One to Eight in that: during the polishing process in Step Two, the polishing head and polishing pad rotate in the same direction, and the polishing pad is a polyurethane polishing pad; during the polishing process in Step Two, the aluminum nitride to be polished is immersed in the polishing liquid. Everything else is the same as in Specific Implementation Methods One to Eight.

[0042] Specific Implementation Method Ten: This implementation method differs from Specific Implementation Methods One to Nine in that the polishing time in step two is 40-80 minutes. Everything else is the same as in Specific Implementation Methods One to Nine.

[0043] The following examples demonstrate the beneficial effects of the present invention:

[0044] Example 1:

[0045] A thermal field-assisted chemical mechanical polishing method for aluminum nitride is performed according to the following steps:

[0046] I. Preparation of polishing fluid:

[0047] Diamond abrasive, dispersant, complexing agent and water are mixed, and then stirred at 800 rpm for 30 min. Then, the mixture is ultrasonically dispersed at 150 W for 10 min to obtain polishing liquid.

[0048] The polishing solution has a pH of 7;

[0049] II. Polishing:

[0050] Aluminum nitride with a diameter of 25 mm and a thickness of 1 mm was fixed onto a polishing head using high-temperature resistant epoxy resin. The polishing was carried out for 1 hour under the following conditions: polishing fluid circulation speed of 30 mL / min, polishing fluid temperature of 50 °C, polishing disc speed of 60 rpm, polishing head speed of 30 rpm, and polishing pressure of 35 KPa, to obtain polished aluminum nitride.

[0051] The diamond abrasive mentioned in step one has a particle size of 250 nm.

[0052] The dispersant mentioned in step one is sodium hexametaphosphate; the complexing agent mentioned in step one is urea.

[0053] The concentration of diamond abrasive in the polishing fluid described in step one is 10 wt%, the concentration of dispersant is 1 wt%, and the concentration of complexing agent is 1 wt%.

[0054] The aluminum nitride to be polished in step two is polycrystalline aluminum nitride; and before polishing, the surface of the aluminum nitride to be polished is cleaned using an ultrasonic cleaner to ensure that there are no impurities.

[0055] In step two, during the polishing process, the polishing head and polishing pad rotate in the same direction, and the polishing pad is a polyurethane polishing pad.

[0056] In step two, the aluminum nitride to be polished is immersed in the polishing solution.

[0057] Example 2: This implementation differs from Example 1 in that the polishing fluid temperature in step two is 80℃. Everything else is the same as in Example 1.

[0058] Example 3: This example differs from Example 1 in that in step one, phosphoric acid with a concentration ≥85wt% is used to adjust the pH of the polishing solution to 4. Everything else is the same as in Example 1.

[0059] Example 4: This example differs from Example 1 in that the pH is adjusted to 6 using phosphoric acid with a concentration ≥85wt% in step one. Everything else is the same as in Example 1.

[0060] Comparative Experiment 1: This comparative experiment differs from Example 1 in that the polishing solution temperature in step two is 28℃. Everything else is the same as in Example 1.

[0061] Comparative Experiment 2: This comparative experiment differs from Example 4 in that the complexing agent is omitted. Everything else is the same as Example 4.

[0062] Figure 1This is a schematic diagram of the polishing equipment used in Example 1. As shown in the diagram, it includes a polishing machine (where the polishing disc contains a retaining ring for circulating the polishing fluid); a magnetic stirring heater, which stirs the polishing fluid via a magnetic rotor and simultaneously heats it; a thermocouple to measure the temperature of the liquid inside the polishing disc; a peristaltic pump to circulate the polishing fluid; and a beaker for holding the polishing fluid. Before polishing, the polishing fluid is continuously circulated by two peristaltic pumps to ensure a constant temperature and volume of the liquid inside the polishing disc. The temperature of the magnetic stirring heater is set, and the temperature of the liquid inside the polishing disc is measured by the thermocouple. Once the temperature stabilizes within ±3°C of the set temperature, polishing begins. After polishing, the surface roughness is measured using a Zygo white light interferometer, and the mass of the samples before and after polishing is weighed using a high-precision balance to calculate the material removal rate.

[0063] Figure 2 The images show the surface roughness morphology of polished aluminum nitride in Example 4 and Comparative Experiment 1. (a) Comparative Experiment 1, (b) Example 4. Comparative Experiment 1 was conducted at room temperature, meaning there was no chemical reaction, and therefore it was mechanical polishing. It can be seen that the mechanically polished surface in Comparative Experiment 1 will have scratches, while the surface of Example 4 after chemical mechanical polishing will not have scratches.

[0064] Figure 3 The figures show a comparison of the removal rates of polished aluminum nitride materials in Examples 1 to 4 and Comparative Experiments 1 to 2. As can be seen from the figures, the removal rates for Examples 1 to 4 are 4.1 μm / h, 5.4 μm / h, 5.2 μm / h, and 4.8 μm / h, respectively, while those for Comparative Experiments 1 to 2 are 3.7 μm / h and 4.7 μm / h, respectively. The synergistic effect of increasing temperature, decreasing pH, and adding complexing agents can achieve surface modification of aluminum nitride. The material removal rate of Example 3 is 40% higher than that of Comparative Experiment 1.

[0065] Figure 4 The figures show a comparison of the surface roughness of polished aluminum nitride in Examples 1 to 4 and Comparative Experiments 1 to 2. As can be seen from the figures, the roughness of aluminum nitride in Examples 1 to 4 is 12.1 nm, 11.2 nm, 9.8 nm and 8.9 nm, respectively, while that in Comparative Experiments 1 to 2 is 18.0 nm and 10.4 nm, respectively. The synergistic effect of increasing temperature, decreasing pH and adding complexing agents can make grain removal more consistent, and the surface roughness can be reduced to as low as 8.9 nm.

Claims

1. A thermal field-assisted chemical mechanical polishing method for aluminum nitride, characterized in that... It is done in the following steps: I. Preparation of polishing slurry: Abrasive, dispersant, complexing agent and water are mixed, and then stirred and ultrasonically dispersed in sequence to obtain polishing fluid; The pH of the polishing solution is 3-7; II. Polishing: The aluminum nitride to be polished is fixed onto the polishing head, and polishing is performed under the conditions of a polishing slurry circulation flow rate of 50mL / min~120mL / min and a polishing slurry temperature of 50℃~90℃, thus completing the aluminum nitride thermal field assisted chemical mechanical polishing method.

2. The aluminum nitride thermal field-assisted chemical mechanical polishing method according to claim 1, characterized in that... The abrasive mentioned in step one is diamond abrasive or alumina abrasive, and the particle size is 200nm~1μm.

3. The aluminum nitride thermal field-assisted chemical mechanical polishing method according to claim 1, characterized in that... The dispersant mentioned in step one is sodium hexametaphosphate; the complexing agent mentioned in step one is urea.

4. The aluminum nitride thermal field-assisted chemical mechanical polishing method according to claim 1, characterized in that... In step one, when the pH of the polishing solution is ≥3 and <7, it is adjusted using phosphoric acid with a concentration ≥85wt%.

5. The aluminum nitride thermal field-assisted chemical mechanical polishing method according to claim 1, characterized in that... The concentration of abrasive in the polishing slurry described in step one is 5wt%~10wt%, the concentration of dispersant is 0.5wt%~1wt%, and the concentration of complexing agent is 0.25wt%~1wt%.

6. The aluminum nitride thermal field-assisted chemical mechanical polishing method according to claim 1, characterized in that... In step one, the stirring and ultrasonic dispersion are carried out sequentially. Specifically, the stirring speed is 600 rpm to 1000 rpm for 20 min to 50 min, and then the ultrasonic dispersion is carried out at an ultrasonic power of 150 W to 300 W for 5 min to 20 min.

7. The aluminum nitride thermal field-assisted chemical mechanical polishing method according to claim 1, characterized in that... The aluminum nitride to be polished in step two is polycrystalline aluminum nitride.

8. The aluminum nitride thermal field-assisted chemical mechanical polishing method according to claim 1, characterized in that... During the polishing process in step two, the polishing disc rotates at 40 rpm to 90 rpm, the polishing head rotates at 20 rpm to 50 rpm, and the polishing pressure is 30 kPa to 50 kPa.

9. The aluminum nitride thermal field-assisted chemical mechanical polishing method according to claim 1, characterized in that... In step two, the polishing head and polishing pad rotate in the same direction, and the polishing pad is a polyurethane polishing pad. In step two, the aluminum nitride to be polished is immersed in the polishing liquid.

10. The aluminum nitride thermal field-assisted chemical mechanical polishing method according to claim 1, characterized in that... The polishing process in step two takes 40 to 80 minutes.

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