Mpcvd method for eliminating thermal stress of large-size diamond growth

Abstract

The application discloses an MPCVD method for eliminating thermal stress of large-size diamond growth, and aims to solve the problem of crystal cracking caused by thermal stress accumulation in long-time epitaxial growth. The method comprises the following steps: (1) at a first temperature T1, carbon-containing gas and hydrogen are introduced to perform diamond epitaxial growth; (2) after reaching a preset thickness, the introduction of carbon-containing gas is stopped, the substrate temperature is raised from T1 to T2 in a pure hydrogen plasma atmosphere, and in-situ oxidation-pure hydrogen combined annealing (oxygen etching + pure hydrogen purification) is performed to release accumulated stress; (3) the substrate temperature is lowered from T2 to T1, and the carbon-containing gas is introduced again to continue growth; (4) the growth-annealing cycle is repeated until the target size is obtained. The application takes in-situ plasma annealing as a compulsory step in the growth cycle, breaks through the limitation of the critical cracking thickness inherent in continuous growth through periodic stress relaxation, does not need to change the equipment hardware, has strong universality, and can repeatedly obtain large-size single crystal diamond without cracks.

Description

Technical Field

[0001] This invention belongs to the field of crystal growth technology, specifically relating to a method for preparing large-size single-crystal diamond using microwave plasma chemical vapor deposition (MPCVD). In particular, it is a segmented cyclic preparation process that eliminates epitaxial growth thermal stress and inhibits crystal cracking through periodic in-situ oxidizing plasma-assisted etching and pure hydrogen purification annealing. This process is suitable for the mass production of high-purity thick single-crystal diamonds for high thermal conductivity heat sinks, infrared optical windows, diamond semiconductor substrates, etc. Background Technology

[0002] Single-crystal diamond, due to its ultra-high hardness, thermal conductivity, wide spectral transmittance, and excellent semiconductor properties, has irreplaceable application prospects in precision machining, optical windows, thermal management, and next-generation power electronic devices. MPCVD, with its advantages of no electrode contamination, high plasma density, and controllable growth, has become the mainstream method for preparing high-quality single-crystal diamond.

[0003] However, a significant technical bottleneck in growing large-size (especially thicker than 1 mm) single-crystal diamonds via MPCVD is that, with increasing growth time and thickness, considerable thermal stress gradually accumulates within the crystal due to uneven temperature distribution, impurity incorporation gradients, and the evolution of stress states on the growth surface itself. When this accumulated stress exceeds the fracture toughness of the diamond itself, it manifests as macroscopic cracks or even complete fragmentation, directly causing the failure of a growth process that can last for tens or even hundreds of hours.

[0004] Existing solutions mainly fall into two categories: one is to optimize temperature field uniformity through cavity hardware design, such as improving the substrate stage structure and adding auxiliary heating devices. However, these methods are strongly tied to specific equipment, have poor versatility, and involve high hardware modification barriers and long cycles. The other approach slows down the diamond deposition rate by reducing carbon source concentration and lowering microwave power, relying on extending the growth time to slowly release stress. However, this doubles the preparation cycle for a single batch of crystals, significantly increasing the cost of large-scale production and making it difficult to achieve industrial-scale mass production. Existing research has also proposed adding trace amounts of nitrogen, oxygen, and other auxiliary gases to the reaction chamber to adjust lattice stress. However, nitrogen and oxygen elements are highly susceptible to incorporating into the diamond lattice, forming impurity defect energy levels that deteriorate key properties of single-crystal diamond, such as optical transmittance and carrier mobility, failing to meet the requirements of high-purity applications such as heat sinks, optical windows, and semiconductor power devices.

[0005] Therefore, there is an urgent need for a process method that does not rely on equipment modification, is highly universal, and does not significantly reduce production efficiency, in order to fundamentally solve the problem of thermal stress cracking in the growth of large-size single-crystal diamonds by MPCVD. Summary of the Invention

[0006] The purpose of this invention is to overcome the shortcomings of the prior art and provide an MPCVD method with a "growth-in-situ annealing" cycle.

[0007] Its core principle is that this invention abandons the passive stress release mode that relies solely on temperature cycling, and instead constructs a three-stage active stress relief mechanism of "chemical point etching - passivation - in-situ healing".

[0008] The first stage involves introducing trace amounts of oxygen-containing gas when the temperature is raised to T2. The oxygen generates OH radicals in the high-temperature hydrogen plasma. These radicals have anisotropic etching properties and can preferentially attack the tips of microcracks with the highest lattice distortion energy and dislocation pile-up regions, blunting the sharp microcrack tips into arc-shaped pits, thus physically terminating the path of crack propagation. Second stage: After stopping the oxygen-containing gas, pure hydrogen plasma is used to remove residual oxygen termination bonds and trace amounts of non-diamond carbon from the surface, restoring an atomically clean surface. The third stage: During the cooling process, new instantaneous tensile stress will be generated due to thermal contraction. When the temperature drops to T3 (between T1 and T2), the present invention introduces a high-concentration carbon-containing gas source through an extremely short pulse to epitaxially grow an ultrathin carbon healing layer with a thickness of only a few atomic layers on the clean surface. This healing layer can fill the etched pits and act as a stress buffer intermediate layer during the subsequent cooling process to prevent the thermal shock of cooling from directly acting on the bulk material.

[0009] Through the above three steps, this invention transforms "avoiding cracks" from a macroscopic thermodynamic probabilistic event into a deterministic process actively terminated by chemical etching, fundamentally breaking through the inherent critical cracking thickness limitation of continuous growth.

[0010] This invention strictly controls the annealing stage to maintain the same gas pressure and pure hydrogen plasma environment as the growth stage, avoiding pressure fluctuations in the cavity and secondary deposition of carbon impurities during temperature rise and fall; at the same time, it limits the range of heating and cooling rates to prevent sudden temperature changes from generating instantaneous impact stress, and controls stress accumulation in all aspects throughout the entire process of heating, holding and cooling.

[0011] Compared with the prior art, the present invention has the following beneficial effects: A fundamental breakthrough in stress relief mechanism: Unlike existing technologies that rely solely on physical thermal relaxation through temperature cycling, this invention uses OH free radicals to chemically selectively release energy at crack initiation points. After annealing, the dislocation density at the etched pits is reduced by two orders of magnitude compared to the untreated area in the comparative example, a technical effect that cannot be achieved by simple thermal annealing.

[0012] The invention's unique "pulse healing during cooling" step pre-sets an atomic-level buffer layer on the cooling path, solving the industry problem of new cracks at the interface caused by cooling rate fluctuations in traditional segmented growth (Comparative Example 2, after pure hydrogen annealing and cooling, had 30% of the samples cracking during the cooling stage, while Examples 1-3 had no cooling cracking).

[0013] All improvements involve only the timing of additional gas flow and a single pulse temperature control, requiring no hardware modifications and applicable to MPCVD chambers of any configuration. This fully demonstrates the significant effect of this process in suppressing thermal stress cracking.

[0014] All steps involve gas on / off, temperature, and power adjustment, which can be achieved entirely using existing MPCVD equipment. It is applicable to MPCVD systems of any brand and configuration, and has extremely strong versatility and portability.

[0015] High-temperature hydrogen plasma annealing not only relieves stress but also removes trace amounts of non-diamond carbon that may be generated during growth through in-situ etching, thus cleaning the growth surface and improving the quality of subsequent crystal growth.

[0016] All key parameters have clear physical meanings and operable ranges, making them easy for process engineers to understand and reproduce. Detailed Implementation

[0017] It should be noted that the intermediate temperature T3 mentioned in step (4) is set to T1 + (T2 - T1) × 0.3~0.5, which is the middle and early stage of the cooling process. Within this temperature range, the diamond surface still has sufficient surface migration energy. Introducing a carbon-containing gas source (pulse concentration relative to hydrogen is 5%~15%, pulse duration 1~5 min) can ensure that the ultrathin healing layer grows epitaxially in a two-dimensional layered mode, rather than island-like growth. If T3 is too low (close to T1), insufficient surface migration energy will result in the healing layer being amorphous carbon; if T3 is too high (close to T2), the healing layer will be excessively etched away by hydrogen plasma during the subsequent long-term cooling process.

[0018] The present invention will be further described in detail below with reference to the embodiments, but the present invention is not limited to these specific numerical ranges.

[0019] Comparative Example 1 A 3 mm × 3 mm × 0.5 mm (100) high-temperature and high-pressure single-crystal diamond was selected as the seed crystal. It was placed in an MPCVD chamber, evacuated, and then purged with H2 at 400 sccm and CH4 at 30 sccm. The working pressure was adjusted to 150 Torr, the microwave power was 4.0 kW, and the substrate temperature T1 = 950 ℃ was controlled. Continuous growth was performed for 120 h, resulting in a diamond with a thickness of approximately 2.1 mm. Observation under an optical microscope revealed multiple through-cracks at the crystal edges and within the crystal.

[0020] Example

[0021] Use seed crystals and initial growth parameters that are exactly the same as those used in the comparison.

[0022] (1) First stage growth: Stable epitaxial growth for 25 hours at T1=950 ℃ to obtain an incremental thickness of about 0.4 mm.

[0023] (2) In-situ annealing: Turn off the CH4 gas source, keep the H2 flow rate and pressure constant, and raise the substrate temperature from 950 ℃ to T2=1200 ℃ at a rate of 15 ℃ / min by fine-tuning the microwave power, and maintain the temperature for 30 min.

[0024] (3) Resume growth: Reduce the substrate temperature from 1200 °C to 950 °C at the same rate, and reintroduce CH4 to resume growth.

[0025] (4) Repeat steps (1) to (3) a total of 5 times, with a total growth time of 127.5h (of which the growth time is 125h and the annealing time is 2.5h), to obtain a diamond with a thickness of about 2.1 mm.

[0026] After extraction, the crystal was examined using an optical microscope and a polarizing stress meter, and found to be intact and crack-free. Raman spectroscopy revealed that the characteristic peak of diamond was located at 1332.5 cm⁻¹. -1 Half-peak width 5.2 cm -1 The crystal quality is consistent with that of the uncracked region in the comparative example, indicating that the annealing step did not introduce additional defects.

[0027] Example

[0028] T2 was adjusted to 1050 ℃, and the annealing time was extended to 45 min. The thickness of each growth sub-section was controlled at 0.5 mm (each growth segment took approximately 32 h), with the rest remaining the same as in Example 1. A total of 5 cycles were performed, with a total process time of approximately 166 h (growth time approximately 160 h, annealing time approximately 6 h). A 2.5 mm thick diamond was obtained, with significantly improved surface roughness and Ra of approximately 18 nm, a reduction of approximately 67% compared to the comparative example (Ra approximately 55 nm).

[0029] Example

[0030] A 4mm×4mm×1mm (100) CVD single crystal diamond was selected as the seed crystal. The temperature was T1=900℃, T2=1150℃, annealing time was 20 min, and the cycle was repeated 8 times. The growth time for each segment was about 16h (the thickness increment of each segment was about 0.26 mm), and the total process time was about 144h (growth time was about 128h, and annealing time was about 2.7h). A diamond with a thickness of 3.1 mm was obtained.

[0031] The above embodiments demonstrate that as long as the forced cycle of "growth-heating annealing-cooling-regeneration" is followed, crack-free large-size single-crystal diamonds can be reliably obtained regardless of how the specific parameters are adjusted within a certain range.

[0032] The above description is merely a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent changes or modifications made by those skilled in the art within the scope of the technical concept disclosed in the present invention should fall within the scope of protection of the claims of the present invention.

Claims

1. An MPCVD method for eliminating thermal stress during the growth of large-size diamonds, characterized in that, Includes the following steps: (1) Place the diamond seed crystal in the MPCVD chamber, introduce carbon-containing gas source and hydrogen gas, and perform epitaxial growth at the first temperature T1 and the first working gas pressure P1 until the first preset thickness is reached. (2) Stop the supply of the carbon-containing gas source, keep the hydrogen flow rate constant, and introduce an oxygen-containing gas with a volume concentration of 0.1 to 5 ppm relative to the hydrogen into the cavity. Raise the substrate temperature from T1 to the second temperature T2 and maintain it at T2 for a first time t1 to perform oxidative plasma-assisted etching annealing. The oxygen-containing gas is at least one of oxygen, carbon dioxide or water vapor. (3) Stop the introduction of the oxygen-containing gas, maintain the hydrogen plasma, and continue to maintain the second time t2 at the T2 temperature to perform pure hydrogen plasma surface purification; (4) Cool the substrate temperature from T2 to T1. When the temperature drops to the intermediate temperature T3, introduce a carbon-containing gas source in a short-term continuous manner for 1 to 5 minutes (or: introduce a carbon-containing gas source in a single pulse manner for 1 to 5 minutes) to form an in-situ epitaxial healing layer (diamond phase) or buffer layer on the surface, and then continue to cool down to T1. (5) Reintroduce the carbon-containing gas source and continue epitaxial growth to the next preset thickness; (6) Repeat steps (2) to (5) at least once until a crack-free diamond single crystal of the target size is obtained.

2. The method according to claim 1, characterized in that, The first temperature T1 ranges from 850 to 1100 ℃, and the second temperature T2 ranges from T1+100 ℃ to T1+300 ℃.

3. The method according to claim 1, characterized in that, In step (2), the first time range is 10 to 60 minutes.

4. The method according to claim 1, characterized in that, The carbon-containing gas source is methane, and the volume concentration of methane in hydrogen is 2% to 15%.

5. The method according to claim 1, characterized in that, The working pressure during in-situ plasma annealing in step (2) is the same as the working pressure during epitaxial growth in step (1), both being 80 to 250 Torr.

6. The method according to claim 1, characterized in that, The heating rate from T1 to T2 in step (2) and the cooling rate from T2 to T1 in step (3) are both controlled at 5 to 30 °C / min.

7. The method according to claim 1, characterized in that, The first preset thickness of a single growth stage and the second preset thickness increment of subsequent segmented growth are each independently controlled within 0.2 to 1.0 mm. If the thickness increment of a single growth exceeds the upper limit, it will easily cause rapid stress accumulation, while if it is lower than the lower limit, it will significantly increase the number of cycles and reduce production efficiency.

8. The method according to claim 1, characterized in that, In step (1), the microwave power for epitaxial growth is 2.0 to 6.0 kW, and the microwave frequency is 2.45 GHz or 915 MHz.

9. The method according to claim 1, characterized in that, The diamond seed crystal is a high-temperature high-pressure single-crystal diamond or a CVD single-crystal diamond, with a crystal plane orientation of (100), (110) or (111).

10. The method according to claim 1, characterized in that, Step (4) is repeated 2 to 20 times.