The crucial role of high-purity graphite in the semiconductor single-crystal silicon growth furnace and the failure analysis thereof

The Key Role and Failure Analysis of High-Purity Graphite in Semiconductor Monocrystalline Silicon Growth Furnace
In the Czochralski (CZ) single crystal silicon production process, the thermal field system is the core physical platform that determines the quality of the crystal, the accuracy of size, and the stability of electrical performance. Among them, high-purity graphite materials are widely used in the key components of the thermal field - graphite crucibles, graphite insulation tubes, and graphite heaters. These components together form a precise, uniform, clean and stable high-temperature reaction environment within the single crystal furnace. The material quality and structural integrity of these components directly affect the oxygen content, resistivity distribution, dislocation density and micro-defect level of the final silicon wafer.

The graphite crucible serves as the direct container for holding molten polycrystalline silicon and needs to be used for a long time at temperatures above 1420°C, while also withstanding the strong reducing atmosphere of the silicon liquid and thermal stress cycles. The surface condition and the purity of the body directly affect the path of impurity introduction into the silicon melt: On one hand, the inner wall of the crucible undergoes slow sublimation and re-deposition at high temperatures, forming a 'graphite carbon film'; on the other hand, if the graphite matrix contains trace electroactive impurities such as boron (B), phosphorus (P), and aluminum (Al), they will segregate and diffuse in the silicon melt, thereby altering the impurity segregation behavior at the crystal growth interface. For example, boron atoms are prone to substituting for positions in the silicon lattice, significantly reducing the local resistivity; while phosphorus, due to its segregation coefficient being close to 1, is likely to form a gradient enrichment in the axial distribution; although the segregation coefficient of aluminum (k0 ≈ 0.001) is low, its oxide (Al2O3) particles may induce local undercooling in the silicon melt, becoming a dislocation source or a layer dislocation generator.

The graphite heater is responsible for the main heating function. The uniformity of its resistance and the geometric symmetry determine the radial and axial temperature gradients within the furnace. When the heater has microscopic cracks or local areas with excessive impurities, it will cause an abnormal increase in local Joule heat, leading to thermal field distortion, exacerbating the instability of melt convection, and thereby deteriorating the radial distribution uniformity of oxygen (O) in the crystal. It is worth noting that oxygen mainly originates from the high-temperature decomposition of the quartz crucible (placed inside the graphite crucible), but the temperature gradient control ability of the graphite thermal field directly affects the generation rate and escape efficiency of SiO gas on the melt surface. Experimental results show that for every 0.1 ppm increase in the boron content on the surface of the graphite heater, the standard deviation of oxygen concentration at the edge of the silicon wafer increases by approximately 8%; while excessive aluminum content (＞0.3 ppm) is significantly positively correlated with the detection rate of 'vortex defects' in the crystal (R² = 0.91).

The graphite insulation tube achieves axial thermal shielding through a multi-layer nested structure. Its purity and density uniformity affect the longitudinal heat flow distribution within the furnace. The low-density areas are prone to generating thermal short circuits, causing the seed crystal end to be undercooled or the shoulder to be overheated, thereby inducing screw dislocations or twinning. Moreover, if there are trace amounts of alkali metals (such as Na, K) or transition metals (Fe, Ni) remaining in the insulation tube, although they do not directly participate in electrical doping, they will promote the crystallization of the quartz crucible, accelerate its brittle fracture, and release SiOx particles into the melt, becoming a source of particulate contamination.

The industry has extremely strict requirements for the purity of semiconductor graphite components: for the graphite crucibles and heaters used in mainstream 12-inch single-crystal furnaces, the total ash content must be ≤ 5 ppm, with B ≤ 0.2 ppm, P ≤ 0.1 ppm, and Al ≤ 0.15 ppm. Moreover, it must undergo full-element detection by ICP-MS on a batch-by-batch basis. Additionally, it is required to have an isotropic degree (R value) of ≥ 0.95, a volume density of ≥ 1.75 g/cm³, a compressive strength of ≥ 45 MPa, and a history of being subjected to purification treatment at temperatures above 1800°C.

Typical failure cases show that after the 37th run of a 12-inch production line, it was found that the standard deviation of the resistivity in the center area of the silicon wafer suddenly increased to ±3.2%, and a large number of oxygen precipitation nuclei (OSF) appeared simultaneously. After disassembling the furnace for inspection, a local silicon carbide (SiC) conversion layer was found in the bottom ring area of the graphite heater. EDS analysis confirmed that the B content in this area reached 0.8 ppm (three times the limit), which was caused by the previous silicon vapor infiltration into the graphite micropores and an in-situ reaction with boron impurities at high temperatures. In another case, dense 'grain-like' pits appeared on the inner wall of the graphite crucible. SEM-EDS showed that the bottom of the pits was rich in Al and O. It was traced back to the residual aluminum binder mixed in the raw graphite, which formed Al2O3 hard inclusions after multiple high-temperature cycles, scratching the quartz crucible and causing local silicon leakage.

In response to the aforementioned risks, a three-level prevention strategy is recommended: First, strengthen the supplier admission management, requiring the provision of ASTM D4292 ash content test reports and third-party ICP-MS full elemental spectra, and establish a data chain of graphite parts - furnace runs - wafer quality; Second, implement dynamic assessment of the lifespan of thermal field components, combining infrared thermal imaging to monitor the temperature difference on the heater surface, the attenuation curve of resistance values, and the ultrasonic scanning results of the crucible wall thickness, and setting a scientific replacement threshold (for example, if the resistance change of the heater is greater than ±2.5%, it will trigger an alert); Third, before loading the furnace, perform plasma cleaning of the graphite part surface and 1600°C vacuum high-temperature baking (≥ 4 hours) to effectively remove adsorbed impurities and organic residues. At the same time, avoid direct contact between the graphite part and metal tools to prevent iron-nickel contamination; the transportation and storage environment must strictly control humidity (RH < 40%) to prevent oxidation.

In conclusion, high-purity graphite is not merely an ordinary refractory structural material; rather, it is a functional medium that incorporates multiple coupling effects of thermodynamics, mass transfer, and semiconductor physics. Its purity indicators, microstructure, and service history collectively form the fundamental constraints of the inherent properties of single-crystal silicon materials. Only by adhering to the principles of E-E-A-T (professional experience, professional knowledge, authority, and credibility), and integrating the entire chain of material science, equipment engineering, and process verification, can the increasingly stringent quality requirements for silicon-based substrates in advanced manufacturing processes be guaranteed.

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