Silicon Substrate Resistivity Uniformity via Dopant Ratio Control
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Solution Overview
Problem
Existing silicon single crystal substrates for power devices face challenges in achieving uniform resistivity, controlling bulk microdefects (BMDs), and maintaining optimal oxygen and nitrogen concentrations, which are crucial for achieving high breakdown voltages and reducing leakage currents.
Innovation Solution
The method involves growing silicon single crystals by the Czochralski method with tightly controlled boron and phosphorus ratios, specific cooling rates, and annealing processes to achieve uniform resistivity, reduced BMDs in the surface layer, and moderate BMDs in the substrate thickness, ensuring optimal oxygen and nitrogen concentrations.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If the Czochralski method is used to grow silicon single crystals, then the crystals can be manufactured with controlled dopant concentrations, but the dopant concentration varies in the axial growth direction causing resistivity variation
Solution Approach 1:
The patent changes the chemical composition parameters by adding both phosphorus and boron dopants in specific ratios (P/B = 0.3-0.6) to the silicon melt. This compositional modification compensates for the segregation effects during crystal growth, maintaining more uniform dopant concentration and resistivity along the axial direction of the grown crystal.
Solution Approach 2:
The patent uses a composite doping approach by combining two different dopants (phosphorus and boron) with different segregation coefficients in the silicon melt. This composite doping strategy allows the benefits of both dopants to work together, achieving better resistivity uniformity than single-dopant methods.
2Stability of the object's composition
If phosphorus is added to suppress axial resistivity variation, then axial uniformity improves, but radial resistivity variation increases
Solution Approach 1:
The patent optimizes the phosphorus-to-boron ratio parameter (P/B = 0.3-0.6) to balance the competing effects on axial and radial resistivity uniformity. By carefully controlling this ratio along with the absolute dopant concentrations, the patent achieves simultaneous improvement in both axial and radial resistivity uniformity, overcoming the trade-off described in the contradiction.
3Reliability
If the surface layer has few BMDs to reduce leakage current, then device quality improves, but the capability to getter heavy metals decreases
Solution Approach 1:
The patent applies local quality by creating different BMD densities in different regions of the silicon crystal. The surface layer (device formation region) is optimized to have low BMD density for reduced leakage current, while the bulk region maintains moderate BMD density for heavy metal gettering. This spatial differentiation of BMD distribution allows both requirements to be satisfied simultaneously.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach results in silicon single crystal substrates with uniform resistance, reduced BMDs in the surface layer, and moderate BMDs in the thickness center, enhancing the quality of power semiconductor devices by improving resistivity control and reducing leakage currents.
Implementation Method 1
Silicon single crystals used for substrates for power devices are generally manufactured by the Czochralski method (CZ method)
Implementation Method 2
the central portion of the crystal is cooled at ≧6° C./min from 1200° C.-1100° C.
Implementation Method 3
since the segregation coefficient of dopants such as boron and phosphorus with respect to the silicon single crystal is less than 1, dopant concentration in the silicon melt becomes higher as the silicon single crystal grows
Data Source
AI summary
Silicon single crystal substrates having uniform resistance, few BMDs in a surface layer and a moderate number of BMDs in a center of thickness of the substrate are formed from Czochralski silicon single crystals. The substrates have a resistivity in the center of a first main surface not lower than 50 Ω·cm and a rate of change in resistivity in the first main surface not higher than 3%, an average density of bulk micro defects in a region between the first main surface and a plane at a depth of 50 μm of less than 1×108/cm3, and an average density of bulk micro defects in a region lying between a plane at a depth of 300 μm and a plane at a depth of 400 μm from the first main surface not lower than 1×108 /cm3 and not higher than 1×109 /cm3.