Semiconductor Solidification with Dynamic Dopant Addition
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Solution Overview
Problem
The challenge in semiconductor solidification, particularly in the production of photovoltaic cell substrates, is the inhomogeneous distribution of dopants during crystallization, leading to variations in electrical properties and material yield due to the segregation of boron and phosphorus, resulting in unusable portions of the ingot.
Innovation Solution
A process involving the controlled addition of doped semiconductor charges during solidification to maintain constant dopant concentrations, using subcharges with varying dopant concentrations to dilute and stabilize the dopant distribution, thereby reducing segregation-induced variability in resistivity and carrier density.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If dopants are added during silicon crystal growth to regulate the balance of dopant species, then the electrical properties (resistivity homogeneity and conductivity type stability) are improved, but the total number of dopant species increases significantly, degrading carrier mobility
Solution Approach 1:
The invention changes the parameter of dopant addition by introducing a time-dependent addition rate. Dopants are added at a rate that decreases as crystallization progresses, specifically designed to compensate for dopant segregation while avoiding excessive total dopant accumulation. This dynamic parameter adjustment resolves the contradiction by achieving resistivity homogeneity without degrading carrier mobility.
Solution Approach 2:
The invention implements a feedback mechanism where the dopant addition rate is continuously adjusted based on the crystallization progress and the segregation behavior of dopant species. The addition rate is modulated to maintain the desired balance between electron-accepting and electron-donating dopants, ensuring both resistivity homogeneity and acceptable carrier mobility are achieved.
2Stability of the object's composition
If complete mixing of the liquid semiconductor is achieved during solidification, then the dopant distribution follows the Scheil-Gulliver law, but this leads to significant concentration variation and rejection of the upper ingot portion, reducing material yield
Solution Approach 1:
The invention applies preliminary action by adding dopants during the solidification process itself, rather than relying solely on initial doping. This proactive dopant addition compensates for segregation as it occurs, maintaining uniform composition throughout the ingot and eliminating the need to reject the upper portion, thereby maximizing material yield while ensuring compositional stability.
3Reliability
If phosphorus is added to achieve desired resistivity in compensated or n-type silicon, then the electrical conductivity is improved, but phosphorus segregates more than boron, creating inhomogeneous resistivity along the ingot height
Solution Approach 1:
The invention applies dynamics by making the dopant addition rate variable rather than constant. The addition rate is dynamically adjusted during crystallization to compensate for the differential segregation rates of phosphorus and boron. This dynamic control ensures that phosphorus is added at appropriate rates to maintain both electrical conductivity and resistivity uniformity along the ingot height.
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 process achieves a homogeneous resistivity and reduced variability in free carrier density along the ingot height, enhancing material yield and maintaining the electrical properties of the semiconductor, allowing for the use of a larger portion of the solidified semiconductor.
Implementation Method 1
solidifying the molten semiconductor
Implementation Method 2
the concentration of the dopants varies along the direction of crystallization due to the fact that the composition of the solid formed is different from that of the liquid (accumulation of dopants in the liquid in the most general case)
Data Source
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AI summary
The process comprises forming a bath (103) of molten semiconductor from a first charge of semiconductor comprising the dopants, and solidifying the molten semiconductor by melting additional charges of semiconductor and then adding to the molten bath under solid form. The solidification of the semiconductor lowers the variability of the molten bath according to the ratio of the predetermined variability factors as given in the specification. The dopants include electron acceptors with boron atoms and electron donors with phosphor atoms. The process comprises forming a bath (103) of molten semiconductor from a first charge of semiconductor comprising the dopants, and solidifying the molten semiconductor by melting additional charges of semiconductor and then adding to the molten bath under solid form. The solidification of the semiconductor lowers the variability of the molten bath according to the ratio of the predetermined variability factors as given in the specification. The dopants include electron acceptors with boron atoms and electron donors with phosphor atoms. The additional charges are added to the molten bath with high crystallization speed and low addition speed and according to an equation as given in the specification. A charge of semiconductor is added to the mass of solidified semiconductor each time to increase 1% of mass compared to the total mass of solidified semiconductor obtained at the end of solidification process. The solidification is carried out in a Bridgman type furnace. The furnace comprises a closed enclosure, and a crucible (102) under argon atmosphere. The bath is disposed in the crucible of the furnace. The addition of semiconductor charges is carried out using a dispensing device connected to a preheater. The time for the addition of additional charges is determined using a unit for controlling the dispensing device. The concentration of the dopants in the first charge of semiconductor is different from the same type of dopants in additional charges of semiconductor.