Monocrystalline Silicon Ingots Research Articles

THERMAL SIMULATION OF THE CZOCHRALSKI PROCESS FOR SILICON CRYSTAL GROWTH USING FINITE ELEMENT MODELING APPROACH

This study investigates the thermal dynamics of the Czochralski (CZ) process for silicon crystal growth through numerical simulations. The simulation method of this study is based on finite element method (FEM) heat transfer simulation. The FEM simulation was performed using triangular mesh in half cross section of CZ system with real material properties. The analysis of heat transfer mechanisms includes conduction, convection, and radiation which reflect the impact of cooled argon convection on crystal growth. The simulations reveal that convection currents driven by cooled argon has a crucial role to promote uniform cooling which control crystal growth. This leads to enhanced mono-crystalline silicon ingot crystal quality and purity. Ultimately, insights gained from this study can inform optimization efforts in semiconductor manufacturing, facilitating advancements in electronic device fabrication.

Crystallization processes for photovoltaic silicon ingots: Status and perspectives

The choice of the crystallization process plays a crucial role in determining the quality and performance of the photovoltaic (PV) silicon ingots, which are subsequently used to manufacture solar cells. Silicon ingots are typically grown using either the Czochralski (Cz) process or the direction solidification (DS) method, with each technique influencing the microstructure and defects density as well as the final solar cells’ performance. In this work, we describe these two processes with a brief overview of the main challenges. For monocrystalline silicon ingots, we discuss the role of crucible and bubble development as well as structure loss. For multicrystalline silicon ingots, we briefly review some of the methods that have been developed in the past years and present results of high-performance multicrystalline (HPMC) ingots.

Role of Longitudinal Temperature Gradients in Eliminating Interleaving Inclusions in Casting of Monocrystalline Silicon Ingots

Infrared analysis reveals the presence of interwoven inclusions, primarily comprised of silicon nitride and silicon carbide, in the casting process of monocrystalline silicon ingots. This study investigates how the longitudinal temperature gradient affects the removal of inclusions during the casting of monocrystalline silicon ingots through simulations and comparative experiments. Two monocrystalline silicon ingots were cast, each using different longitudinal temperature gradients: one employing smaller gradients and the other conventional gradients. CGSim (Version Basic CGSim 23.1) simulation software was utilized to analyze the melt flow and temperature distribution during the growth process of quasi–monocrystalline silicon ingots. The findings indicate that smaller longitudinal temperature gradients lead to a more robust upward flow of molten silicon at the solid–liquid interface, effectively carrying impurities away from this interface and preventing their inclusion formation. Analysis of experimental photoluminescence and IR results reveals that although inclusions may not be observed, impurities persist but are gradually displaced to the top of the silicon melt through a stable growth process.

Construction of functional grain boundary clusters for casting large-size and high-quality monocrystalline silicon ingots

The construction of functional grain boundary clusters (FGBCs) effectively prevents the overgrowth of mc-Si at the edge of cast mono-Si ingots. This approach significantly increases the mono-Si proportion and greatly enhances the defect distribution.

Analyzing structure loss in Czochralski silicon growth: Root causes investigation through surface examination

A considerable proportion of the grown Czochralski Si ingots are remelted due to the generation of dislocations or the so-called structure loss. The assessment of the failed ingots is a crucial step toward achieving higher yield. Nonetheless, characterizing Si ingots poses a challenge attributed to their inherent high brittleness and the elevated concentration of dislocations, further complicating the cutting process. In this work, we utilize a non-destructive method to categorize the failed ingots and reveal the root causes of this failure. This study is performed on roughly 80 industrial monocrystalline silicon ingots that are grown for solar cell applications. Most of these ingots have experienced structure loss during the body growth. Analyzing the surface features reveals major categories of the structure loss ingots. Approximately, 66% of the examined ingots seem to be linked with temperature fluctuations, specifically a sudden drop in the melt temperature, and contamination by foreign particles. It is found that the primary causes can be categorized into four major groups: melt contamination, thermal fluctuations, melt vibration, and variations in growth rate. The primary root causes are identified within a fishbone diagram.

Suppression of dislocations and twins by inducing asymmetrical grain boundaries for casting high-quality monocrystalline silicon ingot

Cast monocrystalline silicon (CMC-Si) is a promising photovoltaic substrate owing to the advantage of low cost of directional solidification. However, the high dislocation density and other crystal defects, such as twins, have limited its further application. We proposed a new seed pavement method, namely, the construction of asymmetrical large angle grain boundaries, to suppress the generation of dislocations and other crystal defects. Two CMC-Si ingots, Ingot-A for the traditional method of seed pavement and Ingot-B for the new method of seed pavement, have been casted. The results show that, the crystal quality of Ingot-A is remarkably lower than that of Ingot-B. For Ingot-A, a large number of dislocations are induced, such as small angle grain boundaries, which come from the seed junctions due to size error of the seeds. And some twins are brought out at the symmetric grain boundaries, which reduces the proportion of the (100) crystal plane. For Ingot-B, there are large angle orientation relationships between the adjacent seeds. This asymmetrical large angle grain boundaries have not formed small angle boundaries, and successfully prevented the generation of dislocations and twins. The average conversion efficiency of Ingot-B (21.81%) is significantly higher than that of Ingot-A (20.63%).

Growth of low-cost and high-quality monocrystalline silicon ingots by using split recycled seeds

In this work, we proposed a new method of seed recycling for casting monocrystalline silicon ingots. Different regions at the bottom of the monocrystalline silicon bricks were cut for controlling its size and crystal defects, which were used as recycled seeds and reasonable splicing to induce grain boundaries. The results showed that the recycled seeds with higher concentration of metal impurities will result in a longer red zone and the yield of mono-Si ingot would be slightly decreased. The defect density in the recycled seeds increased significantly, but decreased greatly during the seeding and growth. The defect distribution in PL images at the top of ingots grown with recycled seeds was similar to that of ingots grown with new seeds. Furthermore, the average cell efficiency of the solar cells fabricated by the wafers grown with recycled seeds was only 0.1% different from that of the wafers grown with new seeds. Therefore, this low-cost seed recycling method is of great significance for reducing the cost of cast monocrystalline silicon.

СИСТЕМА АВТОМАТИЧНОГО УПРАВЛІННЯ ВИМІРЮВАННЯМ ПРОМИСЛОВО КОНТРОЛЬОВАНИХ ПАРАМЕТРІВ КРЕМНІЮ ДЛЯ ПОРУВАТИХ ПІДКЛАДОК

Purpose. Certification of silicon ingots intended for the manufacture of porous silicon substrates, which includes many operations for measuring silicon parameters. Some measurement operations are automated and some are not automated. The aim of the work is to fully automate the process of creating a quality certificate for monocrystalline silicon by developing an automatic control system for measuring controlled silicon parameters for porous linings. Methodology. We have performed an analysis of existing methods and methods of industrially controlled parameters of silicon. We have performed the analysis of standards for the measurement of parameters and the measurement process in the factory. Results. In this work, the authors performed the classification of industrially controlled parameters in accordance with the methods and measurement equipment. On the basis of the obtained analysis, the authors developed a scheme of the procedure for performing the operations of measuring the parameters of monocrystalline silicon. The authors proposed a block diagram of an automatic control system for measuring industrially controlled parameters of silicon. All equipment included in the sites is integrated into an industrial local area network, which is controlled by an industrial controller. The work shows the system software developed by the authors, developed in the LabView environment. The program allows you to generate a quality certificate for a monocrystalline silicon ingot for porous substrates, to analyze data for a selected period of time. Originality. For the first time, a classification of methods and means of measured parameters of monocrystalline silicon was carried out according to the criterion of measurement automation. The scheme of the order of performing of measurement operations has been improved in order to optimize it for automating the generation of a quality certificate. Practical value. The proposed automatic control system for measuring industrially controlled parameters of silicon is used at the plant “Pure Metals” that produces ingots of monocrystalline silicon. It can be used in factories where product quality certificates are used based on the measurement of many parameters. Figures 3, tables 1, references 11.

Optimal Cooling System Design for Increasing the Crystal Growth Rate of Single-Crystal Silicon Ingots in the Czochralski Process Using the Crystal Growth Simulation

Here, we report a successfully modified Czochralski process system by introducing the cooling system and subsequent examination of the results using crystal growth simulation analysis. Two types of cooling system models have been designed, i.e., long type and double type cooling design (LTCD and DTCD) and their production quality of monocrystalline silicon ingot was compared with that of the basic type cooling design (BTCD) system. The designed cooling system improved the uniformity of the temperature gradient in the crystal and resulted in the significant decrease of the thermal stress. Moreover, the silicon monocrystalline ingot growth rate has been enhanced to 18% by using BTCD system. The detailed simulation results have been discussed in the manuscript. The present research demonstrates that the proposed cooling system would stand as a promising technique to be applied in CZ-Si crystal growth with a large size/high pulling rate.

A new method of determining the slicing parameters for fixed diamond wire saw

The fixed diamond wire is widely used to slicing hard and brittle crystal material, such as mono-crystalline silicon, polycrystalline silicon, sapphire, silicon carbide, and so on. The slicing capability of fixed diamond wire should be matched reasonably with the properly slicing parameters to ensure the high slicing quality and slicing efficiency. While, there is a certain degree difference in slicing capability for different fixed diamond wires because of the location distribution, density, size distribution, and the protrusion height for diamond particles are all different. The worse slicing quality and lower slicing efficiency will be got if the slicing parameters is not matched reasonably with the slicing capability of fixed diamond wire. Hence, it is necessary to determine the proper slicing parameters for different fixed diamond wire according to its own characteristics to machining hard material with higher efficiency and quality. In this paper, a new method of determining the proper slicing parameters for fixed diamond wire is established. The wire bow angle is used as an index to describe the rationality of the determination for slicing parameters. The correctness and accuracy of this method are verified by a multi wire saw slicing mono-crystalline silicon ingot experiment. The wire bow angle and normal cutting force are obtained respectively by theoretical and experimental results. It is found that the experimental results of wire bow angle are around 3°, and this shows that this new method of determining processing parameter is feasible. In addition, the theoretical and experimental results of the normal cutting force are compared. These results all indicate that the slicing parameters determined in this paper match well with the slicing performance of the fixed diamond wire used in the experiment.

Design of parallel compensator and stabilizing controller to mitigate non-minimum phase behaviour of the Czochralski Process

This paper addresses the design of a parallel compensator and a stabilizing controller for the simplified crystal growth dynamics of the Czochralski (CZ) process, i.e., the process for the production of monocrystalline silicon ingots of uniform diameter. The diameter control of the produced ingots is achieved by a CCD camera measurement used to sense the radius of the boundary between the base of the growing crystal and the surrounding glowing meniscus — a raised melt surface connecting the crystal ingot with the flatter melt surface. Due to the intrinsic nature of the process, the bright ring radius measurement signal exhibits a non-minimum phase behaviour. A combination of the parallel compensator and a stabilizing controller is designed, such that the former provides for the mitigation of non-minimum phase behaviour, while the latter combined with the formal yields a suitable closed-loop performance.

Bubble distribution in fused quartz crucibles studied by micro X-Ray computational tomography. Comparing 2D and 3D analysis

Micro X-Ray computational tomography (µ-CT), has been applied to study the bubble distribution across the wall of fused quartz crucibles used for Czochralski pulling of monocrystalline silicon ingots for solar cells. Two crucibles from different suppliers has been used in the study. One set of samples was stored in water for 3 days (“wet” samples) and one set of samples was stored at 200 °C in air for 3 days (“dry” samples) prior to the first bubble analysis. Then the same samples were heated 24 h at 1400 °C and reanalysed. The µ-CT data were processed in 2D and 3D mode. The results showed that the total bubble volume was the same in 2D and 3D, while the number of bubbles was 6–9 times higher in 2D compared to 3D. The explanation was that in 2D each bubble was counted several times, while in 3D, each bubble was counted only one time. After heating the total bubble volume had increased to the double, while the number of bubbles was almost the same as in the un-heated samples. There was no significant difference in bubble size or distribution between wet and dry treated samples.

Efficiency limiting crystal defects in monocrystalline silicon and their characterization in production

Wafers and thicker slices of an entire n-type monocrystalline silicon ingot were studied using production-compatible electrical and optical characterization techniques. We investigated the capability of these techniques to detect efficiency limiting factors in the early phase of solar cell manufacturing. In addition to the standard characterization methods and parameters - carrier lifetime, resistivity, FTIR, photoluminescence (PL) - the OxyMap technique was used to evaluate the thermal history of the wafers, while the distribution of Bulk Micro Defects (BMD) was measured using Light Scattering Tomography (LST). PERT cells and samples with amorphous silicon surface passivation – deposited at low temperature – were produced from neighboring wafers to correlate silicon properties and solar cell efficiency results.An interesting indirect correlation was found between “as-grown” lifetime and cell performance, as both are influenced by the thermal history of the wafer.We also observed a very strong correlation between BMD position, measured by LST on as-grown samples, and defective areas on PERT cells localized by PL measurements. The LST measurements on heat treated samples (simulating the PERT cell process) showed the growth of BMDs in low efficiency areas. It indicates that the detection of harmful defects is possible even in the as-grown material using LST technique.

The development of a purification technique of metallurgical silicon to silicon of the solar brand

The experimental results that show the possibility of obtaining silicon of the solar brand by the recrystallization of metallurgical silicon in fusible metals, e.g., tin, and growing monocrystalline silicon ingots from the obtained silicon scales by the Czochralski method are presented. The experiments on purifying fusible metal (tin) after ending a cycle to obtain silicon scales for the purpose of its repeated use were made. Tin purification was carried out by the method of vacuum decontamination of tin melt, its filtration, and then zone recrystallization. The qualitative and quantitative analysis of the initial materials (silicon and tin) and their structure after various stages of the technological process was carried out by the method of the roentgen-fluorescent analysis on the Elvax light device. The structural features of the obtained silicon scales were considered by means of raster electronic microscopy on the REMMA106I device. The conductivity type and the specific electric resistance of the obtained silicon monocrystalline ingot were measured by the fourprobe method on the PIUS-1UM-K device. It was shown that the grown monocrystalline silicon ingot has silicon content of at least 99.999% weight, n-type conductivity, and specific electric resistance of at least 2.0 Оhm сm. The described parameters correspond to the silicon the solar brand and confirm the possibility of obtaining it from metallurgical silicon by recrystallization in fusible metals, for example, tin.

Characterization of n-type Mono-crystalline Silicon Ingots Produced by Continuous Czochralski (Cz) Technology

Continuous Czochralski (Cz) technology has been developed to address the high cost drivers of the traditional Cz technology for producing n-type wafers which are used to make the silicon based solar cells with the highest energy conversion efficiency. Continuous Cz technology overcomes the shortcomings of the traditional Cz in low ingot output from each crucible and large axial variation in resistivity and interstitial oxygen across the ingot, two of the drivers for the high wafer cost. In this work, five 2-meter long ingots pulled by continuous Cz from a single crucible were characterized for the minority carrier lifetime, resistivity, interstitial oxygen and substitutional carbon concentrations. In addition, the wafers cut from these ingots were made into cells at both Georgia Institute (nPERT with front junction) and ECN (n-Pasha). Equivalent cell performance from these ingots has been achieved.

On the shape of n-type Czochralski silicon top ingots

Industrial scale n-type monocrystalline silicon ingots with different crown shape and shouldering area have been grown and characterized in terms of minority carrier lifetime, resistivity and concentration and distribution of interstitial oxygen (Oi), voids and thermal donors (TD). The properties have been correlated with the process parameters. The results indicate that there is a relationship between the top ingot shape and the corresponding minority carrier lifetime quality of the first part of the ingot body. Oxygen incorporation is directly correlated to the time that the melt free surface is in contact with the purged gas atmosphere as well as the temperature of the melt. The relative P-band position is dependent on the process parameters at the early part of the ingot and the corresponding oxide particles could be included on the first wafers obtained from the ingot. The results also show that the early body quality is not always improved after TD annealing. This suggests that not all types and sizes of TD are dissolved during standard annealing processes. Other types of oxygen-related defects are most likely present and contribute to the lowered minority carrier lifetime found at this region. Both higher oxygen and vacancy concentration contribute to the formation of these defects.

A New Slicing Method of Monocrystalline Silicon Ingot by Wire EDM

A New Slicing Method of Monocrystalline Silicon Ingot by Wire EDM

High efficiency fine boring of monocrystalline silicon ingot by electrical discharge machining

This article deals with high efficiency and high accuracy fine boring in a monocrystalline silicon ingot by electrical discharge machining (EDM). In manufacturing process of integrated circuits, a plasma-etching process is used for removing oxidation films. This process has recently been examined for use of monocrystalline silicon as the electrode to minimize the contamination. However, it is difficult to machine silicon accurately by the conventional diamond drilling method, because the material removal is due to brittle fracture. The machining force in the EDM process is very small compared with that in conventional machining, therefore, the possibility of high efficiency and high accuracy boring holes in silicon ingot by EDM is experimentally investigated. The removal rate of monocrystalline silicon by EDM is much higher than that of steel, while the electrode wear is extremely small. The improvement method leads to a better hole without chipping at the exit of hole or sticking of the insulator on the wall of hole. Furthermore, it is proved that even a high aspect ratio of about 200 boring is possible.