UMG-Si

Upgraded Metallurgical-Grade Silicon (UMG-Si) Process

UMG-Si is a lower-cost route for producing photovoltaic silicon by refining metallurgical-grade silicon (MG-Si) primarily through metallurgical methods rather than the full chlorosilane-based Siemens process. The goal is to remove impurities sufficiently for solar cell use, typically reaching a purity range on the order of 5N to 6N depending on process quality, feedstock control, and intended cell architecture.

Starting Material

MG-Si is typically produced in an electric arc furnace by carbothermic reduction of quartz:

<math>SiO_2 + 2C \rightarrow Si + 2CO</math>

Typical starting purity is about 98–99.5% Si, with the main impurities including:

• Iron (Fe)
• Aluminum (Al)
• Calcium (Ca)
• Titanium (Ti)
• Boron (B)
• Phosphorus (P)
• Carbon (C)
• Oxygen (O)

For solar use, the most difficult impurities are usually boron and phosphorus because they strongly affect carrier lifetime and electrical behavior.

Overall UMG-Si Refining Sequence

A practical UMG-Si route usually combines several steps:

1. Feedstock selection and blending
2. Primary melting and slag refining
3. Gas refining or vacuum refining
4. Directional solidification
5. Crushing, classification, and quality control
6. Optional repeat refining passes

A representative flow is:

Quartz + carbon -> MG-Si furnace product
MG-Si selection/blending -> remelting
Remelting + slag refining -> reduced metallic impurities / some boron removal
Vacuum or gas refining -> phosphorus reduction and some volatile impurity removal
Directional solidification -> segregation of remaining impurities
Top/bottom crop removal -> upgraded silicon
Optional second pass -> higher purity UMG-Si

1. Feedstock Selection and Blending

The first step is to start with the cleanest practical MG-Si. UMG-Si quality depends heavily on input quality because some impurities are much easier to remove than others.

Important feedstock strategies include:

• Use low-boron quartz and low-boron reductants where possible
• Minimize contamination from furnace refractories
• Sort or blend MG-Si batches by impurity level
• Track phosphorus and boron separately, not just total purity

This is critical because a silicon batch with low iron but high boron may still be unsuitable for photovoltaic use.

2. Slag Refining

Slag refining is used to transfer certain impurities from molten silicon into an oxide slag phase. This step is especially important for removing many metallic impurities and can help reduce boron under the right chemistry.

Basic Principle

Molten silicon is contacted with a designed slag, often based on oxides such as:

• SiO2
• CaO
• CaF2
• Al2O3
• MgO

The slag composition is tuned so that selected impurities are more stable in the slag than in the silicon.

A simplified conceptual reaction for boron removal is that dissolved boron in silicon is oxidized and then captured in the slag phase.

What Slag Refining Removes Best

Slag refining is relatively effective for:

• Aluminum
• Calcium
• Some transition metals
• Part of the boron, depending on slag design and oxygen potential

It is generally less effective for phosphorus.

Key Process Variables

• Temperature, typically above the melting point of silicon
• Slag composition
• Slag-to-silicon mass ratio
• Stirring or mixing intensity
• Contact time
• Furnace atmosphere
• Refractory compatibility

Engineering Notes

• Too much oxidation can increase silicon loss into the slag
• Slag chemistry must be balanced between impurity removal and silicon yield
• Boron removal is thermodynamically difficult and often incomplete in a single pass

3. Gas Refining

Gas refining uses reactive gases bubbled through or passed over molten silicon to remove selected impurities.

Common gases include:

• Oxygen-bearing gases
• Water vapor in controlled amounts
• Chlorine-containing gases in some specialized routes
• Reactive gas mixtures intended to oxidize or volatilize impurities

This can help convert certain impurities into species that either move into slag or leave through the gas phase.

Boron Removal by Gas + Slag Coupling

A common concept is:

• Oxidize dissolved boron at the melt surface or bubble interface
• Transfer the oxidized boron into slag
• Remove the slag after sufficient contact time

This works better when gas refining and slag refining are designed together as one integrated refining stage.

4. Vacuum Refining

Vacuum refining is especially useful for phosphorus removal because phosphorus has significantly higher vapor pressure than silicon at refining temperatures.

Basic Principle

Under reduced pressure and elevated temperature, volatile impurities evaporate preferentially from the molten silicon.

This is one of the most important UMG-Si tools because phosphorus is difficult to remove by slag alone.

Best Targets for Vacuum Refining

• Phosphorus
• Some volatile metallic species
• Part of dissolved gases

Key Process Variables

• Temperature
• Absolute pressure
• Melt surface area
• Residence time
• Agitation or thin-film exposure

Limitations

• Vacuum refining is not very effective for many metallic impurities that are not sufficiently volatile
• Boron removal remains challenging
• Equipment cost rises with vacuum scale and high-temperature compatibility

5. Directional Solidification

Directional solidification is one of the core UMG-Si purification steps. It does not destroy impurities directly. Instead, it separates them based on how they partition between solid silicon and liquid silicon.

Basic Principle

As molten silicon solidifies slowly from one end to the other, most impurities prefer to stay in the remaining liquid rather than enter the growing solid crystal.

This is described by the segregation coefficient:

<math>k = \frac{C_s}{C_l}</math>

where:

• <math>C_s</math> = impurity concentration in the solid
• <math>C_l</math> = impurity concentration in the liquid

For many metallic impurities, <math>k < 1</math>, often much less than 1, so the first-solidified portion is much purer than the last-solidified portion.

Process Description

1. Melt refined silicon in a crucible
2. Cool from bottom to top or from one end to the other
3. Control the thermal gradient to produce progressive solidification
4. Allow impurities to concentrate in the final liquid zone
5. Remove the impurity-rich tail end after solidification

Advantages

• Excellent for many metallic impurities
• Relatively straightforward thermally
• Compatible with casting infrastructure for PV ingots

Weaknesses

• Impurities with segregation coefficients near 1 are hard to remove
• Boron is particularly difficult
• Phosphorus is better removed earlier by vacuum refining than left entirely to directional solidification

6. Cropping and Physical Separation

After directional solidification, the ingot is usually cut into regions:

• Top crop
• Middle acceptable zone
• Bottom crop
• Edge losses if contamination occurred near mold walls

The exact impurity profile depends on furnace design and solidification direction, but in general the impurity-rich zone is removed and recycled or downgraded.

This is an important part of the purification system because the process intentionally pushes impurities into sacrificial regions.

7. Optional Multiple Refining Passes

A single UMG-Si refining cycle may not be enough for high-performance solar cells. Industrially, better results often come from combining:

• Slag refining
• Vacuum refining
• First directional solidification
• Re-melting
• Second directional solidification

This multi-pass strategy trades yield and process time for better purity.

Main Impurity-Specific Removal Mechanisms

Impurity	Main Concern	Best UMG-Si Removal Mechanism	Notes
Fe	Recombination center	Slag refining + directional solidification	Usually easier to reduce than B or P
Al	Electrical contamination	Slag refining + directional solidification	Often manageable with proper slag chemistry
Ca	Metallic contamination	Slag refining	Often reduced effectively in remelting stage
Ti	Severe lifetime degradation	Directional solidification + careful feedstock control	Very harmful even at low concentration
B	Acceptor dopant	Slag/gas refining, limited by thermodynamics	One of the hardest impurities in UMG-Si
P	Donor dopant	Vacuum refining	One of the most important vacuum-removal targets
C	Precipitates, defects	Melt control and feedstock/refractory control	Usually managed by upstream process discipline
O	Defects, precipitates	Melt/crucible/atmosphere control	Strongly linked to furnace and crucible design

Typical UMG-Si Equipment

A UMG-Si production line may include:

• MG-Si crushing and sorting system
• Induction or resistance remelting furnace
• Slag refining furnace or ladle refining system
• Gas injection system
• Vacuum refining chamber or vacuum induction furnace
• Directional solidification furnace
• Ingot cutting and cropping equipment
• Analytical lab for trace impurity measurement

Analytical capability is essential. Without good trace analysis, it is difficult to know whether refining is truly working.

Comparison with Siemens Process

Feature	UMG-Si Route	Siemens Route
Main purification principle	Metallurgical separation	Chemical conversion + distillation + CVD
Capital intensity	Lower	Higher
Energy use	Lower	Higher
Boron removal	Difficult	Better control
Phosphorus removal	Moderate with vacuum refining	Better control
Maximum achievable purity	Lower in practice	Higher and more consistent
Process complexity	Thermometallurgical	Chemical plant + deposition plant
Suitability for decentralized production	Better potential	Much harder

Why UMG-Si Is Attractive

UMG-Si is attractive because it may offer:

• Lower embodied energy
• Lower capital expenditure
• Fewer hazardous chlorosilane process steps
• Better fit for simpler industrial ecosystems
• Potential for local or regional production if purity targets are met

This is why it is often discussed as a possible route for lower-cost photovoltaic manufacturing.

Why UMG-Si Is Difficult

The main technical problems are:

• Boron is hard to remove to very low levels
• Phosphorus removal requires strong vacuum process control
• Metallic contamination can be reintroduced from crucibles, slags, or refractories
• Solar cell performance is very sensitive to trace impurities
• Yield drops because impurity-rich sections must be cropped off

In practice, the challenge is not simply reaching a nominal purity number. The challenge is achieving the right impurity profile consistently enough for cell manufacturing.

Representative UMG-Si Process Flow in More Detail

Step	Operation	Purpose	Typical Output
1	MG-Si batch selection	Start with lowest practical impurity inventory	Qualified feed batch
2	Remelting	Homogenize melt	Uniform liquid silicon
3	Slag refining	Remove metallic impurities and some boron	Cleaner melt + spent slag
4	Vacuum refining	Remove phosphorus and volatile species	Lower-P melt
5	Directional solidification	Segregate remaining impurities	Purified ingot with dirty tail
6	Cropping	Remove impurity-rich regions	UMG-Si ingot sections
7	Re-pass if needed	Increase purity further	Higher-grade UMG-Si

Practical Design Considerations

For a serious UMG-Si process, the following are critical:

• Feedstock discipline

Starting chemistry strongly determines final success.

• Thermochemical control

Temperature, oxygen potential, slag chemistry, and pressure must all be tightly managed.

• Contamination control

Refractories, tooling, mold coatings, and crucibles can undo purification gains.

• Analytical control

Trace boron, phosphorus, and transition metals must be measured reliably, often at ppm to ppb-relevant levels.

• Yield management

Higher purity usually means more crop loss and more silicon recycled back into refining.

Simplified Conceptual Summary

UMG-Si works by combining three main physical principles:

1. Chemical partitioning into slag for some impurities
2. Evaporation under vacuum for volatile impurities such as phosphorus
3. Segregation during freezing for impurities that prefer the liquid phase

The process can produce photovoltaic-grade material in some cases, but it is intrinsically more difficult to control than the chlorosilane route because it lacks the molecular-level purification advantage of distillation.

Concise Bottom Line

UMG-Si is the attempt to refine MG-Si to solar use mainly by remelting, slag refining, vacuum refining, and directional solidification. Its main advantages are lower energy use and lower capital cost than Siemens polysilicon. Its main technical limitation is impurity control, especially boron and phosphorus, plus the need for strong process discipline to maintain cell-grade quality.

One-Line Process Summary

MG-Si -> remelt -> slag refine -> vacuum refine -> directional solidify -> crop impurity-rich zones -> UMG-Si