What is Polysilicon or Polycrystalline Silicon

Polycrystalline silicon or  Polysilicon( poly-Si or “poly”) is a material consisting of multiple small silicon crystals.The difference between Silicon Metal and Polysilicon is that Poly is highly pure form of silicon and is much costlier.Polycrystalline silicon can be as much as 99.9999% pure (also known as 4N pure,the 4 denotes the number of 9s after the decimal point).Polysilicon was primarily used by the semi industry till 5-6 years ago and the electronic industry used 9N pure polysilicon.The Solar Industry which has become the main user of polysilicon nowadays can do with 6N pure poly.

Difference between Single Crystal and Multicrystalline Polysilicon

Polysilicon is refined to single crystal polysilicon before it can be used by the semiconductor industry.However solar cells can be made even from multicrystalline polysilicon.Monocrystalline solar cells on the other hand are made of single crystal polysilicon and have higher efficiency than those made from multicrystalline polysilicon.Polycrystalline cells can be recognized by a visible grain, a “metal flake effect”. Semiconductor grade (also solar grade) polycrystalline silicon is converted to “single crystal” silicon – meaning that the randomly associated crystallites of silicon in “polycrystalline silicon” are converted to a large “single” crystal.

Polysilicon Manufacturing Process

There are 3 methods of manufacturing Polysilicon

a) The Czochralski process is the most commonly used process of producing polysilicon and is also known as the Siemens process.In this process High-purity silicon is melted in a quartz crucibles.Then a pure silicon rod known  as a seed crystal is dipped in molten silicon.The process begins when the chamber is heated to approximately 1500 degrees Celsius, melting the silicon. When the silicon is fully melted, a small seed crystal mounted on the end of a rotating shaft is slowly lowered until it just dips below the surface of the molten silicon. The shaft rotates counterclockwise and the crucible rotates clockwise. The rotating rod is then drawn upwards very slowly, allowing a roughly cylindrical boule to be formed. The boule can be from one to two metres, depending on the amount of silicon in the crucible. By precisely controlling the temperature gradients, rate of pulling and speed of rotation, it is possible to extract a large, single-crystal, cylindrical ingot from the melt.

b) Float Zone Method – Note this method is performed by only a few companies and is an alternative high-purity method to crystals grown by the Czochralski process .A polycrystalline rod of ultra-pure electronic grade silicon is passed through an RF heating coil, which creates a localized molten zone from which the crystal ingot grows.A seed crystal is used at one end in order to start the growth

c) The Bridgman–Stockbarger technique involves heating polycrystalline material above its melting point and slowly cooling it from one end of its container, where a seed crystal is located. A single crystal of the same crystallographic orientation as the seed material is grown on the seed and is progressively formed along the length of the container. The process can be carried out in a horizontal or vertical geometry.

Solar Use of Polysilicon (Solar Wafers and Ingots Raw Material)

Once Polysilicon is produced it has to be made into flat wafers which require further industrial processes like pouring the metal in crucible to make ingots (Czochralski Process)and then cutting the ingots by wiresaws to make solar wafers.The solar wafers are then converted into solar cells by processes like diffusion,etching etc.Note silicon solar PV panels growth has exploded in the last 10 years and the costs have fallen dramatically.Note solar cells made from a single crystal of silicon are known as monocrystalline solar cellswhile others are known as multicrystalline solar cells.The efficiency of the 2 types of silicon solar cells differs as does the cost.

Polysilicon Prices

The Solar Crystalline Silicon Industry accounts for almost 85-90% of all the Solar Energy Capacity installed globally and is growing at a rapid rate.Polysilicon which is the main raw material for the industry has been the focus of the industry over the last 2-3 years.The reason was the sharp run up in Poly Prices in 2008 to $400/kg despite costing just $30/kg.The rapid increase in the Solar Module Industry had caught the poly producers by surprise as it normally takes 2-3 years for a Poly Plant to come online.This had led downstream producers with a secure cheap supply of polysilicon as the kings of the market.New Business Models  based on low or no polysilicon usage were invested in.However the 2009 Crash brought down the price to $50/kg killing many of the companies.Polysilicon prices are expected to come down in the future as more polysilicon plants are built to meet the massively growing demand for Silicon Cells.

Polysilicon Deposition

Polysilicon deposition is the process of depositing a layer of polycrystalline silicon on a semiconductor wafer, is achieved by pyrolyzing  SiH₄) at 580 to 650 °C.When the rate at which polysilicon deposition occurs is slower than the rate at which unreacted silane arrives, then it is said to be surface-reaction-limited. Polysilicon layers can be deposited using 100% silane at a pressure of 25–130 Pa (0.2 to 1.0 Torr) or with 20–30% silane (diluted in nitrogen) at the same total pressure.At reduced pressure levels for VLSI manufacturing, polysilicon deposition rate below 575 °C is too slow to be practical. Above 650 °C, poor deposition uniformity and excessive roughness will be encountered due to unwanted gas-phase reactions and silane depletion

Polysilicon Technology and Cost

Manufacturing of Polysilicon was restricted to a small number of companies until a few years ago.Starting a poly fab and expanding it is a tough process and requires both expertise and experience.Till a few years ago only a few European,American and Japanese companies like Mitsubishi Polysilicon used to possess this skill.However polysilicon equipment manufacturers like GT Advanced Technology (GTAT) have started making polysilicon reactors and exporting it to Asian producers.Some of these companies like GCL and OCI have mastered this technology and started producing polysilicon at even lower cost than the Western companies.Newer Polysilicon Technology has been developed like uMg Polysilicon or upgraded Metallurgical Silicon keeping Solar Wafers in Mind.These Technology claims that it can be made at much lower costs of $5/kg than compared to $30/kg cost in standard Siemens process.But the price fall in poly price has made these new technology players in a bind the biggest failure being Timminco.

Polysilicon Industry (Size and Growth)

The Polysilicon Industry has grown by more than 50% in the last few years mainly driven by the demand for photovoltaic wafers made from polysilicon.While the electronic demand for poly has grown by 5-10% per year,the demand from the solar industry has grown exponentially.While the incumbent producers of polysilicon like Hemlock and Wacker have expanded their capacity they have failed to meet the demand.This has led to the rise of giant poly producers from Asia like GCL and OCI Chemicals.From zero in 2008,these companies will become the biggest producers of polysilicon by 2012.Smaller producers though have faced a tough time and some of them like Shunda have closed due to lack of scale and technology expertise.It is expected that the polysilicon industry will continue to grow as Solar Energy usage grows due to demand elasticity.Almost 150,000 tons of polysilicon will be produced in 2011,a huge increase from around 40,000 tons in 2005.The size of the industry is almost $7.5 billion in 2011.