Method for producing silicon mono-crystals and multi-crystalline silicon ingots

Technical field

The present invention relates to a method for producing silicon mono-crystals and multi-crystalline silicon ingots from molten silicon containing both boron and phosphorus dopants.

Background technology

In recent years, photovoltaic solar cells have been produced from ultra pure virgin electronic grade poly-silicon (EG-Si) supplemented by suitable scrap material, cuttings and rejects from the electronic chip industry. As a result of the recent downturn experienced by the electronics industry, idle poly-silicon production capacity has been adapted to make available lower cost grades suitable for manufacturing PV solar cells. This has brought a temporary relief to an otherwise strained market for solar grade silicon feedstock (SoG-Si) qualities. With demand for electronic devices returning to normal levels, a major share of the poly-silicon production capacity is expected to be allocated back to supply the electronics industry, leaving the PV industry short of supply The lack of a dedicated, low cost source of SoG-Si and the resulting supply gap developing is today considered one of the most serious barriers to further growth of the PV industry.

In recent years, several attempts have been made to develop new sources for SoG-Si that are independent of the electronics industry value chain. Efforts encompass the introduction of new technology to the current poly-silicon process routes to significantly reduce cost as well as the development of metallurgical refining processes purifying abundantly available metallurgical grade silicon (MG-Si) to the necessary degree of purity.

When producing PV solar cells, a charge of SoG-Si feedstock is prepared, melted and silicon mono-crystals are pulled or the molten silicon is directionally solidified into multi-crystalline square ingots in a specialized casting furnace. Before melting, the charge containing SoG-Si feedstock which contains negligible amounts of boron and phosphorus, is doped with either boron or phosphorus to produce p-type or n-type ingots respectively. With few exceptions, commercial solar cells produced today are based on p- type silicon ingot material. The addition of the single dopant (eg. boron or phosphorus) is controlled to obtain a preferred electrical resistivity in the material, for example in the range between 0.5-1 .5 ohm cm. This corresponds to an addition of 0.02 - 0.2 ppma of boron when a p-type ingot is desired and an intrinsic quality (practically pure silicon with negligible content of dopants) SoG-Si feedstock is used. The doping procedure assumes that the content of the other dopant (in this example case phosphorus) is negligible (P< 1/10 B).

In Norwegian patent application No. 20035830 filed December 29, 2003 it is disclosed a method for producing directionally solidified Czochralski, float zone or multi-crystalline silicon ingots or thin silicon sheets or ribbon for making wafers based on a silicon feedstock material produced from metallurgical grade silicon by means of metallurgical refining processes. The silicon feedstock contains between 0.2 ppma and 10 ppma boron and between 0.1 and 10 ppma phosphorous. Boron and phosphorus have different distribution coefficient in silicon. Thus the equilibrium distribution coefficient for boron in silicon is 0.8 while the equilibrium distribution coefficient for phosphorus in silicon is only 0.35. Due the low distribution coefficient for phosphorus the content of phosphorus in the remaining silicon melt during directional solidification will increase and when the ratio between boron and phosphorus ([ppma of boron]/[ppma of phosphorus])becomes below a certain value the silicon ingot produced according to Norwegian patent application No. 20035830 will have a type change from p-type to n-type at a position about 2/3 of the ingot height. Thus the multi-crystalline ingots produced will contain both p-type and n-type silicon.

In order to increase this low yield of p-type material of the silicon ingot containing both boron and phosphorus, it is in EP-A 1848843 proposed to add boron during the directional solidification process to maintain a fixed ratio between boron and phosphorus in the remaining melt in order to solidify the multi-crystalline ingot with an increased amount of p-type material. Adding of minor amounts of boron during the directional solidification process is however difficult to control with the risk that too much or too little boron is added. This may affect the resistivity profile of the solidified ingot. In addition the resistivity will generally increase from the lower end to the upper end of the silicon ingot resulting in a transition from p-type material to n-type material towards the end of crystallization.

The same challenge exists when pulling silicon mono-crystals from a silicon melt containing both boron and phosphorus.

In addition to obtain silicon mono-crystals and multi-crystalline silicon ingots where 90-99% of the mono-crystals and the multi-crystalline ingots are of p- type it is important to have a flat resistivity profile over the height of the mono- crystals and the multi-crystalline ingots; that is, as little variation in resistivity from the lower end to the top of the mono-crystals and the multi-crystalline ingots as possible, as this will provide silicon wafers having the same resistivity when mono-crystals and multi-crystalline ingots are cut into wafers. In order to obtain this, the boron and phosphorus contents should have minimal variations over the height of the mono-crystals and the multi- crystalline ingots.

It is well known that phosphorus can be removed from a silicon melt by treatment of the silicon melt under vacuum. However, this way of removing phosphorus from silicon is always done as a separate step before pulling mono-crystals or before producing multi-crystalline ingots by directional solidification. Since phosphorus is more volatile than boron, the boron content of a silicon melt will not be affected during vacuum treatment of silicon melt in order to remove phosphorous. Description of Invention

It is an object of the present invention to provide a method for increasing the amount of p-type material and to obtain a flat resistivity profile when pulling silicon mono-crystals, and when producing directionally solidifying multi- crystalline silicon ingots from a silicon melt containing both boron and phosphorous. The present invention thus relates to a method for increasing the amount of p- type material when pulling silicon mono-crystals and when directionally solidifying multi-crystalline silicon ingots from a silicon melt initially containing between 0.12 ppma and 5 ppma boron and between 0.04 ppma and 10 ppma phosphorus, which method is characterized in that the pulling of mono- crystals and the directionally solidification of multi-crystalline silicon ingots is carried out under a pressure below 600 mbar and where an inert gas is continuously supplied to the surface of the silicon melt and continuously removed from the surface of the silicon melt whereby phosphorus is continuously removed from the molten silicon during the solidification process resulting in a substantially constant ratio between boron and phosphorus in the silicon melt during the drawing of the silicon mono-crystal and during the directional solidification of the multi-crystalline silicon ingot.

According to a preferred embodiment the pressure during the process is kept below 200 mbar and more preferably below 50 mbar.

According to another preferred embodiment the inert gas supplied to the surface of the silicon melt is argon.

In order to further improve the phosphorus removal during pulling of silicon mono-crystals and during directionally solidifying multi-crystalline silicon ingots from the silicon melt it is preferred that the area of the surface of the melt is large compared to the depth of the silicon melt. It is thus preferred that the ratio between the diameter and the height of the melt is at least 1 :1 and more preferably more than 1.5:1 .

Finally, it is preferred that the silicon melt is agitated during the pulling of silicon mono-crystals and during directional solidification of multi-crystalline silicon ingots in order to further improve the phosphorus removal. The agitation of the silicon melt can be done in any known way, but it is preferably by means of an induction coil arranged about the outside of vessel containing the silicon melt. By the method of the present invention it has been found that due to the continuous removal of phosphorus during the solidification process the ratio between boron and phosphorus in the molten silicon and thereby in the silicon mono-crystals and in the directionally solidified multi-crystalline silicon ingots is kept constant resulting in a constant resistivity over the height of the silicon mono-crystals and the height of the directionally solidified multi-crystalline ingots.

Short Description of the Drawings

Figure 1 is a diagram showing the resistivity for a directionally solidified silicon ingot made according to the prior art made from a silicon melt containing both boron and phosphorus Figure 2 is a diagram showing the resistivity for a mono-crystal drawn from a silicon melt according to the method of the present invention and compared with model calculation of the resistivity.

Detailed Description of the Invention Example 1 (Prior art)

A silicon mono-crystal was pulled from a silicon melt initially containing 0.26 ppma boron and 0.27 ppma phosphorous The resistivity in the produced silicon ingot was measured and compared to a model resistivity calculation without phosphorus removal during the directional solidification, and as can be seen from Figure 1 , the ingot changed from p-type material to n-type material at about 75 % of the height of the ingot. Figure 1 further shows that the measured resistivity values fit well with the resistivity values calculated using the model.

Example 2 (invention)

Two silicon mono-crystals were pulled from silicon feedstocks containing respectively 0.33 ppma boron and 0.36 ppma phosphorus and 0.52 ppma boron and 0.36 ppma phosphorus. The pulling of mono-crystals was carried out at a pressure of 20 mbar and argon was continuously supplied to the surface of the silicon melt and continuously removed from the surface of the silicon melt. The resistivity of the produced mono-crystals at different heights where measured and compared to a model resistivity calculation based on phosphorus removal during the pulling of the mono-crystals.

The results are shown in Figure 2. The results in Figure 2 show that the resistivity profile is virtually constant for the first 85 to 90 % of the height of the mono-crystal and that the change from p-type to n-type only takes place about 99% of the height of the mono-crystals. The measured resistivity values fit very well with the resistivity values calculated using the model of phosphorous removal during the pulling of mono-crystals. The measured resistivity figures thus clearly shows that phosphorus has been continuously removed during pulling of the mono-crystals and confirms that a substantially constant ratio between boron and phosphorous was obtained over the height of the mono- crystals.

Thus, by the present invention it is possible to substantially increase the part of a directionally solidified ingot solidifying as p-type and to obtain silicon mono-crystals and multi-crystalline silicon ingots having a constant resistivity over the height of the mono-crystals and the height of the multi-crystalline ingots.