Patent Application
20100258768
The present disclosure relates to methods and systems for use in the fabrication of semiconductor materials such as silicon. More particularly, the present disclosure relates to a method and system for controlling resistivity in the formation of p-type silicon ingots that permits the use of low-grade silicon feedstock, for fabricating silicon that may be ultimately useful in the manufacturing of solar cells and similar products.
The photovoltaic industry (PV) industry is growing rapidly and is responsible for increasing industrial consumption of silicon beyond the more traditional integrated circuit (IC) applications. Today, the silicon needs of the solar cell industry are starting to compete with the silicon needs of the IC industry. With present manufacturing technologies, both IC and solar cell industries require a refined, purified silicon feedstock as a starting material.
Materials alternatives for solar cells range from single-crystal, electronic-grade (EG) silicon to relatively dirty, metallurgical-grade (MG) silicon. EG silicon yields solar cells having efficiencies close to the theoretical limit, but at a prohibitive price. On the other hand, MG silicon typically fails to produce working solar cells. However, there may be other semiconductor materials that could be useful for solar cell fabrication. In practice, however, nearly 90% of commercial solar cells are made of crystalline silicon.
Because of the high cost and complex processing requirements of obtaining and using highly pure silicon feedstock and the competing demand from the IC industry, silicon needs usable for solar cells are not likely to be satisfied by either EG, MG, or other silicon producers using known processing techniques. As long as this unsatisfactory situation persists, economical solar cells for large-scale electrical energy production may not be attainable.
The resistivity is one of the most important properties of silicon (Si) used for manufacturing solar cells. That is because the solar cell efficiency sensitively depends on the resistivity. State-of-the-art solar cell technologies require resistivity values ranging between 0.5 Ωcm and 5.0 Ωcm, typically.
Besides the resistivity range, the type of conductivity is of utmost importance when making solar cells. Conductivity needs to be either p-type or n-type, i.e., either electrons or holes are the majority current carriers. In current cell technologies, p-type silicon material is typically doped with boron (B), which introduces holes or, expressed differently, acts as an acceptor in respective silicon. Alternatively, n-type material may be used. Such material is typically doped with phosphorus (P), which introduces electrons. Expressed differently, phosphorus is acting as a donor.
Feedstock silicon materials based on upgraded metallurgical (UM) silicon very often contain similar amounts of B and P. As a consequence, boron-induced holes and phosphorus-induced electrons can cancel each other, an effect called compensation. The compensation of majority current carriers often leads to a transition from p-type silicon (in the beginning of a crystallization process) to n-type silicon (at the end of a crystallization process). This is a consequence of different segregation behavior of these doping elements: phosphorus has a smaller segregation coefficient than boron. Thereby, in the case of casting ingots for producing multi-crystalline (mc) Si, the process might end up with p-type material only in the bottom and middle parts of such ingots, whereas the top part becomes n-conductive and has to be discarded.
Currently produced feedstock materials based on UMG silicon come often with a base resistivity below the minimum resistivity of 0.5 Ωcm that is typically specified by solar cell manufacturers. There is a simple reason for this: expensive processes for upgrading UMG-Si are primarily concerned with taking out non-metals, including dopant atoms B and P. In order to reduce cost, there is a clear tendency to minimize such processing, i.e., UMG-Si typically still contains high concentrations of dopant atoms. As long as boron is the dominating dopant we get p-type material with relatively low resistivity.
Compensation of boron by phosphorus—increasing with ongoing crystallization due to different incorporation of B and P at solidification—results in increasing resistivity with ongoing crystallization. So, the typically very low resistivity at the beginning of crystallization increases with ongoing crystallization. However, as already stated, there is the general problem of too heavy resistivity increase due to overcompensation of B by P, resulting in a transition of conductivity from p-type to n-type. The initial addition of boron for suppressing such a transition is not practical because one would even further reduce the resistivity in bottom and middle parts of, e.g., an ingot of mc-Si.
Accordingly, a need exists to control the compensation effect of the material, in order to increase the portion of p-type silicon material in ingots thereby increasing the yield of such material.
Techniques are here disclosed for providing a combination of interrelated steps at the ingot formation level for ultimately making economically viable the fabrication of solar cells on a mass production level. The present disclosure includes a method and system for forming multi-crystalline silicon ingots, which ingots possess uniformly p-type semiconductor material along essentially all of the ingot axial length. With the disclosed process and system, silicon ingots may be formed directly within a silicon melt crucible. For example, using mc-Si ingots formed from the processes here disclosed, solar wafers and solar cells can be made with improved performance/cost ratio, based on this mc-Si material.
According to one aspect of the disclosed subject matter, a semiconductor ingot-forming method and system permit controlling resistivity in the formation of a silicon ingot by preparing UMG silicon feedstock for being melted to form a silicon melt. The present disclosure assesses the concentrations of boron and phosphorus in said UMG-Si feedstock material. Our approach of choice is analyzing the initial incorporation of B and P by in-situ measuring the resistivity in the moment when the ingot formation process starts. Based on this assessment, a predetermined amount of an element of group III of the periodic system, which can be Ga, Al, a combination of Ga and Al, or another group III element, is added to the UMG-Si feedstock material during the crystallization of a large-size ingot. The predetermined quantity of such group III element(s) associates with the assessed B and P concentrations.
The present disclosure includes melting the UMG-Si feedstock and the added group III element(s) to form a molten silicon solution including the predetermined amount of group III element(s); performing a directional solidification of the molten silicon solution for forming a silicon ingot and, by virtue of the adding a predetermined amount of the group III element(s), maintaining the homogeneity the resistivity of the silicon ingot throughout the silicon ingot. Below certain resistivity it becomes advantageous to add P or other group V elements to the group III element(s). This way the useful resistivity range of reasonably yielding p-type ingots made of compensated UMG-Si feedstock can be extended.
In one embodiment, the present disclosure also includes methods for repeatedly measuring the dopant concentrations in the silicon melt during ingot formation, by testing resistivity in a sample. The dopant concentrations may then be repeatedly adjusted by adding discrete amounts of dopant in order to keep the resistivity of the ingot within a predetermined range.
In another embodiment, dopant concentrations in the silicon melt may be continuously adjusted by a flow of dopant, rather than repeatedly tweaked by discrete additions of dopant. In this embodiment, the repeated resistivity measurements may be fed into a resistivity model that outputs the required flow rate of a group III or group V element, which may in turn be fed into the melt.
These and other advantages of the disclosed subject matter, as well as additional novel features, will be apparent from the description provided herein. The intent of this summary is not to be a comprehensive description of the claimed subject matter, but rather to provide a short overview of some of the subject matter's functionality. Other systems, methods, features and advantages here provided will become apparent to one with skill in the art upon examination of the following figures and detailed description. It is intended that all such additional systems, methods, features and advantages be included within this description, be within the scope of the accompanying claims.
The features, nature, and advantages of the disclosed subject matter may become more apparent from the detailed description set forth below when taken in conjunction with the drawings in which like reference characters identify correspondingly throughout and wherein:
The method and system of the present disclosure provide a semiconductor ingot formation process for producing a multi-crystalline silicon ingot. As a result of using the disclosed subject matter, an improvement in the properties of low-grade semiconductor materials, such as upgraded, partially purified metallurgical silicon occurs. Such improvement allows, for example, the use of UMG-Si in producing solar cells as may be used in solar power generation and related uses. The method and system of the present disclosure, moreover, particularly benefits the formation of solar cells using UMG or other non-electronic grade feedstock silicon materials, but can be used for electronic grade feedstock silicon too.
Directional solidification (DS) leads to characteristic axial distribution of impurities (I), controlled by impurity-specific segregation characteristics. The so-called segregation coefficient S that describes segregation characteristics is described by the following simplified expression:
S(I)=f(C(I)crystal/C(I)melt)
where,
At B/P ratios close to 2 and below, we have seen a transition from p-conductivity to n-conductivity in respective ingots, leading to yield loss of usable p-type material of at least 10%. Typically, the portion of n-type material and the corresponding yield loss of p-type material can be up to almost 50%. The methods of the present disclosure may significantly improve the distribution of axial resistivity in ingots made from feedstock material with such compensation-related transition of conductivity. These methods allow the complete suppression of the transition to n-type material, so that up to 100% of respective ingots can be used. More typical is a yield of approximately 95% of usable p-type material.
As conceptual diagram 10 shows, because of the different segregation coefficients of boron and phosphorus, at some point in the silicon ingot formation, the concentration of phosphorus exceeds the concentration of boron. Thus, a silicon melt that began as a p-type semiconductor material will become an n-type semiconductor material. This is shown at line 20. Line 20, therefore, depicts that since the silicon becomes an n-type semiconductor, the resulting silicon ingot exhibits a p-n junction.
In the following
In contrast to line 66, line 68 demonstrates the effect of adding an amount of gallium sufficient to counteract the effect of the increasing concentration of phosphorus relative to boron (as shown in
In contrast to line 76, line 78 demonstrates the effect of adding an amount of gallium sufficient to counteract the effect of the increasing concentration of phosphorus relative to boron. Line 78 shows a slight decrease in resistivity in the silicon ingot due to the addition of gallium. However, the addition of gallium further provides the beneficial effect of essentially reducing the difference between Na and Nd throughout the almost entire silicon ingot formation. Thus, essentially until the ingot is approximately at the 0.95 completion point, resistivity ranges between 0.43 Ωcm and 0.98 Ωcm and so the silicon remains usable p-type semiconductor material. The p-n junction is completely avoided.
In plot 80, ordinate 82 for the values of Nd-Na ranges from 1×1015 to 5×1017 per cm−3. Along abscissa 84 ingot formation completeness ranges from 0.0 to 1.0. With an initial resistivity of the silicon ingot of 0.2 Ωcm, line 86 shows that without the disclosed process, at approximately 0.8 completeness, Nd essentially equals Na. Line 90 demonstrates the effect of adding an amount of gallium sufficient to counteract the effect of the increasing concentration of phosphorus relative to boron. However, almost throughout the whole ingot the resulting resistivity is below the already initially very low resistivity of 0.2 Ωcm (resistivity below this value is less useful). Thus, when using feedstock silicon leading to resistivities at the low end of the useful range, with gallium (or aluminum) alone it is practically impossible to bring the material into a more useful resistivity range, even though the p-n junction is still completely suppressed.
Line 88, however, shows a different result. Line 88 shows the result of also adding a certain amount of phosphorus to the feedstock silicon, in addition to the already added gallium or aluminum. As line 88 shows, the effect is to initially increase the resistivity and avoid the p-n junction. Thus, the silicon remains as p-type semiconductor material, and within approximately 95% of the ingot the resistivity ranges from 0.17 Ωcm to 1.42 Ωcm. Only a very small percentage of this material is in the less useful range <0.2 Ωcm, as opposed to the case of adding only the group III element Ga (or, similarly, Al).
Summarizing, at relatively high ingot resistivity (beyond≈0.4 Ωcm) an addition of only aluminum or gallium can advantageously counteract compensation of B due to P. These elements of group III of the periodic system have to be added to the feedstock silicon before melt-down for starting crystallization. Contrary to the case of adding boron, when adding Al or Ga an excellent homogenization of the resistivity along the crystallization axis is obtained, in conjunction with avoiding strong resistivity reduction in the early stage of crystallization (which happens if simply adding B instead of Al or Ga). A mixture of Al and Ga is also possible.
At relatively low ingot resistivity (below≈0.4 Ωcm) one can start adding a combination of Ga and P or, alternatively, of Al and P to advantageously counteract compensation. At very low resistivity (approaching≈0.2 Ωcm) such a combination of a group III element and P becomes mandatory. Applying a certain Ga/P ratio or, alternatively, a certain Al/P ratio (whereby Ga can be partially substituted by Al, and vice versa) can now be exploited to make use of feedstock material with very low resistivity, down to a minimum resistivity of approximately 0.15 Ωcm. Such low-grade material is associated with low production cost.
The present disclosure provides methods for controlling resistivity at growing silicon ingots from compensated feedstock silicon material, comprising, in one embodiment, the steps of:
In plot 100, ordinate 102 for the values of Na and Nd ranges from 1×1016 to 1×1018 per cm−3. Along abscissa 104 ingot formation completeness ranges from 0.0 to 1.0. Line 106 shows Na, and line 108 shows Nd.
As shown in plot 100, the concentration of acceptor atoms in the silicon melt may be increased stepwise at intervals during ingot formation. This result may be achieved by measuring the dopant concentration of the remaining molten silicon at intervals during crystallization, and thereby inferring the necessary amount of dopant to be added. One way of measuring the molten silicon's dopant concentration is to insert a dip rod (e.g., one made of quartz) into the molten silicon, allowing a portion of the molten silicon to cling to the dip rod and then solidify; the resistivity of this solidified piece of silicon may be measured once it has cooled, and the relative concentration of dopant atoms may be inferred in known ways. The resistivity measurement of the solidified silicon may be accomplished by either contact or contactless (e.g., inductive) measuring. The resistivity measurement may be calibrated by comparing it against samples having known resistivities, in order to eliminate inaccuracies from the measurement.
This procedure may allow all or most of the ingot to be formed as p-type silicon, while also allowing the resistivity to be more tightly controlled, and maintained between predetermined levels. In the ingot shown in plot 100, for example, the transition to n-type silicon is suppressed until approximately 91% formation, shown by line 105. The relative concentration of donor and acceptor atoms is kept reasonably tightly controlled up until approximately the same point, such that, by far, most of the silicon ingot is p-type, with a resistivity in a usable range.
However, in some cases it may be undesirable for an ingot to have an axial resistivity profile that varies discontinuously or almost discontinuously, such as the profile shown in plot 120. To avoid such discontinuities, the present disclosure provides methods as follows for continuously varying the amount of dopant flowing into the molten silicon.
Line 146 shows the donor concentration as it increases due to the less-than-one segregation coefficient of phosphorus. Line 148 shows the acceptor concentration, which increases both because of the less-than-one segregation coefficient of the acceptor dopants, and also because new dopant atoms are continuously being fed into the molten silicon. Line 150 shows what would be the net dopant concentration in the ingot if no additional dopant were added to the melt during ingot formation; line 150 assumes that the initial concentrations of donor and acceptor atoms are the same as in lines 146 and 148 (i.e. shown by the intersections of lines 146 and 148 with ordinate 142), respectively. Line 151 marks the p-n junction and transition to n-type material that would occur in line 150 if no additional dopant were added to the melt during ingot formation. Line 152, by contrast, shows the relatively flat net dopant concentration that may be achieved by the continuous-flow methods of the present disclosure.
View 172 shows another embodiment of the present disclosure, with a higher concentration of donor atoms than acceptor atoms. This view shows that the present disclosure may be adapted to producing n-type silicon with resistivity constrained to a predetermined range, simply by changing the initial concentrations of dopant atoms, and then successively adding acceptor dopant in the correct quantities to keep resistivity to a desired range. N-type silicon may be desirable in some circumstances, but it may need to have resistivity in a certain range in order to be useful. The present disclosure provides a simple way of producing low-cost n-type silicon with resistivity constrained to a predetermined range. One of ordinary skill will recognize that the continuous-flow methods of the present disclosure may also be adapted to producing n-type silicon with a relatively flat axial resistivity profile.
The semiconductor processing features and functions described herein provide for resistivity control in the formation of p-type and n-type semiconductor ingots. Although various embodiments which incorporate the teachings of the present disclosure have been shown and described in detail herein, those skilled in the art may readily devise many other varied embodiments that still incorporate these teachings. The foregoing description of the specific embodiments, therefore, is provided to enable a person skilled in the art to make or use the claimed subject matter. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other embodiments without the use of the innovative faculty. Thus, the claimed subject matter is not intended to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.
| Number | Date | Country | |
|---|---|---|---|
| Parent | 11769109 | Jun 2007 | US |
| Child | 12618577 | US |
