This invention relates generally to semiconductor sheets, and more specifically semiconductor sheets and methods of fabricating the same.
It is known to cut sheets of semiconductor material into wafers of predetermined sizes. Wafers formed of semiconductor materials are used for a variety of applications, and as such, there is an ever-increasing demand for such wafers. For example, at least some known solar-electric systems employ a semiconductor substrate, typically fabricated from silicon (single crystal or polycrystalline). The use of solar-electric systems has increased sharply in the past decade, and as such, the need for semiconductor wafers has also increased in the past decade. Although the expression “solar-electric” is used herein, persons of skill in the art will recognize that the discussion applies to a variety of photovoltaic materials, systems, and phenomena.
A wide variety of fabrication methods are used to produce semiconductor wafers from silicon feedstock. For example, at least some known multicrystalline silicon wafers used in solar cells have been produced by melting a high-purity material to which a dopant, such as phosphorus, boron, gallium, and/or antimony is added, has been added, in an inert atmosphere. The resulting silicon melt is deposited and is cooled to form a multicrystalline ingot. The ingot is then sliced to a desired wafer size. In another fabrication method, a layer of granular silicon is applied to a belt or a setter. The silicon, along with the belt or setter, is then subjected to a thermal sequence to form a silicon wafer or sheet of silicon. The silicon wafer and/or sheet of silicon is then removed from the belt or setter, and is then sized by sawing or scribing.
A significant portion of the cost of such semiconductor wafers is the raw semiconductor material itself. For example, in the case of solar-electric systems, a limiting factor for the use of such systems is the cost of the semiconductor material in the semiconductor wafers (in particular, the cost of the silicon) needed for such systems. Various purities of silicon feedstock are available for use in producing semiconductor wafers. The purity of silicon feedstock is determined by the level of impurities, such as, boron, phosphorus, iron, titanium, and tungsten, that are present in the silicon feedstock. Some applications of semiconductor wafers, for example, high-power electronic devices, require a higher degree of silicon feedstock purity than other applications, for example, solar-electric systems. Because the cost of silicon feedstock increases as the purity of the silicon feedstock increases, the use of solar-electric devices may be limited by the cost of the silicon.
In one embodiment, a method of fabricating a sheet of semiconductor material is described. The method includes forming a first layer of silicon powder that has a lower surface and an opposite upper surface. The method also includes depositing a second layer of silicon powder across the upper surface of the first layer, wherein the second layer of silicon powder has a lower surface and an opposite upper surface and has a lower melting point than the first layer of silicon powder. The method also includes heating at least one of the first and second layers of silicon powder to initiate a controlled melt of one of the first and second layers of silicon powder, and cooling at least one of the first and second layers of silicon powder to initiate crystallization of at least one of the first and second layers of silicon powder.
In another embodiment, a method of fabricating a semiconductor wafer is described. The method includes forming a first layer of silicon powder that has a lower surface and an opposite upper surface. The method also includes depositing a second layer of silicon powder across the upper surface of the first layer, wherein the second layer of silicon powder has a lower surface and an opposite upper surface and has a lower melting point than the first layer of silicon powder. The method also includes heating at least one of the first and second layers of silicon powder to initiate a controlled melt of one of the first and second layers of silicon powder, and cooling at least one of the first and second layers of silicon powder to initiate crystallization of at least one of the first and second layers of silicon powder to form a silicon sheet. The method also includes cutting the silicon sheet to form at least one semiconductor wafer, wherein the at least one semiconductor wafer is sized to facilitate use in a predetermined application.
In another embodiment, a sheet of semiconductor material having a lower surface and an opposite upper surface is described. The sheet of semiconductor material has a higher concentration of metal impurities at the lower surface than any other portion of said sheet. The sheet of semiconductor material is fabricated by a process that includes: forming a first layer of silicon feedstock that has a lower surface and an opposite upper surface; depositing a second layer of silicon feedstock across the upper surface of the first layer, wherein the second layer of silicon feedstock comprises a silicon powder and a metal impurity and has a lower melting point than the first layer of silicon feedstock; heating at least one of the first and second layers of silicon feedstock to initiate a controlled melt of at least one of the first and second layers of silicon feedstock; and cooling at least one of the first and second layers of silicon feedstock to initiate crystallization of at least one of the first and second layers of silicon feedstock. The heating and cooling facilitates segregation of the metal impurity towards the lower surface of the first layer of silicon feedstock.
Currently, silicon is one of the most commonly used semiconductor materials, also referred to as feedstock, used in the fabrication of semiconductor wafers. Accordingly, as used herein, the terms “semiconductor” and “semiconductor materials” refer to silicon based components and silicon materials. However, as will be readily appreciated by one of ordinary skill in the art, other semiconductor materials in addition to the silicon materials and/or including non-silicon materials can be fabricated using the apparatus and methods described herein Also, although only the use of silicon powder feedstock is described for use in fabricating a silicon sheet is described herein. Alternatively, crystallized silicon feedstock may be used without deviating from the present invention.
In an exemplary embodiment, a user positions at least one layer of silicon powder, for example, first layer 18 and/or second layer 20 within fabrication apparatus 10. In an alternative embodiment, wherein surface 16 is a conveyor belt, the conveyor belt passes under a hopper 22 that deposits a desired quantity of silicon powder, together with any desired additives, onto the conveyor belt to create a first layer of silicon powder 18. A second hopper 24 may be used to dispense additional silicon powder onto the conveyor to create a second layer of silicon powder 20.
In the exemplary embodiment, upper climate zone 12 and lower climate zone 14 each include a heat source (not shown in
An inert atmosphere is preferably maintained within an interior 26 of fabrication apparatus 10. Specifically, as defined herein, the interior 26 includes upper climate zone 12 and lower climate zone 14. In the exemplary embodiment, the interior 26 of fabrication apparatus 10 is substantially sealed to facilitate preventing the inert materials from escaping from the fabrication apparatus 10 and to prevent contaminants from entering.
Silicon layers 18 and 20 are then subjected to a thermal treatment. For example, upper climate zone 12 and lower climate zone 14 can be independently controlled. As such, each silicon layer 18 and/or 20 can be subjected to an independent thermal treatment such that any desired thermal profile, described in more detail below, can be implemented.
In an exemplary embodiment, the methods described below are performed within fabrication apparatus 10 to fabricate a silicon sheet from silicon feedstock. The two climate zones 12 and 14 facilitate greater control over the thermal energy applied to silicon layers 18 and/or 20. In certain stages described below, it is advantageous to apply a higher magnitude of thermal energy to an upper surface of silicon layer 20 than to a bottom surface of silicon layer 18. In other stages described below, it is advantageous to extract thermal energy from an upper surface of silicon layer 20 while applying thermal energy to a bottom surface of silicon layer 18. Fabrication apparatus 10, and more specifically, the two climate zones 12 and 14 in combination with surface 16, facilitate control over the application and removal of thermal energy to silicon layers 18 and/or 20.
In the exemplary embodiment, first silicon powder 34 and second silicon powder 42 are different. For example, the melting point of either the first powder 34 and/or the second powder 42 is modified, such that the melting points of each powder 34 and 42 are different. More specifically, in the exemplary embodiment, the melting point of second powder 42 is modified through the addition of an impurity (not shown in
Specifically, in the exemplary embodiment, second layer 32 melts into a liquid and re-crystallizes before first layer 30 is liquified. In other words, the dissimilar melting points enable a manufacturer to melt and solidify one layer, i.e., layer 32, of the multiple layers of silicon powder preferentially. In one alternative embodiment, the melting point of one of layers 30 and 32 is modified by oxidizing the silicon powder. In another alternative embodiment, the melting point of one of the layers 30 and/or 32 is modified using any other modification means that enables sheet 28 (shown in
As the thermal energy 50 applied to bottom surface 36 is increased, first powder 34 begins to melt. As first powder 34 melts, liquified silicon powder 42 mixes with first powder 34. The melting of silicon powder 34 is aided by the impurity-rich liquified silicon powder 42 that has seeped into the upper surface 38 of first powder 34.
The process and apparatus described above enable the fabrication of a silicon sheet from multiple layers of silicon powder, which are modified to have slightly dissimilar melting points, by melting the deposited silicon layers in a controlled-atmosphere furnace. The different melting points of the multiple layers of silicon powder facilitates enhanced control of melting locally and the melting and partial re-crystallization of one silicon layer before a different silicon layer is melted. More specifically, in the exemplary embodiment, a layer of silicon powder having a lower melting point is spread across a layer of silicon powder having a higher melting point. When thermal energy is applied to the layers of silicon powder from above the layers, the upper layer melts first, seeps partially into the bottom layer, and then re-crystallizes from the top when the heat is extracted from above the layers.
As the upper layer melts, impurities in the layer segregate towards the bottom of the wafer and the upper layer of silicon crystallizes from the top, leaving upper layer of silicon substantially impurity-free. Thermal energy is then applied to the layers from beneath the layers. The bottom layer of silicon, which includes its own impurities and those segregated from the upper layer, melts into a liquid, which enables impurities to segregate towards the bottom of the wafer. Because the thermal energy is removed from above the layers, crystallization continues from top to bottom, segregating the impurities further towards the bottom of the wafer.
By melting the layers of silicon powder in different fabrication stages, limitations posed by the high surface tension of silicon are mitigated, such that a thinner wafer from a powder layer that may not otherwise be possible may be produced. In the exemplary embodiment, the layers of silicon powder are heated, initially from above, followed by heat extraction and heating from below. Effects from surface tension are avoided by maintaining part of the silicon sheet as a solid throughout the process. Also, the presence of impurities used to modify the melting point of the upper layer of silicon assists the segregation of other undesirable impurities such as, but not limited to, Titanium, Tungsten, and Chromium. This is highly beneficial since silicon feedstock with boron and phosphorous content is more readily available and lower in cost when compared to substantially pure silicon.
The above-described methods and apparatus provide a cost-effective and reliable means to facilitate the production of thin silicon sheets from low cost powdered silicon feedstock. The addition of an impurity reduces the melting point of one layer of silicon feedstock. Having multiple layers of silicon that each has a different melting point, allows melting and partial re-crystallization of one silicon layer before other silicon layers are melted. By controlling the application and removal of thermal energy, the added impurities, along with any other impurities present in the silicon feedstock, segregate towards a bottom of the silicon sheet where the impurities have substantially no detrimental effect on the performance of solar cells made from silicon sheets produced. Furthermore, as the impurities segregate towards the bottom of the silicon sheet, the top of the silicon sheet is left substantially free of the impurities. As a result, a substantially pure silicon sheet is produced in a cost-effective manner.
Exemplary embodiments of a process and apparatus for forming a sheet of silicon using two chemically different layers of powders spread prior to melting is described above in detail. The process and apparatus are not limited to the specific embodiments described herein, but rather, steps of the process and components of the apparatus may be utilized independently and separately from other steps and components described herein.
While the invention has been described in terms of various specific embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the claims.
The United States Government may have rights in this invention pursuant to Contract No. 70NANBB3H3061.