Classifications

• • C—CHEMISTRY; METALLURGY
  • C30—CRYSTAL GROWTH
  • C30B—SINGLE-CRYSTAL GROWTH; UNIDIRECTIONAL SOLIDIFICATION OF EUTECTIC MATERIAL OR UNIDIRECTIONAL DEMIXING OF EUTECTOID MATERIAL; REFINING BY ZONE-MELTING OF MATERIAL; PRODUCTION OF A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; SINGLE CRYSTALS OR HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; AFTER-TREATMENT OF SINGLE CRYSTALS OR A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; APPARATUS THEREFOR
  • C30B15/00—Single-crystal growth by pulling from a melt, e.g. Czochralski method
  • C30B15/002—Continuous growth
• • C—CHEMISTRY; METALLURGY
  • C30—CRYSTAL GROWTH
  • C30B—SINGLE-CRYSTAL GROWTH; UNIDIRECTIONAL SOLIDIFICATION OF EUTECTIC MATERIAL OR UNIDIRECTIONAL DEMIXING OF EUTECTOID MATERIAL; REFINING BY ZONE-MELTING OF MATERIAL; PRODUCTION OF A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; SINGLE CRYSTALS OR HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; AFTER-TREATMENT OF SINGLE CRYSTALS OR A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; APPARATUS THEREFOR
  • C30B15/00—Single-crystal growth by pulling from a melt, e.g. Czochralski method
  • C30B15/02—Single-crystal growth by pulling from a melt, e.g. Czochralski method adding crystallising materials or reactants forming it in situ to the melt

Definitions

• Zisld ⁇ l ⁇ hs invention This invention relates to semiconductor grade silicon and, more particularly, to a method of producing large single crystal doped silicon ingots on a continuous basis.
• Semiconductor grade silicon is a foundational to the large and growing semiconductor, computer, instrumentation and electronic industries.
• Semi ⁇ conductor grade silicon is characterized by the requirement of ultrahigh purity, a level of purity not required and largely unattainable in other fields of chemical and metallurgical technology.
• Another char- acteristic of semiconductor grade silicon is the requirement, in some applications, that the silicon contain minute but precisely known or controlled amounts of specific impurities. This is known as "doping" silicon and the product is referred to as “doped" silicon.
• Exemplary of the elements with which silicon is doped are boron, phophorous, arsenic and antimony.
• Boron is ⁇ typical "electron acceptor” and phosphorous is a typical "electron donor”.
• the semiconductor industry is well developed now and there are thousands of semiconductor devices marketed directly or assembled into computers, radios, televisions, controllers and nearly an infinite variety of other electronic devices.
• Single crystal silicon ingots are produced using a very well-known and widely used classical technique for growing single crystals known as the Czhrochalski method, sometimes referred to as the CZ method.
• Single crystal ingots of metals are grown according to the Czchrochalski method by contacting a small single crystal of the metal to be grown with a molten body of the metal and drawing the single crystal away from the molten body of metal slowly while rotating the single crystal.
• the single crystal is kept at a temperature lower than the melting point of the crystal.
• the layer of molten metal adjacent the crystal in immediate contact with the crystal, only a few atoms thick, deposits on the single crystal seed and the seed grows.
• the atoms of the molten metal deposit in the same crystal structure as the seed and, thus, a larger single crystal is formed. This process continues with layer upon layer of single crystal being deposited upon the growing ingot until a large ingot is formed. These ingots may be very large, weighing upward of 100 kg typically and may be several inches in diameter and a few feet in length. If the seed and the molten metal are of high purity metal, such as semiconductor silicon in the present context, then the result is a single crystal of ultrahigh purity semiconductor grade silicon in which the crystal structure is virtually perfect. A number of adaptations and variations for growing single crystals are known, see, for example, U.S. Patents Nos. 3,998,598, 4,282,184, 4,410,494, 4,454,096 and 4,458,152.
• the Czhrochalski method is not without limitations, however, and while the production of perfect ultrahigh purity silicon is one of technology's crowning achievements, the method is subject to serious problems.
• the Czhrochalski method is generally performed as a batch process. A given quantity of silicon from any convenient source is melted in a crucible and drawn on a seed crystal until the molten metal is depleted. If the silicon feedstock were perfectly pure, and if there were no impurities introduced in the process, then a perfect single crystal of perfectly uniform purity could, in theory, be produced. Such is not possible, however, and the presence of even minute impurities creates non- uniformity in the ingot purity.
• partitioning occurs wherein the impurities preferentially migrate into the crystal being grown or remain preferentially in the melt — the latter being more common.
• the partitioning effect favors build up of impurities in the melt.
• the concentration cf impurities in the molt increases. Since the partition effect results in a relatively constant percentage of the impurity present in the mel being partitioned into the crystal, as the level of impurity in the melt increases during growth of the crystal, the level of the impurity in the crystal also increases. Thus, the impurity level in the crystal increases as the crystal is grown. (The converse would occur if the impurities partitioned preferentially int
• a principle feature of the present invention is a method of growing single crystal silicon ingots b a modified Czhrochalski process on a continuous or sem continuous basis wherein the ingots have a uniform level of dopant along the length of the ingot.
• the material is • difficult to handle and cannot be fed into a crucible in a precisely metered manner.
• the act of providing, or attempting to provide, a continuous feed tends to disturb the CZ furnace heat balance and stability thus causing or increasing the risk of a defective crystal growth.
• the silicon is crushed into fine powder, it acquires an enormous surface area and acquires substantial impurities simply from the crushing [ ⁇ ration. With the large, irregular surface area of such a product, it is virtually impossible to feed silicon in crushed or comminuted form into a furnace without introducing large amounts of oxygen and other adsorbed and absorbed impurities into the melt. Product quality always suffers and the risk of large numbers of very expensive rejected ingots is significantly increased.
• An important feature of the present invention is that it solves all or most of the problem of the prior art respecting continuous growth of silicon ingots and thus opens the way for the economic production of extremely high quality, ultrahigh purity, perfect single crystal silicon ingots on a substantially continuous basis.
• the present invention may be de cribed as a method for continuously producing semiconductor grade silicon by (a) producing spherical, low surface to volume ratio, nonagglomerated individual monodisperse particles of silicon having a diameter of from about 1/2 to about 2 mm, (b) forming a molten body of silicon, (c) continuously feeding the monodisperse particles of silicon of step (a) into the molten body of step (b) , and (d) drawing a single crystal of silicon continuously from the molten body of silicon o step (b) to thereby form an ingot of ultrahigh purity silicon having substantial uniform composition along the entire length of the ingot.
• the present invention is most advantageous as a 8 method for continuously producing an ingot of silicon having a constant leval of doping along the length thereof by (a) forming a body of molten silicon, (b) continuously feeding into said molten body ultrahigh purity spherical, low surface to volume ratio, nonagglomerated individual monodisperse particles of silicon having a diameter of from about 1/2 to about 2 mm, ( ⁇ ) producing spherical, low surface to volume ratio, nonagglomerated individual monodisperse particles of silicon containing a predetermined quantity of dopant and having a diameter of from about 1/2 to about 2 mm, (d) continuously feeding into said molten body the particles of step ( ⁇ ), and (e) continuously drawing a single crystal of silicon having a constant level of dopant from the molten body of dopant containing silicon to thereby produce an ingot of semiconductor grade silicon 'having a.uniform concentration of dopant therein along the length of the ingot.
• the present invention may also be described as a method of producing on a continuous basis doped silicon ingots having constant composition along the length thereof, comprising feeding two streams of spherical, low surface to volume ratio, nonagglomerated individual monodisperse particles of silicon into a molten body of silicon, one of said streams being of higher purity than the other stream, the other stream comprising silicon containing dopant, and continuously pulling a single crystal from the molten body.
• the present invention encompasses the very important and unexpected discovery of a unique form of silicon, and singularly striking and unobvious use of such silicon in a new method for producing perfect single crystal silicon ingots on a continuous basis.
• silicon can be produced in the form of spherical, low surface to volume ratio, nonagglomerated individual monodisperse particles of silicon having a diameter of about 1 mm average with a maximum size distribution from about 1/2 to about 2 mm in diameter.
• This product is a free flowing "shot-like" product with each sphere being very nearly perfect, and approximately the same size as every other sphere.
• Semiconductor grade as used here is as defined in the semiconductor device fabrication industry, e.g., 0.1 ppb boron, 0.3 ppb phosphorus, etc.
• This feedstock is formed by thermal decomposition or hydrogen reduction of bromosilanes at 600°C to 1000°C at about atmospheric pressure in a "fluidized bed" reactor to achieve a product diameter from 1/2 millimeter to 2 millimeters.
• Suitable substrate particles for feedstock to the fluid bed reactor can be created by crushing or attrition of larger particles, The form of the small substrate particles is unimportant - only their purity and freedom from contamination.
• the spherodicity of the product of Step 1 of the Schumacher Silicon Process is developed during deposition onto these "substrate” particles fed into the Step 1 fluid bed reactor.
• the deposition results of course from the thermal decomposition or hydrogen reduction of the bromosilane compound to produce silicon and various by-products depending on the exact bromosilane and decomposition reduction method chosen.
• Sintering avoidance is accomplished in the bromosilane system by its low decomposition temperature, being lowest for thermal decomposition, and raised by hydrogen dilution as a result of some hydrogen reduction, e.g.
• T_ being higher than T-, .
• Sintering is a result of surface diffusion under the driving force of surface curvature which causes a distribution of particles to eliminate --..iriall particles and grow large particles, reducing the overall, net surface to volume ratio, and binds together the particles in contact with one another to again reduce the system surface/volume ratio.
• Step 1 reactor is a fluid bed reactor
• contact times between particles are of a short duration, so that sintering does not become a problem until somewhat higher temperatures, at about 1000°C - 1050°C due to this and the monodisperse character of the product.
• Step 2 doped silicon is produced for addition of donors and acceptors to the continuous CZ melt.
• Step 2 operates identically to Step 1, except that the bromosilane working fluid employed in the process is not as pure as possible, but is taken from that part of the process where donors or acceptors are concentrated.
• Make-up working fluid and BBr- ⁇ ) or PBr z,> are added to the feed streams to cause sufficient boron or phosphorus doped polycrystalline silicon of extremely low metals content (semiconductor grade) to be produced in a fluid bed reactor designated for this purpose.
• Step 3 of the SSP is the continuous or semicontinuous production of constant composition single crystal silicon lvia the CZ process of seed withdrawal from the melt.
• current technology which employs batch melting of a specific quantity (5-10 up to 60 or 100 or more kg) of semiconductor grade polycrystalline silicon produced via the Siemen's Process, or a fluid bed process which does not utilize bromosilane chemistry and is, therefore, incapable of producing continuous CZ feedstock from Steps 1 and 2, a partioning of impurities occurs across the solid-liquid phase boundary, since impurities, including donors and acceptors, exist in different equilibrium concentrations in a liquid in equilibrium with its solid phase.
• concentration -of impurities vary with time in the melt and, therefore, with position in the solid solidified from that melt.
• An important feature of the present invention is the step of adding a controlled number of spherical, low surface to volume ratio, nonagglomerated individual monodisperse particles of silicon having a mean diameter of about 1 mm to the silicon melt at a constant rate, typically sphere-by-sphere to subtantially exactly replenish the melt as the single crystal grows, but without disrupting or disturbing the mass or thermal balance and stability of the crucible or melt. Since the
• solid feedstock is introduced in particles characterized as spherical, thus having the minimum possible surface to volume ratio, and being nonagglomerated individual monodisperse particles of silicon having an approximately uniform diameter of about 1 mm, the effect on the mass of the melt, and upon the heat balance, i.e., the heat required to melt the added silicon and compensate for losses to the single crystal and to the environment, is a function of the rate of introduction of the individual particles. Since the rate of introduction of individual particles, one or more at a time, of substantially identical heat capacity can be controlled and kept constant, there are no disturbances or "spikes" in the heat absorbed in the system or required of the heating source. It is difficult to overestimate the importance of this facet of the invention, as it makes possible the .very reliable growth of perfect single crystal silicon on a continuous basis.
• Melt composition is maintained essentially constant with respect to the desired constituent of the product ingot doping with additions of SSP using Step 2 product as feedstock.
• the Schumacher Silicon Process comprises the following steps to produce a superior quality wafer for semiconductor device manufacturing, including integrated circuits and silicon photovoltaic solar cells.
• This uniformity is developed by the continuous, or semi-continuous, pulling of CZ crystal from a melt in which constant thermal gradients and constant constitutional gradients exist, and only build up of minor constituents takes place. About 300 ft. of crystal can be pulled prior to too much build up of minor constituents.
• additions to the melt must, in fact, melt prior to coming into contact with the growing solid-liquid interface, so that additions are made behind a weir or other arrangements to provide sufficient time for such additions to melt, prior to being brought by convection to the region of the growing solid-liquid interface.
• the weir arrangement is the only feature distinguishing the SSP crucible from ordinary CZ
• Crystal may be pulled from a shallow melt, or a deep melt.
• Various forms of heating, inductive, resistance, R F, microwave, etc. may be used.
• Electromagnetic fields to control wall contact and melt convection paths may be employed. All of these are known in the art of CZ pulling of single crystal silicon of semiconductor grade.
• the apparatus in which SSP is operated differs from the standard crystal pull furnace only in that arrangement made for handling of the ingot as it is withdrawn from the melt.
• the standard tower will handle only a limited length ingot.
• an arrangement is made to keep the crystal aligned to within .the critical angle with the melt by side supports which also serve to support the load, at least partially.
• the product ingot is then much longer than typical ingots, and.has a much reduced variation with position of both minor and major solute species than conventional CZ ingots.
• This ingot is then sliced, lapped, and polished into wafers by conventional means. These wafers show little variation in doping, defects, or impurity concentration from one wafer to the next, and from one end of the ingot to the other, clearly a vast improvement in the state of the art.
• Example 1 Ultrahigh purity monodisperse spherical particles of silicon were manufactured according to the process described in U.S. Patent No. 4,084,942.
• the particles are of high density, of nearly uniform size, from 1/2 to 2 mm and principally about 1 mm in diameter, and formed a free flowing product which was free of fines and dust.
• a controlled concentration of dopant e.g., boron tribromide or phosphorous tribromide
• the dopant concentration may be at any desired and effective level, generally in the range from 0.001 to 1 ppm, as this product will be used as feedstock to the silicon melt to contribute a low level of dopant.
• Higher dopant concentrations may be used as well, since any ratio of feedstocks can be used to control the dopant concentration in the silicon melt.
• a body of molten silicon is made up of the desired ratio of feedstocks from the proceeding steps, namely ultrahigh purity silicon and doped silicon, to form a melt of uniform composition.
• the composition is maintained uniform during the. entire operation of the process by continuously feeding in the ratio of feedstocks required.
• a melt having 5 ppb dopant is maintained by feeding equal quantities of ultrahigh purity silicon, less than 0.1 ppb, and doped silicon having 10 ppb dopant.
• a single crystal of silicon, which- may be of ultrahigh purity is contacted into the melt and withdrawn while being rotated, according to the classical Czhrochalski technique, thus growing a single crystal ingot having a uniform composition along the entire length of 5 ppb.
• the single crystal may be drawn continuously from the melt for very long periods of time. Calculations indicated that a single crystal up to 300 feet long is entirely feasible, although equipment design and handling convenience suggest that a crystal of this long may not be efficiently handled.
• the method of this invention comprises producing single crystal ingots continuously by forming a molten body of silicon metal of two feedstocks of silicon, one feedstock containing a predetermined level of dopant; continuously drawing a single crystal ingot of doped silicon from said molten body of silicon, said ingot being
• the concentration of dopant is uniform along the length of the ingot, while continuously feeding said feedstocks into said molten body of silicon to thereby maintain the concentration of dopant uniform in said body during the drawing of the single crystal therefrom.
• continuous, or continuously, as used herein means carrying out the process while repeating the steps set forth either periodically, or without interruption.
• a continuous process sometimes referred to as semi-co tinuous, would involve the repeated periodic introduction of silicon material feedstock while the crystal was being drawn from the melt, as well as constantly adding silicon material feedstock during drawing of every feedstock. Every feedstock inherently contains some impurity or additive, hence, the terms ultrapure silicon and doped silicon are used in the normal technical meaning of these terms.
• the two feedstocks could, within the scope of the invention, contain, respectively, two concentrations of the same dopant or concentrations, the same or different, of two or more dopants.
• the method of the invention may be described as including the steps of feeding a first feedstock into a molten body of silicon metal, said first feedstock comprising ultrahigh purity semiconductor grade silicon; feeding a second feedstock comprising ultrahigh purity semiconductor grade silicon to which a known amount of semiconductor dopant has been added; and, while carrying out the above-stated steps, drawing a single crystal of doped silicon from said molten body.
• the method of the invention preferably includes the steps of introducing into said molten body a first silicon composition characterized in being spherical, low surface to volume ratio, nonagglomerated individual, monodisperse silicon particles having a diameter of about 1 mm, and introducing into said molten body a second silicon composition characterized in being spherical, low surface to volume
• the doped silicon particles preferably contain boron, antimony, arsonic, or phosphorous.
• the silicon particles and the doped silicon particles have a mean diameter of about 1 mm, and the particle feedstocks are substantially free of sintered particles, particles substantially over 2 mm in diameter, and fine particles substantially under 1/2 mm in diameter.

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• Crystals, And After-Treatments Of Crystals (AREA)
• Liquid Deposition Of Substances Of Which Semiconductor Devices Are Composed (AREA)

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• 1984
  • 1984-12-04 JP JP50011985A patent/JPS62501497A/ja active Pending
  • 1984-12-04 WO PCT/US1984/001980 patent/WO1986003523A1/en not_active Application Discontinuation
  • 1984-12-04 EP EP19850900367 patent/EP0205422A4/de not_active Withdrawn

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