1. Field of the Invention
The present invention relates to a double crucible for growing a silicon single crystal which is made of double structure by disposing a ring-shaped partition wall having introducing through-holes at its lower part inside it.
More particularly, it relates to a double crucible capable of growing a silicon single crystal having few micro-defects 0.3 .mu.m or more in diameter which are called Crystal Originated "Particles" (COP) in case of being used in a silicon single crystal continuous pulling and growing apparatus.
2. Description of the Prior Art
A silicon single crystal continuously growing furnace using the so-called CZ method which grows a high purity silicon single crystal for semiconductor from molten silicon inside the inner crucible while resupplying the silicon and dopant which are consumed for this growth from the outer crucible into the inner crucible is known.
On the other hand, a silicon single crystal growing apparatus of batch type has a demerit that such quality as concentration of oxygen, crystal growth interface and concentration of dopants in a single crystal bar varies in the longitudinal direction since an amount of molten silicon inside the quartz crucible varies with growth of the single crystal. The silicon single crystal continuously growing apparatus does not have such demerit but has a merit of high productivity.
In the continuously growing furnace, as shown in FIG. 7, a heat insulator 2 and a heater 3 are disposed concentrically with furnace 1 inside, a main quartz crucible 6 in the shape of bottomed cylinder is engaged with a graphite susceptor 4 fixed on the upper end of a rotatory shaft 5 in the middle of the furnace 1. The lower end of a partition wall 7 in the shape of ring concentrically with the main crucible 6 is fixed on the inner bottom of the main crucible 6. The ring-shape partition wall 7 is provided in the direction of vertical line. An outer crucible 8 and an inner crucible 9 for respectively storing molten silicon M are formed by the main crucible 6 and the partition wall 7. Plural introducing holes 10 and 10 for connecting the outer crucible 8 to the inner crucible 9 are provided at the lower part of the partition wall 7. The molten silicon M in the outer crucible 8 which has been molten by the heater 3 can move from the outer crucible 8 to the inner crucible 9 through the introducing holes 10. A rotatory lifting mechanism 11 is provided above the top of the furnace body 1 and a seed S suspended with a wire 12 from the rotary lifting mechanism 11 disposed above the main crucible 6, The rotary lifting mechanism 11 is made so as to grow a high purity silicon single crystal at the lower end of a single crystal bar T grown from the seed S by lifting the single crystal bar T while rotating it.
In order to resupply materials of the single crystal which have been consumed for this growth, high purity polycrystalline silicon grains and a specified amount of dopant are supplied to the outer crucible 8 from a material feeding mechanism 13 provided outside the furnace 1 through a chute 14 bored through the wall of the furnace 1. These materials become high purity molten silicon after being molten by sensible heat of the molten silicon M inside the outer crucible. The molten silicon flows into the inner crucible 9 through the introducing holes 10 of the partition wall 7 as being accompanied with a specified concentration of dopant, is mixed with the molten silicon M stored in the inner crucible 9, and maintains the molten silicon containing a specified concentration of dopant. A silicon single crystal having the dopant of desired concentration is grown and lifted from the molten silicon.
A silicon wafer cut from the single crystal bar lifted by the continuously growing apparatus is polished and then generally is cleaned by the so-called SC-1 cleaning process using a cleaning solution made of NH.sub.4 OH, H.sub.2 O.sub.2, and H.sub.2 O on the basis of the standard RCA method. This SC-1 cleaning solution removes organic contaminants and metal impurities forming part of particles by a strong oxidizing action of H.sub.2 O.sub.2 and a solving action and compound producing reaction of NH.sub.4 OH.
Recently, shallow etch pits have been observed after the SC-1 cleaning and various papers have been presented which consider that the etch pits are caused by hole clusters introduced during the crystal growth (for example, J. Ryuta et al., "Crystal-originated singularities on Si wafer surface after SC1 cleaning", Jpn. J. Appl. Phys., 29, L1947-L1949 and T. Abe et al., "Behavior of point defects in FZ silicon crystals", Semiconductor Silicon 1990, pp. 105-116 Electrochem. Society (1990). The above-mentioned etch pits are detected by counting them by means of a laser particle counter on the market. The etch pits counted as particles are called Crystal Originated "Particles" (COP).
In case of continuously growing a silicon single crystal by means of the prior art, although the quality of the single crystal is good without varying in the longitudinal direction of the crystal bar as mentioned above, a ratio (V.sub.OUT /V.sub.IN) of an amount of molten silicon stored in the outer crucible (V.sub.OUT) to an amount of molten silicon stored in the inner crucible (V.sub.IN) is 0.3 or so in the double crucible shown in FIG. 7, and therefore it has been desired that the heat capacity of the outer crucible is made large enough for the molten silicon in the outer crucible to heat the molten silicon in the inner crucible. Due to the insufficient heat capacity of the crucible, in case of making a silicon wafer by lifting a silicon single crystal by means of the above-mentioned double crucible, generation of COP of 0.3 .mu.m or greater in diameter has been unavoidable on the silicon wafer polished and cleaned by means of the SC-1 cleaning. Although the cause of this has not been made fully clear, it is thought that the cause is the above-mentioned insufficient heat capacity.
In order to increase the ratio of V.sub.OUT /V.sub.IN, it is thought to increase the capacity of the outer crucible. In this case, when the capacity of the inner crucible is not changed, the main crucible is increased in the radial direction of it and at the same time the heater and the heat insulator need to be made greater in capacity and as a result the furnace has been made larger in size. In case of increasing V.sub.OUT /V.sub.IN by making the upper inner diameter of the partition wall 7 smaller than the lower inner diameter of the partition wall 7, as shown in FIGS. 8 to 10, without changing the size of the main crucible, in other words, without increasing an amount of molten silicon in the main crucible, D/A is made so small as 1.2 and therefore there has been a demerit that it is hard to keep the crystal growth, supposing that A is the diameter of a silicon single crystal being grown and D is the inner diameter of the above-mentioned partition wall at a level of the molten silicon.
On the other hand, in recent years semiconductor integrated circuits have been made remarkably highly integrated, and with this trend circuits made on a silicon wafer are also made finer. And in case of using a wafer having COP of 0.3 .mu.m or greater in diameter, the COPs have prevented the high density integration.
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