CN1016191B - Fabrication method of silicon single crystal substrate with high oxygen content for semiconductor devices - Google Patents

Abstract

A method for the production of silicon substrates, including growing silicon monoliths at relatively high production rates. It has been found that the growth rate of the silicon crystal has a great influence on the generation of crystal defects in the silicon crystal or silicon substrate. In addition, the oxygen content in silicon crystals or silicon substrates is significantly higher than in ordinary silicon crystals or silicon substrates. The high growth rate of silicon crystals inhibits the segregation of oxygen from the crystals, which can reduce the number of defects or stacking faults in the crystals during heat treatment of semiconductor devices. In a preferred method according to the invention, the growth rate of the silicon crystal is ≥ 1.2 mm/min. The optimum oxygen content in the grown silicon crystal is selected to be ≥1.8×10 ¹⁸ /cm ³ .

Description

The present invention generally relates to silicon single crystal substrates capable of adsorbing large quantities of metal impurities. In addition, the present invention also relates to a method for producing a silicon single crystal substrate with a relatively high oxygen content. In particular, the present invention relates to a process for producing silicon wafers with high oxygen content by crystal growth process and its equipment.

Silicon single crystal substrates have been widely used in the production of various semiconductor devices. In such semiconductor devices, it is generally desirable to minimize leakage current. It is known that leakage current can be reduced by a so-called internal gettering (I.G.) effect. The I.G. effect can be realized by defects formed in the internal structure of the silicon substrate.

As is well known, silicon substrates are obtained from silicon single crystals produced by growing single crystals of silicon from molten polycrystalline silicon, such as by the Czechlaus method (hereinafter abbreviated as "CZ" method). In the CZ method, silicon single crystals are slowly pulled out from a molten bath of polycrystalline silicon. Silicon substrates are obtained by cutting or "flaking" polished single crystals of silicon.

The silicon single crystal produced contains a large amount of oxygen. Oxygen in silicon single crystals can cause defects or crystal dislocations, such as dislocation loops, stacking faults, etc., due to the segregation of oxygen in the silicon substrate during heat treatment. Defects in finished semiconductor devices degrade the device's nominal characteristics, specifically, its breakdown voltage and increase its leakage current. As a result, the yield of semiconductor devices is significantly lowered.

On the other hand, it has been found that defects in semiconductor devices can function to adsorb metallic impurities by the so-called gettering or I.G. effect. For example, in semiconductor devices, the surface of the silicon substrate is the main active area, for example, insulated gate field effect transistors (MOS-FET's) or integrated circuits using MOS-FET's, in addition to the main Defects outside the active area Both show the I.G. effect, adsorption of metal impurities from the active area. This helps to reduce the leakage current of the semiconductor device.

However, achieving a consistent I.G. effect in mass production is difficult. For example, in the case of using the traditional CZ method to grow silicon single crystals, the density of defects in the crystals often makes the top (that is, the beginning of crystal growth) and the tail (that is, the end of crystal growth) appear equivalent due to the temperature hysteresis. big difference. In addition, although a high oxygen content is beneficial for improving the I.G. effect, when the oxygen content is too high, defects are formed even on the surface of the semiconductor device, and as a result, the performance of the semiconductor deteriorates as described above. Moreover, in some semiconductor production methods, care must be taken to strictly control the oxygen content, or, in view of the heat treatment conditions required for the production of some semiconductor devices, special I.G. treatment must be implemented.

Therefore, in the process of efficiently manufacturing silicon substrates for semiconductor devices, how to obtain a sufficient I.G. effect to reduce leakage current without causing harmful effects on defects in the finished semiconductor device, especially after heat treatment The rather high oxygen content is a coherence issue.

Accordingly, a general object of the present invention is to provide a silicon substrate and a method of producing the same which can overcome the above-mentioned problems.

Another object of the present invention is to provide a silicon substrate which contains a relatively high oxygen content without degrading the properties of the silicon substrate due to oxygen segregation, dislocation loops, stacking faults and the like.

Still another object of the present invention is to provide a method for producing a silicon substrate as a raw material for semiconductor device production, which method can provide a high yield without degrading the characteristics of the finished product.

In order to achieve the above and other objects, there is employed a method of producing a silicon substrate comprising a method of growing a silicon single crystal at a higher than conventional rate. It has been found that the growth rate of the silicon single crystal has a significant effect on the generation of defects in the silicon single crystal. Furthermore, according to the present invention, the oxygen content in the silicon single crystal or silicon substrate is remarkably higher than that in all conventional silicon single crystals or substrates. Accelerate the growth of silicon single crystal, and obviously inhibit the segregation of oxygen in the single crystal. It is possible to reduce the number of defects or dislocations generated in single crystals during heat treatment in the production of semiconductor devices.

In a preferred method according to the present invention, the growth rate of the silicon single crystal is greater than or equal to 1.2 mm/min. In addition, the optimum oxygen content in the grown silicon single crystal is greater than or equal to 1.8 x 10 ¹⁸ /cm ³ .

According to the present invention, a silicon substrate with an oxygen content greater than or equal to 1.8×10 ¹⁸ /cm ³ can obtain a leakage current less than or equal to 1×10 ^-10 .

According to the viewpoint of the present invention, a method for the production of a silicon substrate containing a relatively high oxygen content for semiconductor devices comprises the following steps:

Silicon single crystals are grown from molten silicon at a relatively high growth rate, and the selected growth rate can avoid the segregation of oxygen from the single crystal during the subsequent heat treatment of semiconductor device production; and from silicon single crystals Made of silicon substrates.

The optimum growth rate of silicon single crystal is greater than or equal to 1.2 mm/min. On the other hand, the optimum oxygen content in the silicon substrate is greater than or equal to 1.8 x 10 ¹⁸ /cm ³ . Further preferably, the growth rate of the silicon single crystal is approximately in the range of 1.5 mm/min to 2.1 mm/min.

In a preferred embodiment, the silicon single crystal growth process includes the following steps: placing silicon in a crucible; heating to keep the silicon in a fluid state; and gradually pulling out the silicon single crystal from the molten silicon in the crucible.

In the step of heating the silicon, the heat applied to it must be sufficient to prevent curing of the silicon surface. Preferably, during the stage of heating the silicon, more heat is applied to the surface of the silicon than to the rest of the molten silicon.

In another embodiment, the method further includes the step of applying a magnetic field to the silicon. In addition, the preferred method may further comprise means of driving the crucible to rotate. The rotational speed of the crucible can be controlled in order to adjust the oxygen content in the silicon substrate.

According to another viewpoint of the present invention, in order to realize the production method of above-mentioned silicon substrate, a kind of equipment that is used to grow a kind of single crystal silicon that contains relatively high oxygen content as the silicon substrate raw material of semiconductor device comprises: a crucible, for It is used for silicon loading; a heating device is used to heat silicon to keep silicon in a fluid state; and a pulling device can pull silicon single crystal from molten silicon in the crucible at a relatively high speed, so that the subsequent manufacturing of semiconductors During the heat treatment of the device, oxygen segregation from the substrate can be prevented. Preferably, the pulling speed of the silicon single crystal is greater than or equal to 1.2 mm/min. In addition, the optimum oxygen content in the silicon substrate is greater than or equal to 1.8×10 ¹⁸ cm ³ . The heater provides enough heat to prevent the surface of the molten silicon from solidifying. Therefore, the heating means supplies more heat to the surface of the molten silicon than to the rest of the molten silicon.

The apparatus also includes means for applying a magnetic field to the molten silicon. In addition, the apparatus further includes drive means for rotating the crucible. The crucible drive rotates the crucible at a variable speed in order to regulate the oxygen content in the silicon wafer.

According to still another aspect of the present invention, a semiconductor device is produced using a silicon substrate having an oxygen content greater than or equal to 1.8 x 10 ¹⁸ /cm ³ and a leakage current value of less than 1 x 10 ^-10 ampere.

The present invention will be more fully understood from the following detailed description, together with the accompanying drawings of preferred embodiments of the invention. However, they do not limit the present invention, and the specific examples are only for illustration and understanding of the present invention.

In the attached picture:

Fig. 1 is the sectional view that realizes the silicon single crystal growth equipment that adopts according to the preferred embodiment of silicon single crystal production method of the present invention;

Figure 2 is a perspective view of the heater in Figure 1;

Fig. 3 is a three-dimensional spatial graph illustrating the interrelationship between crystal growth rate, oxygen content and stacking fault density;

Fig. 4 is the relationship curve of heat treatment time and oxygen content;

Fig. 5 and Fig. 6 represent respectively, carry out the result of leakage current measurement with a batch of diode samples that are made according to the production method of silicon substrate of the present invention and the silicon substrate produced by known conventional method;

Fig. 7 is a cross-sectional view of an improved specific silicon single crystal growth apparatus used for carrying out the preferred embodiment of the silicon single crystal production method according to the present invention.

Referring now to the accompanying drawings, FIG. 1 shows a silicon single crystal growth apparatus used for carrying out a preferred embodiment of the method for growing a silicon substrate according to the present invention. As seen in FIG. 1, a preferred embodiment of the method for producing a silicon substrate includes a method for growing a silicon single crystal as a raw material for the silicon substrate. According to the priority The selected method, the growth of silicon single crystal adopts CZ method.

In the single crystal growth apparatus of the present invention, silicon 3 is molten, and it is contained in a quartz crucible 2, and the quartz crucible 2 is arranged in a graphite crucible 1. Graphite heater 4 and heat insulating material 9 surround crucible 1 . Several additional cooling jackets 10a, 10b and 10c surround the insulating material 9 in turn. The cooling jacket 10b has a viewing port 12 through which the pulled single crystal 6 can be observed. An exhaust pipe 13 is provided on the bottom plate of the cooling jacket 10b to discharge the inert gas which is introduced into the jackets 10a, 10b and 10c from above as a protective atmosphere. A shaft 8 fixed on the bottom surface of the crucible 1 can freely pass through the small opening 10d on the bottom plate of the cooling jacket 10a, and the crucible 1 can be raised or lowered by rotating with this shaft. The lower edge of the heater 4 is fixed to a ring plate 14 which itself is fixed to a pair of shafts 15 which freely pass through two small openings 10e and 10f in the bottom plate of the cooling jacket 10a. The shaft 15 can be used to raise or lower the heater 4 . A cylindrical molybdenum heat shield 16 with an inner diameter slightly larger than the outer diameter of the single crystal 6 is placed on the liquid silicon 3 and surrounds the single crystal 6 . In the heat shield 16, the seed crystal is clamped by the chuck 7 mounted on the bottom of the tie rod 17, so that the cylindrical single crystal 6 can grow from the seed crystal 5.

In the CZ method, assuming that the solid-liquid interface between the single crystal 6 and the liquid 3 is flat, and there is no radial temperature gradient in the single crystal 6, the maximum growth rate of the single crystal can be expressed as follows:

V _max = (k)/(hρ) ( (dT)/(dX) )

Among them, k represents the thermal conductivity of the single crystal 6, h represents the heat of solidification, ρ represents the density, and dT/dx represents the temperature gradient of the single crystal solid phase at the solid-liquid interface. Specifically, X refers to the distance along the long axis of single crystal 6. In the above formula, since k, h and ρ are inherent properties of the material, it is necessary to increase the temperature gradient dT/dx. To increase or obtain the maximum single crystal growth rate Vmax. However, in the above-mentioned CZ method, since the single crystal 6 is heated by radiant heat from the surface of the liquid 3, the inner wall of the crucible 2, and the heater 4, the temperature gradient dT/dx value is necessarily limited so that The actual growth rate is always smaller.

From the above discussion, it can be seen that to increase the growth rate of silicon single crystal, it can be achieved by reducing the heat supplied by the heater 4 to the molten silicon 3, that is, by reducing the temperature of the molten silicon. While this has a proportional effect on reducing the temperature gradient, the amount of heat radiated to the single crystal can be reduced to a considerable extent by the Stefan-Boltzmann law, with the net effect of increasing the temperature gradient as a result dT/dx. However, reducing the amount of heat supplied by the heater 4 to obtain a higher growth rate means that the surface of the molten silicon tends to solidify because it is cooled by exposure to the atmosphere of the protective gas in the furnace. This limits the range in which the molten silicon 3 can be cooled.

The heater 4 of the preferred silicon single crystal growth equipment is designed to supply enough heat to the surface of the molten silicon 3 to keep the silicon in a liquid state. In particular, the heater 4 is designed to be structured so as to provide more heat to the surface of the molten silicon 3 than to the rest of the molten silicon, resulting in a minimum temperature of the molten silicon 3 .

FIG. 2 shows the structure of the heater 4 . The heater 4 is made of a heat transfer material such as graphite and is generally in the form of a circular sleeve with a tapered portion 4a at the upper end. The heater 4 has alternating upper cutouts 4b and lower cutouts 4c, and the extension of each cutout is parallel to the vertical axis of the heater 4 . This construction provides a cylindrical casing with a serpentine circuit suitable for use as a heating element. Furthermore, the top corner of the lower cutout 4c bifurcates into two short cutouts 4d and 4e which extend at an angle of 45° relative to the cutout 4c. Current passes through each portion defined by the adjoining of the upper cutout 4b and the lower cutout 4c, and heat is generated due to ohmic (resistance) loss.

In order to make the single crystal grow along the seed crystal from the molten silicon material with the single crystal production equipment of the above-mentioned structure, for example, the two crucibles 1 and 2 can be rotated clockwise by the shaft 8 while leaning on the rod 17 The growing single crystal 6 is turned counterclockwise, or vice versa. At the same time, the pull rod 17 is gradually lifted by a transmission mechanism (not shown), so as to pull out the single crystal from the melt. In addition, both crucibles 1 and 2 are also gradually raised so that the surface of the liquid 3 can be maintained at a predetermined position relative to the heater 4 .

The above-mentioned equipment has the following advantages: the upper end of the heater 4 is tapered, and in addition, the bifurcation of 4d and 4e is formed at the top of the lower slit 4c, so the cross-sectional area of the tapered part 4a is smaller than that of the heater. other parts of 4. In particular, the cross-sectional areas near the bifurcations 4d and 4e are very small. Therefore, when current is passed through the heater 4, the tapered portion 4a of the heater 4 is heated to a higher temperature than the other portions of the heater 4. As a result, the temperature difference between the melting surface 3a vertically opposite to the tapered portion 4a, the inner wall of the crucible 2, and the maximum temperature in the melt 3 is small.

In addition, since the tapered portion 4a increases the total resistance of the heater 4 compared with the conventional one, the temperature of the heater 4 will be higher assuming that the same amount of current is applied. Therefore, in this device, the current through the heater 4 can be smaller than in conventional heaters of similar design.

As already explained, in order to accelerate the maximum growth rate Vmax, the temperature gradient dT/dX of the solid-phase single crystal 6 at the solid-liquid interface must be increased. Therefore, it would be best to reduce the heat output of the heater 4 because the single crystal is heated by the heater 4 radiation.

In the apparatus according to the present invention, although the heat output of the heater 4 is reduced in order to increase the temperature gradient (dT/dX), due to the above, the maximum temperature difference between the fusion surface 3a and the melt 3 is very small, Solidification of the surface of the melt 3 on the inner wall of the crucible 2 can thus be avoided. The result is a significantly faster growth rate of 0.2 mm/min than conventional devices. In addition, the single crystal 6 can be grown continuously, thereby increasing the yield and reducing the cost of single crystal production.

In a preferred embodiment of the method for producing or fabricating a silicon substrate according to the present invention, the apparatus described above is used. It has been found in the present invention that the crystal growth rate has a great influence on the generation of crystal defects, especially stacking faults. Therefore, in the present invention, in order to obtain a silicon single crystal having an oxygen content of more than 1.8 x ¹⁰¹⁸ / ^cm3 , it is prescribed that the crystal growth rate must be higher than 1.2 mm/min. This silicon single crystal is cut into silicon substrates. The growth rate of the selected silicon single crystal is higher than that of the traditional method, which can prevent the segregation of oxygen in the subsequent heat treatment, thus also avoiding the loss of the quality of the grown silicon single crystal. Therefore, it is feasible to increase the oxygen content. An oxygen content of 1.8 x 10 ¹⁸ /cm ³ or higher can be achieved in the present invention, and therefore, an enhanced IG effect can be obtained.

Discuss below adopts the device of Fig. 1 and Fig. 2, the finished product of silicon substrate produced according to the best method of the present invention.

Using CZ method to control and grow silicon single crystal. From this single crystal is cut into wafers. The surface of the wafer was mirror-polished, and then subjected to two heat treatments at a temperature of 1100° C. for two hours each in an atmosphere of dry oxygen. Afterwards, the wafer is etched to a depth of 13 microns using a so-called dry etch method in order to expose the stacking faults. In order to complete this test, the growth rate of silicon single crystal in the CZ process was changed to prepare various samples. At the same time, various samples with different oxygen contents were prepared. The stacking fault densities of these samples were measured. Figure 3 shows the results of these measurements.

The results shown in Fig. 3 show that basically no stacking faults are formed when the silicon single crystal growth rate is greater than or equal to 1.2 mm/min. In addition, it was further confirmed that no stacking faults occurred during heat treatment of silicon wafers or silicon substrates, including surface polishing.

In addition, changes in oxygen content due to heat treatment at 750°C were measured. Figure 4 shows these measurements as a graph of oxygen content versus heat treatment time. In the figure, curves 21 to 23 show the relationship between the oxygen content and the heat treatment time when the crystal growth rate is greater than 1.2 mm/min. The initial oxygen contents of curves 21 to 26 are: 1.644×10 ¹⁸ /cm ³ , 1.667×10 ¹⁸ /cm ³ , 1.709×10 ¹⁸ /cm ³ , 1.866×10 ¹⁸ /cm ³ , 2.019×10 ¹⁸ /cm ³ and 1.737×10 ¹⁸ /cm ³ . Although when the silicon substrate or silicon single crystal is heat-treated, the oxygen content will eventually decrease due to the escape of oxygen, it is obvious that due to the adoption of the present invention, when the initial content of oxygen is very high, even in a relatively long period of time After heat treatment, the oxygen content represented by curves 24 to 26 changes little and only after a long time does a measurable loss of oxygen occur.

As can be seen from Figure 3 and Figure 4, it is obvious that the result of high-speed crystal growth has fewer stacking faults.

In another experiment, using the present invention and the traditional method, silicon substrates were prepared respectively, n ⁺ -P junctions were formed on the silicon substrates, and then diodes were made, and the leakage current of each diode Pn junction was measured. In this case, a p-type region was formed on an n-type silicon substrate, and at the same time, an n ⁺ region having an area of 2.4 x ^10-12 cm/ ^cm2 was formed. Apply a test voltage of 5 volts to the n ⁺ region for measurement. The test results of the silicon substrates are shown in Fig. 5. These silicon substrates are made of silicon single crystals grown by the CZ method, the crystal growth rate is greater than or equal to 1.2 mm/min, and the oxygen content is 2.0×10 ¹⁸ /cm ³ . On the other hand, Fig. 6 shows the results of testing a silicon substrate made of a silicon single crystal grown at a rate of 0.6 to 0.9 mm/min by a conventional silicon single crystal growth method. In FIG. 5 and FIG. 6, the abscissa is the measured leakage current, and the ordinate is the number of samples showing the indicated leakage current. Comparing Fig. 5 with Fig. 6, it can be seen that, in the case of the silicon substrate manufactured by the present invention, its leakage current is indeed reduced to 10 ^-11 amperes or lower. This may be the result of the obvious IG effect caused by high oxygen content.

Apparently, according to the preferred method of the present invention, a silicon single crystal with a high oxygen content can be provided. Furthermore, the oxygen content can also be accurately selected from a wide range by a crystal growth method in which a magnetic field is applied to molten silicon in a quartz crucible and the crucible is rotated as necessary. An example of such a crystal growth method using a magnetic field will be described with reference to FIG. 7. FIG.

In the figures, the overall device is generally indicated by the reference number 31 . Quartz crucible 32 contains molten silicon from which crystals grow. The crucible 32 rotates around its central axis at an adjustable rotational speed. A heater 34 surrounds the crucible 32 . The heater 34 may be an electric cartridge heater 35 similar to the heater 4 in the above embodiment. As required, a cylindrical heat insulator or a water-cooled jacket 36 can be assembled outside the heater. Outside the jacket 36, a DC magnetic field generator 37 made of permanent magnets or electromagnets is installed. The silicon single crystal seed is indicated at reference numeral 38, and the puller is shown at reference numeral 39. When the seed crystal rotates around the rotating shaft of the crucible, the pulling chuck 39 pulls the silicon single crystal seed crystal 38 upward.

The electric power supplied to the heater 34 may be a direct current with a fluctuation of 4% or less, or an alternating current of 1 kHz or more, or a pulse current. Such currents have proven sufficient to avoid unwanted resonance phenomena between the heater 34 and the magnetic field.

The single crystal silicon seed crystal 38 is pulled out from the molten silicon surface at a predetermined speed, so as to guide the growth of silicon single crystal 40 . In this case, changing the rotational speed of the crucible 32 and thus also in particular the oxygen content in the finished crystal 40 is changed. The reason for this is as follows. The actual viscosity of molten silicon in the crucible increases due to the applied magnetic field. Due to the rotation of the silicon in the opposite direction to the rotation of the crucible, frictional contact occurs between the molten silicon 3 and the inner wall of the crucible 32 . Thus, the crucible 32, especially the oxygen in the inner wall of the quartz crucible will Dissolved in molten silicon 33. Since the amount of dissolved oxygen increases with the increase of the frictional contact, that is, with the increase of the rotation speed of the crucible relative to the molten silicon 33, the oxygen content in the growing crystal 40 is increased. Furthermore, it has been shown that, provided that the rotational speed of the crucible is sufficiently high, a higher crystalline oxygen content can be obtained with the application of a magnetic field than without the application of a magnetic field.

As mentioned above, the present invention has many advantages due to the ability to maintain a high oxygen content. For example, when the crystal is being pulled, the temperature hysteresis effect can be basically limited; due to the high oxygen content, during heat treatment, an extremely high I.G. effect can be obtained; in addition, the crystal stacking fault on the substrate surface can also be suppressed. Due to these advantages, semiconductor elements formed on such silicon substrates can obtain many unique advantages, such as reduction of leakage current, improvement of breakdown voltage, increase of characteristic uniformity, improvement of yield, and the like.

Although the invention has been discussed in terms of preferred embodiments in order to facilitate a better understanding of the invention, it must be understood that the invention may be implemented in various ways without departing from the principles of the invention. Therefore, it must be understood that the invention includes all possible implementations and modifications of the illustrated devices which can be equipped without departing from the principles of the invention as set forth in the appended claims.

Claims (2)

1. A production method for preparing silicon substrates with high oxygen content, including growing silicon single crystals by pulling method, polishing and heat treatment of silicon single crystal slices, wherein, the steps during growth include putting silicon into a crucible and heating, using the upper Tapered heater, the cross-sectional area of the tapered part of the heater is smaller than that of other parts, so that the heat supplied by the silicon surface is more than the rest of the molten silicon, and it also includes further measures to rotate the crucible, and the speed can be controlled to adjust the silicon crystal The oxygen content in the silicon _substrate is grown at a high pulling speed of about 1.5-2.1 mm/min, so that the oxygen content of the silicon substrate is greater than or equal to 1.8×10 ¹⁸ /cm ³ . 2. A device for growing a silicon single crystal with a high oxygen content used as a raw material for a silicon substrate of a semiconductor device, comprising: A crucible for silicon, A heater is used to heat the silicon to keep the silicon in a fluid state. The heater supplies enough heat to prevent the surface of the molten silicon from solidifying. There is a tapered portion at the upper end of the heater. The cross-sectional area of the tapered portion is smaller than the cross-sectional area of other parts, the heater supplies more heat to the surface of the molten silicon than to other parts of the molten silicon; and a pulling device for pulling the silicon single crystal from the molten silicon in the crucible at a relatively high speed, in order to prevent the oxygen content in the silicon substrate from decreasing during the heat treatment of the process for preparing the above-mentioned semiconductor device later, Wherein the pulling speed of the silicon single crystal is 1.5mm/min to 2.1mm/min; the oxygen content of the silicon substrate is greater than or equal to 1.8× ¹⁰¹⁸ / ^cm3 ; Wherein the driving device of the crucible can rotate the crucible at a variable speed so that the oxygen content in the silicon substrate can be adjusted.

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