Patent Application
20100330325
The present invention relates to a sintered silicon wafer with superior mechanical properties.
In the silicon semiconductor manufacturing process, a wafer prepared based on single crystal pulling method is primarily used. This single-crystal silicon wafer has increased in size with the times, and it is expected to become 400 mm or larger in the near future. In addition, a so-called mechanical wafer for testing is now required in order to establish the apparatus and peripheral technology necessary for the semiconductor manufacturing process.
Generally speaking, since this kind of mechanical wafer is subject to a fairly high precision testing, it needs to possess properties that are similar to the mechanical properties of single-crystal silicon. Thus, although previously it would be used for testing, it appears to be a reality that the single-crystal silicon wafer was being used as is. However, since a single-crystal silicon wafer of 400 mm or larger is extremely expensive, an inexpensive wafer having similar properties as single-crystal silicon is in demand.
Meanwhile, as a component of such semiconductor manufacturing equipment, a proposal has also been made for application of a sputtering target formed from a rectangular or disk-shaped silicon plate. The sputtering method is being used as a means for forming thin films, and there are several sputtering methods including the bipolar pulsed DC sputtering method, radio-frequency sputtering method, magnetron sputtering method and the like. Thin films of various electronic parts are being formed using the sputtering characteristics unique to the respective methods.
This sputtering method is a method that a substrate as the anode is placed vis-a-vis a target as the cathode, and an electrical field is generated by applying a high voltage between the foregoing substrate and target under an inert gas atmosphere; and is applying the principle that the ionized electrons and inert gas collide in the electrical field to form a plasma, the cations in the plasma collide with the target surface to hammer out the constituent atoms in the target, and the discharged atoms adhere to the opposite substrate surface so as to form a film.
A sintered compact of polycrystalline silicon is proposed for this kind of sputtering target, and the target of this sintered compact is required to be of considerable thickness and to be of a large-size rectangle or disk shape in order to improve the deposition efficiency. Moreover, a proposal has also been made for using this sintered compact of polycrystalline silicon as a board for supporting the single-crystal silicon wafer. Nevertheless, polycrystalline silicon entails significant problems in that the sinterability is inferior, and the obtained products have low density and low mechanical strength.
In light of the above, in order to improve the properties of the foregoing target of silicon sintered compact; proposed is a silicon sintered compact formed by compression-molding the silicon powder obtained by being heated within a temperature range of 1200° C. or higher but lower than the melting point of silicon under reduced pressure to deoxidize and subsequently sintering the molded material, wherein the crystal grain size of the sintered compact is set to be 100 μm or less (for instance, refer to Patent Document 1).
However, although the density will relatively increase and the strength will also increase if the thickness of the target manufactured as described above is thin, for example 5 mm or less; the density will continue to be a low density (less than 99%) and the mechanical strength will also deteriorate if the thickness becomes any thicker. Thus, there is a problem in that it is not possible to manufacture a large-size rectangular or disk-shaped target.
In light of the foregoing circumstances, the present applicant has previously proposed a silicon sintered compact of which the average crystal grain size is 50 μm or less and the relative density is 99% or more, and its production method (refer to Patent Document 2). Although this silicon sintered compact possesses numerous advantages including high density and high mechanical strength, further improvement in these properties is being demanded.
The present invention was devised in view of the foregoing circumstances, and provides a sintered compact wafer having a fixed strength and similar mechanical properties as single-crystal silicon even in cases of sintered silicon wafer of a large-size disk shape.
In order to achieve the foregoing object, the present inventors discovered that it is possible to obtain a sintered silicon wafer with improved mechanical strength by devising the sintering conditions and adjusting the crystal grain size.
Based on the foregoing discovery, the present invention provides:
(1) A sintered silicon wafer, wherein the maximum crystal grain size is 20 μm or less and the average crystal grain size is 1 μm or more but not more than 10 μm;
(2) The sintered silicon wafer according to the above (1); wherein, when a wafer surface is divided into any plural sections and the average grain size is measured for each section, the variation in the average grain size of each section is ±5 μm or less; and
(3) The sintered silicon wafer according to the above (1) or (2), wherein the wafer has a diameter of 400 mm or more and the following mechanical properties 1) to 3) measured by collecting a plurality of test samples from the sintered silicon wafer:
1) the average deflecting strength based on a three-point bending test is 20 kgf/mm2 or more but not more than 50 kgf/mm2;
2) the average tensile strength is 5 kgf/mm2 or more but not more than 20 kgf/mm2; and
3) the average Vickers hardness is Hv 800 or more but not more than Hv 1200.
Accordingly, it is possible to provide a sintered compact wafer having significantly improved strength even in cases of sintered silicon wafer of a large-size disk shape, and provide a sintered silicon wafer having similar mechanical properties as single-crystal silicon used as a mechanical wafer. In addition, since the strength is high, superior characteristics are yielded such as being able to prevent the generation of cracks and chipping, easily being processed into complex shapes, considerably improving the yield rate, and reducing manufacturing costs.
The present invention provides a sintered silicon wafer in which the maximum crystal grain size is 20 μm or less and the average crystal grain size is 1 μm or more but not more than 10 μm. Consequently, it is possible to make the average deflecting strength (bending strength) of the wafer based on the three-point bending test to be 20 kgf/mm2 or more but not more than 50 kgf/mm2, the average tensile strength to be 5 kgf/mm2 or more but not more than 20 kgf/mm2, and the average Vickers hardness to be Hv 800 or more but not more than Hv 1200 even with sintered silicon wafers having a diameter of 400 mm or larger. These are conditions that also coincide with the mechanical properties of a single-crystal wafer.
The greatest weakness of a sintered silicon wafer is the deterioration in the deflecting strength (bending strength), and the present invention is able to overcome this weakness.
When improving the foregoing mechanical properties, the miniaturization of the crystal grain size is extremely important. With a sintered silicon wafer in which the maximum crystal grain size exceeds 20 μm and the average crystal grain size is less than 1 μm or exceeds 10 μm, it is not possible to achieve the foregoing mechanical properties; namely, the average deflecting strength based on the three-point bending test being 20 kgf/mm2 to 50 kgf/mm2, the average tensile strength being 5 kgf/mm2 or more but not more than 20 kgf/mm2, and the average Vickers hardness being Hv 800 to Hv 1200.
It is important to adjust the variation in the crystal grain size of the sintered silicon wafer; specifically, when a wafer surface is divided into any plural sections and the average grain size is measured for each section, it is important that the variation in the average grain size of each section is ±5 μm or less. This is intended to achieve the uniformity in the wafer structure, which is directly related to the uniformity in the foregoing mechanical properties, and is able to effectively prevent nicks or cracks.
Since this kind of silicon sintered compact wafer has high mechanical strength and superior workability, not only can it be used as a mechanical wafer (or dummy wafer), but it can also be used as various components such as a sputtering target or a holder for semiconductor manufacturing equipment.
Upon manufacturing components, superior effects are yielded in that it is possible to prevent the generation of cracks and chipping of the sintered silicon wafer, easily process the sintered silicon into complex shapes, considerably increase the yield rate, and reduce manufacturing costs.
Based on the above, the present invention provides a sintered silicon wafer having a diameter of 400 mm or greater in which the average deflecting strength of the wafer based on the three-point bending test is 20 kgf/mm2 or more but not more than 50 kgf/mm2, the average tensile strength is 5 kgf/mm2 or more but not more than 20 kgf/mm2, and the average Vickers hardness is Hv 800 or more but not more than Hv 1200. Conventionally, a sintered silicon wafer having a diameter of 400 mm or greater and comprising the foregoing properties has not existed.
As a method of producing a silicon sintered compact, for instance, silicon powder prepared by pulverizing coarse grains of high-purity silicon of 5N or higher in a jet mill is baked within a temperature range of 1100 to 1300° C., preferably less than 1200° C., under reduced pressure to deoxidize, subject to primary sintering by hot pressing, and then subject to HIP treatment within a temperature range of 1200 to 1420° C. at a pressure of 1000 atmospheres or greater.
Here, by using high-purity silicon powder and adopting the foregoing deoxidation conditions based on the pulverization and baking of the powder, and the temperature condition and the pressurized condition for HIP treatment; the crystal grain size can be adjusted, and the sintering conditions are adjusted so that the maximum crystal grain size becomes 20 μm or less and the average crystal grain size becomes 1 μm or more but not more than 10 μm. During the sintering process, it is particularly effective to use silicon powder having an average grain size of 10 μm or less.
In addition, deoxidation is important, and sufficient deoxidation is required for obtaining a silicon sintered compact of fine crystals. The reason why the baking temperature was set to be within a range of 1000 to 1300° C., preferably less than 1200° C., is because the elimination of oxygen will be insufficient if the temperature is less than 1000° C.
Deoxidation will proceed at 1200° C. or higher, but necking (phenomenon where powers adhere to each other) will increase, and, even if the necking is undone during the hot press, there are drawbacks in that there will be variations in the grain size distribution, and the working hours will become long. Thus, it is essential that the upper limit of the temperature is set to 1300° C.
In the HIP conditions, if the temperature is less than 1200° C. and the pressure is less than 1000 atmospheres, similarly a high-density silicon sintered compact cannot be obtained, and if the temperature is 1420° C., it will exceed the melting point of Si. With respect to the retention time of the respective processes, it is preferable to perform baking for roughly 5 hours, hot pressing for roughly 10 hours, and the HIP treatment for roughly 3 hours. Prolonged HIP treatment is undesirable since the crystal grains will become coarse. With that said, however, these times may be changed as needed according to the processing conditions, and the present invention is not limited to the foregoing length of time.
The present invention is now explained in detail with reference to the Examples. These Examples are merely for facilitating the understanding of this invention, and the present invention shall in no way be limited thereby. In other words, various modifications and other embodiments based on the technical spirit claimed in the claims shall be included in the present invention as a matter of course.
Silicon powder having an average grain size of 7 μm prepared by pulverizing silicon coarse grains having a purity of 6N with a jet mill was subject to baking treatment under reduced pressure at a temperature of 1000° C. for 5 hours to deoxidize.
Subsequently, hot press was performed by setting the temperature to 1200° C. and simultaneously setting the surface pressure to 200 kgf/cm2, and this was thereafter subject to HIP at a temperature of 1200° C. and an applied pressure of 1400 atmospheres to obtain a silicon sintered compact having a diameter of 400 mm.
The crystal grain size can be arbitrarily adjusted by using fine high-purity silicon, selecting the baking (deoxidation) condition, and respectively selecting the HIP temperature and the applied pressure. The silicon sintered compact obtained thereby was ground into a silicon wafer.
The silicon sintered compact wafer of Example 1 had an average crystal grain size of 7 μm and a maximum crystal grain size of 16 μm. The mechanical strength of the sintered silicon wafer was measured. The measured mechanical strength is the average value of the five points arbitrarily sampled from the wafer.
Consequently, the average bending strength of the five sampled points was 26 kgf/mm2, the average tensile strength was 14 kgf/mm2, and the average Vickers hardness was Hv 1000. It satisfied the properties required as a mechanical wafer. Incidentally, the characteristic values were rounded off to the whole number. The results are shown in Table 1.
As described above, since the silicon sintered compact wafer possesses sufficient strength, cracks or chipping did not occur even when the diameter of the wafer was increased to 420 mm, 440 mm, 460 mm, 480 mm . . . .
Incidentally, although silicon having a purity of 6N was used since the inclusion of impurities in the silicon sintered compact wafer is undesirable, there is no particular problem so as long as the purity level is 5N or higher. In addition, the mechanical properties were not affected so as long as the purity level was 5N or higher.
Fine silicon powders having a purity of 5N and 6N and an average grain size of 1 μm to 10 μm were, as with Example 1, baked within a temperature range of 1100 to 1300° C. under reduced pressure to deoxidize, and subsequently hot pressed within a temperature range of 1200 to 1420° C. at a surface pressure of 200 kgf/cm2 or greater, and the silicon obtained thereby was further subject to HIP treatment within a temperature range of 1200 to 1420° C. at a pressure of 1000 atmospheres or higher so as to produce sintered silicon in which, as shown in Table 1, the maximum crystal grain size is 20 μm or less and the average crystal grain size is within the range of 1 μm to 10 μm.
The results are similarly shown in Table 1. As shown in Table 1, the average deflecting strength was 21 to 33 kgf/mm2, the average tensile strength was 12 to 17 kgf/mm2, and the average Vickers hardness was Hv 830 to Hv 1120. In all cases, the average deflecting strength based on the three-point bending test was 20 kgf/mm2 or more but not more than 50 kgf/mm2, the average tensile strength was 5 kgf/mm2 or more but not more than 20 kgf/mm2, and the average Vickers hardness was Hv 800 or more but not more than Hv 1200. These possessed the mechanical properties of the present invention and could be used as a mechanical wafer.
Next, based on representative Example 1 of the present invention, the variation in the average grain size of each section was observed when the silicon wafer surface was divided into any plural sections and the average grain size was measured for each section. The results are shown in Table 2.
According to the observation, a sintered silicon wafer in which the variation was ±5 μm or less had an average deflecting strength of 25 to 26 kgf/mm2, an average tensile strength of 13 to 14 kgf/mm2, and an average Vickers hardness of Hv 970 to Hv 1000, and it is evident that the smaller the variation, the smaller the differences based on location and the better the mechanical properties. Accordingly, it is desirable to suppress the foregoing variation to ±5 μm or less in order to stabilize the mechanical properties and improve the quality of the silicon wafer.
Nevertheless, it should be understood that the range of this variation will not cause any significant problem so as long as the maximum crystal grain size is 20 μm or less and the average crystal grain size is within a range of 1 μm or more but not more than 10 μm in the present invention.
A sintered silicon wafer having an average crystal grain size of 12 μm and a maximum crystal grain size of 25 μm was prepared by using silicon powder having a purity of 5N and an average grain size of 10 μm, and respectively selecting the baking (deoxidation) condition, HIP temperature and applied pressure. As with Example 1, the mechanical strength was measured. The results are shown in Table 3. The measurement value of the mechanical strength is an average value of the five sampled points.
As shown in Table 2, the deflecting strength (bending strength) was 16 kgf/mm2, the tensile strength was 10 kgf/mm2, and the Vickers hardness was Hv 790. It did not satisfy the bending strength and the Vickers hardness required as a mechanical wafer. This is considered to be because it did not satisfy the conditions of the present invention; specifically, the maximum crystal grain size being 20 μm or less and the average grain size being 1 to 10 μm.
Secondly, a silicon ingot having a maximum crystal grain size of 8 mm and an average crystal grain size of 2 mm was prepared with the melting method, and this was cut out to obtain a silicon wafer. The mechanical strength of this cast silicon wafer was measured as with Example 1. The results are shown in Table 3. The measurement value of the mechanical strength is an average value of the five sampled points.
As shown in Table 3, the average deflecting strength was 8 kgf/mm2, the average tensile strength was 5 kgf/mm2, and the average Vickers hardness was Hv 780. The deterioration in the average deflecting strength and the average tensile strength was considerable, and it did not satisfy the mechanical properties required as a mechanical wafer. The deterioration in properties is considered to be a result of the coarsening of the crystal grain size.
Sintered silicon wafers having the average crystal grain size and maximum crystal grain size shown in Table 2 were prepared by using silicon having a purity of 5N and respectively selecting the baking (deoxidation) condition, HIP temperature and applied pressure. As with Example 1, the mechanical strength was measured. The results are shown in Table 3. The measurement value of the mechanical strength is an average value of the five sampled points.
As shown in Table 3, with Comparative Examples 3 to 6, the average deflecting strength was 12 to 19 kgf/mm2, the average tensile strength was 7 to 11 kgf/mm2, and the average Vickers hardness was Hv 720 to Hv 820. These did not satisfy the bending strength and the Vickers hardness required as a mechanical wafer.
Meanwhile, with Comparative Example 7, the tensile strength and the Vickers hardness became greater than a single-crystal silicon wafer, and there was a problem in that the mechanical properties were no longer similar to those required as a mechanical wafer. Thus, the sintered silicon wafer of Comparative Example 7 was also inappropriate. This is considered to be because these did not satisfy the conditions of the present invention; specifically, the maximum crystal grain size being 20 μm or less and the average grain size being 1 to 10 μm.
The present invention is able to obtain a sintered compact wafer having significantly improved strength and similar mechanical properties as single-crystal silicon even in cases of sintered silicon wafer of a large-size disk shape, and this is effective as a mechanical wafer. In addition, since this silicon sintered compact wafer has high mechanical strength, it can also be used as a sputtering target or various components of semiconductor manufacturing equipment.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2007-184756 | Jul 2007 | JP | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/JP2008/062172 | 7/4/2008 | WO | 00 | 1/8/2010 |
