Silicon wafer manufacturing method

Abstract

To provide a silicon crystal having the minimum concentrations of both carbon and iron and simultaneously no-defect, and also having a size of more than 50 μm and the minimum pinhole density.SOLUTION: A silicon single crystal has: an oxygen concentration of more than 2×1017 at / cm3; a diameter of more than 100 μm; a pinhole density of less than 1.0×10-51 / cm3; a carbon concentration of less than 5.5×1014 at / cm3, preferably, less than 4×1014 at / cm3; an iron concentration of less than 5.0×109 at / cm3, preferably, less than 1.0×109 at / cm3; a COP density of less than 1000 defects / cm3; an LPIT density of less than 1 defect / cm2; and a crystal diameter of more than 200 mm, preferably, more than 300 mm.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a method for manufacturing silicon wafers, the method comprising the steps of melting polysilicon in a crucible, pulling a single crystal on a seed crystal from the heated melt in the crucible according to the Czochralski method, and slicing wafers from the pulled single crystal. [Background technology]

[0002] The crucible is usually made of a material containing silicon dioxide, such as quartz. The crucible is generally filled with crushed and / or granular material, generally made of polycrystalline silicon, which is heated by means of side heaters arranged around the crucible and a bottom heater arranged below the crucible. After a thermal stabilization stage of the melt, a single crystal seed is immersed in the melt and pulled up. At the same time, silicon crystallizes at the end of the seed, which is wetted by the melt. The crystallization rate is substantially influenced by the rate at which the seed is pulled up (crystal rise rate) and the temperature of the interface where the molten silicon crystallizes. With proper control of these parameters, the first segment to be pulled is called the neck to remove dislocations, the second is pulled up as a single crystal cone-shaped segment, and the last is pulled up as a single crystal cylindrical segment from which wafers are subsequently cut.

[0003] In crystal pulling methods, as described for example in US Pat. No. 5,954,873 A, the relevant process parameters are set in order to obtain a radially homogeneous distribution of defects in the crystal.

[0004] In particular, it is noted that agglomerates consisting of vacancies (voids), also called crystal originated particles (COPs), do not form or form below the detection limit. In the following, the detection limit is 1000 defects / cm. 3 This density is understood to be the detection limit of COP.

[0005] At the same time, we note that aggregates consisting of interstitial silicon atoms, called LPITs, do not occur at the detection limit or occur only below the detection limit. In the following, we consider 1 defect / cm 2 This LPIT density is understood to be the detection limit.

[0006] Hereinafter, this semiconductor material will be referred to as "defect-free." The inclusion and subsequent release of gases in the crucible material, gases surrounding the crushed and / or granular material, silicon oxides formed in the melt, as well as gases diffusing into the melt, are considered possible causes of the formation of voids in the single crystal, called pinhole defects (not to be confused with COPs). Pinholes are formed when a gas bubble reaches the interface between the growing single crystal and the melt, around which the single crystal crystallizes. If, when cutting the wafer, the cutting plane intersects the void, the wafer formed typically has a circular depression or hole with a diameter that can be from a few micrometers to a few millimeters. Wafers with such voids cannot be used as slices of substrates for the manufacture of electronic components.

[0007] For example, the density of pinholes formed in a rod piece can be measured using a scanning ultrasound method, for example as described in DE 10 2006 032 431 A1. In this case, pinholes with a diameter of about 50 μm are detected. In this method, determining the exact size of each pinhole involves a relatively large measurement error.

[0008] US9665931A1 describes a method for determining the density of pinholes in a wafer and their respective sizes, which can be used to determine the size of the pinholes very accurately.

[0009] In order to be able to accurately measure the size of pinholes in a rod piece, the rod piece to be measured is preferably measured according to DE 10 2006 032 431 A1, with the coordinates of any pinholes found being stored simultaneously.

[0010] The area containing the relevant pinhole is then preferably sliced ​​into wafers and analyzed by means of the method described in US 9665931 A1. The size of the pinholes thus found can be determined with a measurement error of a few percent.

[0011] The inventors have discovered that semiconductor materials that contain a relatively high density of pinholes with diameters greater than 50 μm are indeed defective semiconductor materials, and so it is of primary importance to avoid pinholes with diameters greater than 50 μm.

[0012] A series of proposals have already been published on how pinhole formation can be suppressed. Most of these proposals focus on improving the properties of the crucible material. When using suitable crucible materials, the size of the pinholes formed is less than 50 μm.

[0013] It has also been proposed (e.g. EP 2 471 980 A1) to optimize the crucible settings to avoid damaging the crucible and thus avoiding the formation of bubbles in the melt which result in pinholes in the single crystal.

[0014] Other proposals focus on suppressing or eliminating pinholes during melting of the ground and / or granular material, for example US2011 / 304081A1 recommends careful handling of the crucible using an appropriate power distribution of the heater over time to reduce the density of pinholes appearing in the crystal.

[0015] To achieve the desired (defect) properties of the pulled crystal, a suitable distribution of the thermal power with fixed ratios is required, which is in contrast to the method proposed for example in DE 10339792 A1, where it is not possible to achieve both goals (low density of pinholes and desired defect properties).

[0016] In the method for producing a silicon single crystal described in JP5009097A2, the pressure in the crystal pulling system is reduced to 5 to 60 mbar when the polysilicon is melted, and is set to 100 mbar or more when the crystal is pulled.

[0017] In the method for producing a silicon single crystal described in US2011 / 214603A1, the output of the heater during melting is set higher than the output during the subsequent crystal pulling, and the pressure during melting is set to 30 mbar or less, which is lower than the pressure during the subsequent crystal pulling.

[0018] However, the methods described have been found to have drawbacks, in particular the pulled single crystals have been found to have increased impurity values ​​with respect to carbon and iron, in addition the high density of large pinholes that appeared did not meet expectations.

[0019] The inventors have found that with the methods proposed in the prior art it is not possible to produce single crystals according to the Czochralski method that have a very low density of large pinholes and very low iron and carbon contamination and at the same time the desired defect characteristics (free of defects). Summary of the Invention [Problem to be solved by the invention]

[0020] The object of the present invention is to provide a method that makes it possible to produce defect-free crystals with a minimum density of pinholes larger than 50 μm in size and with minimum carbon and iron impurities.

[0021] It is yet another object of the present invention to provide silicon crystals having minimal concentrations of both carbon and iron, which are at the same time defect free and have a minimal density of pinholes greater than 50 μm in size. [Means for solving the problem]

[0022] The above object is achieved by the methods and products set forth in the claims. The features given in relation to the above-described embodiments of the method according to the invention can also be applied mutatis mutandis to the product according to the invention. Conversely, the features given in relation to the above-described embodiments of the product according to the invention can also be applied mutatis mutandis to the method according to the invention. These and other features of the embodiments according to the invention are set out in the drawings and in the claims. The individual features can be realised separately or in combination in the embodiment of the invention. Moreover, they can describe advantageous embodiments which can be protected independently. [Brief description of the drawings]

[0023] [Figure 1] 1 shows the relationship between the flow rate f [l / h] of an inert gas and the applied pressure p [mbar]. A person skilled in the art will understand that the unit l / h (liters per hour) means standard liters per hour, i.e. the volume per hour that a gas has at standard pressure. [Diagram 2] A typical brightness profile measured by the camera during silicon heating is shown in brightness value b versus time, both in relative units. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0024] During heating, the measured brightness initially remains constant within error limits (201). As the solid to liquid phase transition begins, the brightness signal increases sharply (202).

[0025] Once the silicon has completely melted, the measured brightness becomes constant again (203), but at a higher level than at the start (201).

[0026] Melting of polysilicon is understood to mean the process by which polysilicon goes from room temperature in the solid state to a temperature above its melting point in the liquid state. The end of the melting process is defined as the time to place the seed crystal for crystal pulling, which is then started.

[0027] Table 1 summarizes the results of measurements of pinhole, carbon, and iron concentrations in crystals pulled according to the prior art (Comparative Examples 1 and 2) and the present invention (Examples 3, 4, and 5).

[0028] DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS OF THE PRESENT EMBODIMENT Rods with a nominal diameter of either 300 mm or 200 mm were pulled according to the Czochralski method, which involved depositing polycrystalline silicon in a quartz crucible provided for crystal pulling, as known from the prior art.

[0029] The procedure for producing defect-free crystals was used for crystal pulling. This can be achieved by a CUSP magnetic field, a horizontal magnetic field, or a moving magnetic field, and furthermore, the rotation of the crystal and the rotation of the crucible are set appropriately for this purpose.

[0030] The results shown in Table 1 were obtained from crystals pulled using a horizontal magnetic field. In addition, the crystal rotation and crucible rotation were varied to obtain different oxygen concentrations.

[0031] It should be noted that the type of magnetic field used is not important, what is most important is that a central upward melt flow is achieved to pull a defect-free crystal.

[0032] Each single crystal rod thus pulled was further divided into rod pieces using a band saw, and wafers were cut from the rod pieces according to the prior art and examined for pinhole and defect characteristics as well as impurities (carbon, iron).

[0033] The carbon concentration in the silicon was determined using gas fusion analysis, as described, for example, in DE 10 2014 217 514 A1.

[0034] Iron concentrations were measured using ICPMS (Inductively Coupled Plasma Mass Spectrometry) technique, which can also be measured by NAA (neutron activation analysis) with appropriate calibration.

[0035] Example 1 of Table 1 shows the results achievable by conventional means known from the prior art, where an excessively high pinhole density was identified.

[0036] In order to reduce the density of pinholes in the single crystal rod, the inventors first tried to reduce the pressure during polysilicon melting to a level where the gases that may be generated could not cause pinholes, as described in JP5009097A2. In this regard, the inventors found that this method was only suitable to a limited extent as a method for adequately reducing the density. The results are summarized in Example 2 of Table 1.

[0037] It became apparent that this approach resulted in a clear increase in the contamination of the single crystal rods with carbon and iron (Table 1, Example 2).

[0038] Therefore, the inventors have found that further measures must be taken to solve these problems.

[0039] When preparing the crucible, care is taken to use polysilicon, preferably with a very low impurity level, as described in DE 10 2010040293 A1.

[0040] Particularly preferably, the average surface area per unit mass is 2 cm 2 Use less than 1000 g of silicon.

[0041] More particularly preferably, the surface area per unit mass at a distance of more than 2 cm and less than 5 cm from the wall of the crucible is less than 1 cm 2Prepare a crucible using less than 1000 g of polysilicon per unit mass. Fill the remainder of the volume of the crucible with a surface area of ​​1 cm per unit mass. 2 / g or larger than 5cm 2 Prepare using polysilicon with a thickness of less than 1 / g.

[0042] During the melting of the polysilicon, the pressure in the crystal pulling system is preferably set to 10 mbar or less. At the same time, the total flow rate f [l / h] of the purge gas in the pulling system is preferably set to 10 mbar or less. Set p[mbar] to be greater than p[mbar] multiplied by 160.

[0043] FIG. 1 shows a preferred region of pressure p and flow rate f at (102). Particularly preferably, the total flow rate f [l / h] of the purge gas of the pulling system is set to be greater than the pressure p [mbar] multiplied by 400, particularly preferably by 720. At the same time, the pressure is preferably set to less than or equal to 10 mbar.

[0044] FIG. 1 shows at (101) a preferred region of pressure p and flow rate f. In general, it is advantageous to keep the flow rate, f, as high as possible while at the same time keeping the pressure as low as possible. The maximum flow rate at a given pressure then depends only on the pump power.

[0045] The purge gas used during melting includes gases from the following list of gases: argon, helium, nitrogen, or combinations thereof. Preferably, argon with a purity of greater than 99.99% by volume is used.

[0046] Example 3 in Table 1 shows the crystallographic results achieved by the above-described means according to the present invention. In yet another embodiment, the pressure (and therefore the flow rate of the purge gas) was increased when the polysilicon first became liquid, in this regard the pressure increase was 4 mbar, preferably 8 mbar, particularly preferably 12 mbar.

[0047] In this regard, the melting process was observed using a camera which determined the moment when the silicon first became liquid using suitable digital image processing methods.

[0048] The inventors have found that the time point at which the brightness of the evaluated image data can be significantly increased can be correlated very well with the time point at which the transition from the solid phase to the liquid phase begins.

[0049] Figure 2 shows, by way of example, the brightness as a function of time. It has become apparent that in order to obtain a better effect on the density of pinholes and the concentrations of carbon and iron, it is necessary to increase the pressure, preferably at the point between regions (201) and (202).

[0050] Example 4 in Table 1 shows the crystallographic results achieved by the above-described means according to the present invention. In yet another embodiment, polysilicon with a chlorine content of 1 ppba was used for the setup.

[0051] While one skilled in the art would expect that chlorine would release iron from the system at high temperatures and contaminate the silicon, the inventors have surprisingly found that in this case, using polysilicon with a chlorine content greater than 1 pbba has an even better effect on iron contamination.

[0052] Example 5 in Table 1 shows the crystallographic results achieved by the above-described means according to the present invention. The above description of the exemplary embodiments should be understood as illustrative. Therefore, the present disclosure is intended, firstly, to enable a person skilled in the art to understand the present invention and the related advantages, and secondly, to encompass the variations and modifications of the described structures and methods that are obvious to the understanding of a person skilled in the art. Therefore, all such variations and modifications and equivalents are intended to be covered by the scope of protection of the claims.

[0053] [Table 1]

Claims

1. A silicon single crystal, comprising: Oxygen concentration is 2 x 10 １７ at / cm ３ Higher than The density of pinholes with a diameter of 100 μm or more is 1.0×10 －５ 1 / cm ３ is less than Carbon concentration is 5.5 x 10 １４ at / cm ３ is less than Iron concentration is 5.0 x 10 ９ at / cm ３ is less than COP density is 1000 defects / cm ３ is less than The density of interstitial silicon atoms is 1 defect / cm ２ is less than A silicon single crystal having a crystal diameter greater than 200 mm.

2. The density of pinholes with a diameter of more than 50 μm is 1.0×10 －５ 1 / cm ３ The single crystal of claim 1 , wherein the average molecular weight is less than 1.

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