Abstract:
A wafer etching and impurity analysis method is presented where a wafer is held in a vessel having gas introduction and exhaust ports, a solution including a mixture of hydrofluoric acid and nitric acid alone or together with sulfuric acid is bubbled with a carrier gas without being heated, which generates a gas containing vaporized hydrofluoric acid and nitric acid, and the inside of the vessel is purged so that the amount of gas supplied is kept constant at all times. The molar ratio of the hydrofluoric acid and nitric acid (and sulfuric acid if used) in the solution is set to a specific value. The amount of gas introduced is specified. All or a specific portion of the wafer is cooled to a specific temperature between 0 and 20° C. Consequently, the gas is condensed on the surface of the wafer, which allows the required portion of the wafer to be etched and the amount of required in-plane etching to be uniform. The method produces a wafer surface with good smoothness after etching, reduces the amount of recovery liquid needed, and reduces the amount of admixed silicon during impurity analysis, the concentration time, and so forth.

Description:
INCORPORATION BY REFERENCE  
       [0001]    This is a U.S. national phase application under 35 U.S.C. §371 of International Patent Application No. PCT/JP02/10904 filed Oct. 21, 2002, and claims the benefit of Japanese Patent Application No. 2001-325863 filed Oct. 24, 2001. The International Application was published in Japanese on May 1, 2003 as WO 03/036706 A1 under PCT Article 21(2). 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    This invention relates to an etching method with which a recovered solution of decomposition residue after a silicon wafer surface has been etched is analyzed and the metal impurities contained in the silicon wafer are quantitatively analyzed. The invention further relates to an etching method and apparatus and an impurity analysis method. The method produces an amount of in-plane etching that is uniform, and there is little contamination during the concentration of the recovered solution. Therefore, a high-precision and high-sensitivity analysis is possible.  
         BACKGROUND OF THE INVENTION  
         [0003]    As devices have become smaller and the level of integration has risen, it has become increasingly important in the field of semiconductor manufacturing to reduce the amount of metal impurities present in a silicon wafer, as these impurities can markedly diminish the performance of a device.  
           [0004]    Also, since some silicon wafers (p +  wafers) cannot be evaluated from their electrical characteristics, there has been a need for a high-precision and high-sensitivity chemical analysis method. Methods for evaluating the metal impurities in a silicon wafer by chemical analysis include a direct dissolution process, a sandwich process, and an indirect dissolution process.  
           [0005]    Direct dissolution is a method in which a chemical is dropped onto a silicon wafer and the wafer is directly dissolved in the chemical, such as by dropping hydrofluoric acid and nitric acid onto a silicon wafer, etching, and recovering these acids. A sandwich process involves dropping hydrofluoric acid and nitric acid onto a base surface, placing a silicon wafer over this, and then etching and recovering these acids. Indirect dissolution is a method in which hydrofluoric acid and nitric acid are heated, a silicon wafer is etched with the gas generated by this heating, and the subsequent decomposition residue is recovered with a chemical.  
           [0006]    To concentrate the recovered solution, a suitable amount of aqua regia, sulfuric acid, or the like is mixed into the recovered solution, and a hot plate, microwaves, or the like is used for concentration. Analysis is performed by atomic absorption spectrometry (AAS), inductively coupled plasma-mass spectrometry (ICP-MS), or the like.  
           [0007]    However, with direct dissolution, the liquid is dropped over the entire surface of the silicon wafer, so a large amount of chemical must be used, metal impurities present in the chemical itself pose a substantial background to the analysis values, and the metal impurities in the sample cannot be analyzed to a high degree of sensitivity.  
           [0008]    With a sandwich process, a silicon wafer is placed on drops of liquid, after which the wafer is slid aside and the liquid drops are recovered. With this method, very little liquid is dropped (just several hundred microliters), so uniform in-plane etching is difficult, and there is considerable variance in the amount of etching from one silicon wafer to the next.  
           [0009]    With indirect dissolution, a gas of hydrofluoric acid and nitric acid is generated at room temperature vaporization pressure or a vaporization pressure produced by heating, and this vapor is used for dissolution. Some of the problems with this method are that it is difficult to control the amount of gas generated, which affects the amount of etching of the silicon wafer and prevents uniform etching of the surface, and etching unevenness makes it difficult to recover the subsequent decomposition residue with a chemical.  
           [0010]    In particular, an indirect dissolution method that has been proposed as a method for etching a silicon wafer surface layer or a method for analyzing metal impurities is a method in which a silicon wafer is held horizontally in a sealed vessel, nitric acid and hydrofluoric acid are put in separate vessels, each vessel is heated to produce separate gases of nitric acid and hydrofluoric acid, and the silicon wafer is cooled and its surface layer is etched (Japanese Laid-Open Patent Application H 8 -330271). A problem with this method, however, is that adequate etching performance cannot be achieved (approximately 0.1 μm/hr) because the nitric acid and hydrofluoric acid gases are supplied separately.  
           [0011]    In another method that has been proposed, a silicon wafer is held upside-down by a vacuum chuck, nitric acid and hydrofluoric acid are put into the same vessel underneath this wafer and heated, the nitric acid and hydrofluoric acid gas thus generated etches the silicon wafer, and the condensed liquid on the wafer surface is recovered and subjected to atomic absorption spectrometry (Japanese Laid-Open Patent Application H6-213805).  
           [0012]    A problem with this method, however, is that the nitric acid and hydrofluoric acid gas is generated by heating the vessel of the nitric acid and hydrofluoric acid, and it is difficult to control the amount of gas generated, which affects how much the silicon wafer is etched and prevents the surface from being etched uniformly.  
           [0013]    In addition, there is a method in which the condensed liquid is analyzed directly with an atomic absorption spectrometer, but the condensed liquid contains a large HF and silicon component, which poses a substantial background during analysis, the peak shape for the sample becomes abnormal, etc., making quantitative analysis difficult.  
           [0014]    Also, with conventional methods, because the metal impurities in decomposition residue are quantitatively analyzed by atomic absorption spectrometry or inductively coupled plasma-mass spectrometry, a large quantity of silicon is included in the recovered solution, and therefore has to be removed. In order to remove this silicon, it is dissolved in a mixture of aqua regia, sulfuric acid, or the like and concentrated.  
           [0015]    However, since this concentration method involves the use of a large quantity of chemical, there is the danger that the metal impurities contained in the chemical will contaminate the concentrate, and there is also the possibility that long-term concentration will result in contamination from the atmosphere.  
         SUMMARY OF THE INVENTION  
         [0016]    It is an object of the present invention to solve the above-mentioned problems encountered in etching by an indirect dissolution method, and to provide a method for etching a silicon wafer. With this method, the amount of in-plane etching can be uniform, and there is little contamination during the concentration of the recovered solution. Therefore, a high-precision and high-sensitivity analysis can be possible. It is a further object of the present invention to provide an apparatus used in the above-mentioned etching method, as well as an impurity analysis method that utilizes the above-mentioned etching method.  
           [0017]    As a result of various studies aimed at achieving more uniform etching in an indirect dissolution process, the inventors discovered that the gas can be supplied in a constant quantity to control the etching process by holding a silicon wafer horizontally in a vessel (chamber) that has a gas introduction and exhaust system, bubling a solution comprising a mixture of hydrofluoric acid and nitric acid, or of hydrofluoric acid, nitric acid, and sulfuric acid, with a carrier gas such as N 2  without being heated to generate a gas of hydrof luoric acid and nitric acid, and supplying this gas to the vessel to purge its inside.  
           [0018]    Also, the inventors perfected the present invention by adjusting the hydrofluoric acid and nitric acid in an etching solution—which is bubbled with a carrier gas without being heated—to a specific molar ratio, and regulating the amount of gas by the above-mentioned means as the gas is introduced, and cooling all or a specific portion of the silicon wafer to between 0 and 20° C. to condense the gas on its surface. As a result, the required portion of the wafer can be etched, with a uniform amount of the required in-plane etching, so the wafer surface will have good smoothness after etching, the amount of recovery liquid can be reduced, and the amount of admixed silicon during impurity analysis, the concentration time, and so forth can be decreased.  
           [0019]    Specifically, the present invention is directed to a silicon wafer etching method, wherein a carrier gas is bubbled through a mixed solution of hydrofluoric acid and nitric acid or of hydrofluoric acid, nitric acid, and sulfuric acid, the hydrofluoric acid and nitric acid gas thus generated is introduced into a vessel with a gas inlet and outlet, and released to the outside of the vessel, while condensing the gas on the surface of a cooled silicon wafer that is held horizontally inside the vessel, thereby etching the surface. The amount of etching is controlled by controlling the amount of gas introduced and released.  
           [0020]    The invention is further directed to:  
           [0021]    a method in which the mixed solution is composed of hydrofluoric acid and nitric acid, and the nitric acid is used in an amount of 0.351 to 2.805 mol per mole of hydrofluoric acid,  
           [0022]    a method in which the mixed solution is composed of hydrofluoric acid, nitric acid, and sulfuric acid, and the nitric acid is used in an amount of 0.351 to 2.805 mol and the sulfuric acid in an amount of 0.163 to 0.490 mol per mole of hydrofluoric acid,  
           [0023]    a method in which the cooling temperature of the silicon wafer is 0 to 20° C.,  
           [0024]    a method in which the cooled portion of the silicon wafer is the entire wafer, the outer periphery of the wafer, the middle part of the wafer, or a specific part of the wafer, and  
           [0025]    a method in which the amount of etching is 0.02 to 10.0 μm.  
           [0026]    The present invention also proposes an etching apparatus comprising means for generating an etching gas, a vessel having a gas introduction and exhaust system, and cooling and holding means for coolably holding a silicon wafer in the vessel.  
           [0027]    Further, the present invention is directed to a method for analyzing the impurities of a silicon wafer, including an etching step of introducing a gas containing vaporized hydrofluoric acid and nitric acid, generated by bubbling a carrier gas through a mixed solution containing hydrofluoric acid and nitric acid, into a vessel having a system for introducing and exhausting said gas, and condensing the gas on the surface of a cooled silicon wafer held inside said vessel to etch the surface, a recovery step of recovering decomposition residue by scanning the entire surface of the etched wafer with a mixed solution of hydrofluoric acid and aqueous hydrogen peroxide, and an analysis step of concentrating the recovered solution and analyzing the metal impurities in said solution. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0028]    [0028]FIG. 1 is a diagram illustrating the simplified structure of the silicon wafer etching apparatus used in the present invention;  
         [0029]    [0029]FIGS. 2A and 2B are graphs of the relation between silicon wafer etching time and etching amount, measured using a mixed solution of hydrofluoric acid and nitric acid;  
         [0030]    [0030]FIG. 3 is a graph of the relation between silicon wafer etching time and etching amount, measured using a mixed solution of hydrofluoric acid, nitric acid, and sulfuric acid;  
         [0031]    [0031]FIG. 4 is a diagram illustrating the silicon wafer in-plane etching amount distribution, measured by ADE;  
         [0032]    [0032]FIG. 5 is a graph of the silicon wafer etching amount and the in-plane uniformity (CV %) of the etching amount measured by ADE;  
         [0033]    [0033]FIG. 6 is an oblique view of an image produced by AFM measurement of a silicon wafer that has been etched to a depth of 0.2 μm;  
         [0034]    [0034]FIG. 7 is a graph of the silicon wafer etching amount and the surface roughness measured by AFM; and  
         [0035]    [0035]FIG. 8 is a graph of the metal impurity recovery percentage in the 2 μm etching of a silicon wafer that has been quantitatively contaminated (2 ng). 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0036]    The silicon wafer etching method pertaining to the present invention is characterized by the use of a vessel that allows the introduction and exhaust of gas, and is characterized in that a mixed solution of hydrofluoric acid and nitric acid is made into a gas by being bubbled with a carrier gas such as N 2  without being heated, the generated gas of hydrofluoric acid and nitric acid is introduced into the above-mentioned vessel and exhausted outside the vessel, a silicon wafer is held horizontally inside the vessel, the above-mentioned gas is condensed on the required portion of the silicon wafer surface which has been cooled to a specific temperature, thereby etching this surface, and the amount of etching is controlled by controlling the amount of gas introduced and exhausted.  
         [0037]    [0037]FIG. 1 shows an example of the apparatus for etching a silicon wafer pertaining to the present invention. A reaction vessel  1  has a lid  2  at the top, and a cooling apparatus  5  is built into a stage  4  used to hold a silicon wafer  3  horizontally inside the reaction vessel  1 . An etching chemical container  6  is disposed on the outside of the reaction vessel  1 , and a solution  7  comprising a mixture of a hydrofluoric acid solution and a nitric acid solution is contained in the chemical container  6 . N 2  gas (used here as the carrier gas) is blown into the chemical container  6  through a bubbling pipe  8 , the solution gas thus generated enters the reaction vessel  1  through an introduction pipe  9  and an introduction port  1   a , and at the same time this gas is guided out of the reaction vessel  1  by an exhaust pump or the like through an exhaust port  1   b.    
         [0038]    Specifically, with a conventional method, the etching gas was generated by heating the solution used for etching, which was dangerous and also made it difficult to adjust the flow of etching gas, but with the present invention, the gas is generated merely by bubbling the solution  7 , which is obtained by mixing etching-use hydrofluoric acid and nitric acid, with N 2  gas, without heating the solution  7 , and is introduced into the reaction vessel  1  while being exhausted to the outside, so that a constant amount of gas is always supplied, which allows the etching gas to be supplied safely and in a precise constant amount.  
         [0039]    The silicon wafer  3  is cooled by the cooling apparatus  5  on the stage  4 , and the etching gas condenses on the cooled surface of the silicon wafer  3  and etches (decomposes) the silicon wafer  3 . The following reactions occur in this etching.  
         Si+4HNO 3 [[↑]]→SiO 2 +4NO 2 ↑+2H 2 O  (1)  
         SiO 2 +4HF[[↑]]→R SiF 4 ↑+2H 2 O  (2)  
         [0040]    The reactions in (1) and (2) above can be accelerated by exhausting the SiF 4  as the etching gas is supplied.  
         [0041]    With a conventional method, solid diammonium silicon hexafluoride ((NH4)2SiF6) was produced by the following reaction (3), in which the tiny amount of NH3 gas generated from the HNO3 gas reacted with the HF gas (the etching gas) and the SiF 4  gas produced in reaction (2).  
         SiF 4 [[↑]]+2HF[[↑]]+2NH 3 [[↑]]→(NH 4 ) 2 SiF 6   (3)  
         [0042]    With a conventional method, the above-mentioned reaction (3) occurred because the gas produced by the initial reactions remained in the reaction vessel  1 . With the present invention, the reactions (1) and (2) are accelerated by exhausting the SiF 4  while the N 2  gas is introduced and exhausted, which keeps the production of (NH 4 )2SiF 6  by reaction (3) to a minimum, so that substantially all of the decomposition residue following the reaction becomes the metal impurity salt to be analyzed.  
         [0043]    Furthermore, it is possible to increase the etching rate by adding sulfuric acid to the mixed solution, which suppresses the supply of water into the chamber by the dehydrating action of the sulfuric acid, and increases the supply of HF gas and HNO 3  gas into the chamber by the following reaction (4).  
         H 2 SO 4 +4H 2 O→H 2 SO 4 .4(H 2 O)  (4)  
         [0044]    The effect of the above is that the etching gas can be supplied in a precise constant amount, it is possible to control the amount of etching of the silicon wafer by controlling the N 2  gas, and etching within a range of 0.02 to 10.0 μm can be accomplished.  
         [0045]    Also, with the present invention, the amount of silicon remaining in the decomposition residue on the silicon wafer surface can be kept extremely small. As a result, the desired analysis can be performed right away, without having to perform a chemical treatment for sublimating away the silicon during concentration, which was necessary in the conventional analysis of decomposition residue.  
         [0046]    According to the present invention, the smoothness of the etched silicon wafer surface was a CV of 5 to 10% in 69-point measurement of a 20 mm square in-plane portion of an 8-inch wafer as shown in the example (FIG. 5) involving ADE measurement, thus the etching could be preferably performed. The surface roughness is as shown in the example (FIG. 7) involving AFM measurement, and was no more than 10% of the etching amount. Etching of 0.2 μm was excluded because the surface roughness was the same as the inherent surface roughness of the wafer.  
         [0047]    As discussed above, etching as in the present invention results in excellent smoothness of the etched silicon wafer surface, so the mixed solution of hydrofluoric acid and hydrogen peroxide used as the solution of the recovery means can be used in a smaller amount, such as 50 to 150 μL.  
         [0048]    Thus reducing the recovery solution to an amount of 50 to 150 μL also reduces the amount of liquid involved in concentration, and a smaller amount of recovered liquid allows the liquid to be concentrated on a hot plate in only 10 to 20 minutes. The advantage here is that there is less contamination from the chemical liquid and from the atmosphere during concentration.  
         [0049]    In the present invention, if the mixed solution is a mixture of hydrofluoric acid and nitric acid, it is preferable for the nitric acid to be used in an amount of 0.351 to 2.805 mol per mole of hydrofluoric acid. Specifically, the etching reaction will not proceed on the wafer surface if the amount is less than 0.351 mol, but the etching rate will drop sharply if 2.805 mol is exceeded.  
         [0050]    In the present invention, if the mixed solution is a mixture of hydrofluoric acid, nitric acid, and sulfuric acid, it is preferable for the nitric acid to be used in an amount of 0.351 to 2.805 mol, and the sulfuric acid 0.163 to 0.490 mol, per mole of hydrofluoric acid. The relationship between hydrofluoric acid and nitric acid was described above, but if the amount of sulfuric acid is less than 0.163 mol per mole of hydrofluoric acid, the etching rate will be the same as with a mixed solution of hydrofluoric acid and nitric acid, and no increase in this rate can be expected, but exceeding 0.490 mol is also undesirable because there will be more variance in the in-plane etching of the wafer.  
         [0051]    In the present invention, it is preferable for the cooling temperature of the silicon wafer to be 0 to 20° C. Condensation will be uneven and there will be more variance in the in-plane etching of the wafer below 0° C., but exceeding 20° C. is also undesirable because condensation will be difficult and the etching rate will drop.  
         [0052]    In the cooling of the silicon wafer, cooling just the portion to be analyzed, such as just the outer periphery or just the middle part of the wafer, allows the etching liquid to be condensed only where needed, which in turn allows just the portion to be analyzed to be selectively etched.  
         [0053]    The etching method and analysis method pertaining to the present invention are not limited to a CZ wafer or epitaxial wafer, and a SIMOX wafer or an SOI wafer (laminated wafer) can also be analyzed, and these methods are also effective in the analysis of depth profiles of metal impurities.  
       EXAMPLES  
     Example 1  
       [0054]    Using the silicon wafer etching apparatus shown in FIG. 1, the silicon wafer  3 , which had been quantitatively contaminated (2 ng) with a standard solution of Fe—Ni—Cu, was placed on the stage  4  in the reaction vessel  1 , and the stage  4  and the silicon wafer  3  were cooled to 10° C. with the cooling apparatus  5 .  
         [0055]    Meanwhile, the solution  7 , which consisted of 100 mL of a 50% hydrofluoric acid solution and 200 mL of a 68% nitric acid solution, was put in the etching chemical container  6 , N 2  gas was passed through as a carrier gas at a rate of 1 L/min, and exhaust was performed at 100 hPa.  
         [0056]    Various holding times were tested in this etching gas introduction and exhaust state. After this, just N 2  gas was allowed into the reaction vessel  1  for 15 minutes to replace the system with N 2  gas, after which the lid  2  was opened and the silicon wafer  3  was taken out, and the amount of etching was measured.  
         [0057]    [0057]FIGS. 2A and 2B show the relation between the amount of etching and the etching time of three silicon wafers (indicated in the graphs as white circles, white triangles, and black circles; the same applies hereinafter), measured using a mixed solution of hydrofluoric acid and nitric acid. It is clear from FIGS. 2A and 2B that with the present invention, because a specific amount of etching gas is introduced while the reaction vessel  1  was exhausted, the amount of gas introduced can be controlled, there is an accurate, positive correlation between etching time and etching amount, and the amount of etching can be accurately controlled over a range of 0.02 to 10.0 μm.  
         [0058]    [0058]FIG. 3 shows the relation between the amount of etching and the etching time, measured using a mixed solution of hydrofluoric acid, nitric acid, and sulfuric acid. The etching rate here is about four times that with the mixed solution of just hydrofluoric acid and nitric acid, and it is clear that the etching rate is greatly increased.  
       Example 2  
       [0059]    The various silicon wafers before and after the etching performed for various holding times in Example 1 were measured for thickness (ADE measurement), and the in-plane distribution of the amount of etching of the wafer during etching was examined.  
         [0060]    [0060]FIG. 4 shows the wafer in-plane etching amount distribution, and FIG. 5 shows the etching amount and the in-plane uniformity (CV %) of the etching amount. As a result, it can be seen that the wafer in-plane etching uniformity was good (a CV % of no more than 10%) for all wafers.  
         [0061]    The surface roughness after etching was also examined by AFM measurement. FIG. 6 shows an image produced by AFM measurement of a wafer that has been etched by 0.2 μm. FIG. 7 is a graph of the wafer etching amount and the surface roughness. As a result, it can be seen that the surface roughness was good, since the Rms is no more than 10% of the etching amount at all etching amounts.  
         [0062]    In Example 1, etching (2 μm) was performed by holding for 1 hour, after which the entire silicon wafer surface was scanned with 100 μL of a mixed solution of 10% hydrofluoric acid and 25% aqueous hydrogen peroxide, and the decomposition residue produced by the etching reaction was recovered.  
         [0063]    This recovered solution was concentrated for 15 minutes at 220° C. on a hot plate, and dried until solid. No chemicals (hydrofluoric acid, nitric acid, or sulfuric acid) were used during the concentration step.  
         [0064]    1 mL of a 0.2% nitric acid solution was added to the dried and solidified sample, and ICP/MS-ETV analysis was performed, which gave detection values of about 100% the known contamination amounts of the various elements shown in FIG. 8. It is clear from these results that the present invention allows the metal impurity concentration on a wafer surface to be analyzed to a high degree of sensitivity.  
       INDUSTRIAL APPLICABILITY  
       [0065]    With the present invention, the flux of an etching gas introduced into a reaction vessel can be controlled very precisely, which makes it possible to keep the etching amount within a range of 0.02 to 10.0 μm, and a silicon wafer can be etched with excellent in-plane etching uniformity and without any contamination, only a small amount of solution is used to recover the decomposition residue, concentration can be completed in a short time without contamination, there is no effect on ICP-MS analysis, and high-sensitivity and high-precision evaluation is possible.  
         [0066]    Also, with the present invention, in the cooling of the silicon wafer, just the portion to be analyzed can be cooled, so the etching liquid can be condensed just on this cooled portion, and just the portion to be analyzed can be selectively etched, allowing the decomposition residue to be recovered and the impurities analyzed for just this portion.