Patent Publication Number: US-2003221703-A1

Title: Method of removing germanium contamination on semiconductor substrate

Description:
BACKGROUND OF THE INVENTION  
       [0001] 1. Field of the Invention  
       [0002] The present invention relates to a method of removing Germanium (Ge) contamination. More particularly, the invention relates to a method of removing Ge contamination existing on a semiconductor substrate, where the Ge contamination is usually adhered onto the substrate during a fabrication process sequence of semiconductor devices. This method makes it possible to remove effectively Ge contamination existing on a semiconductor substrate to thereby prevent the performance degradation of semiconductor devices due to the Ge contamination.  
       [0003] 2. Description of the Related Art  
       [0004] Conventionally, various cleaning processes or methods have been developed and used actually to remove metal contamination adhered onto a semiconductor substrate during a fabrication process sequence of semiconductor devices. With the prior-art cleaning methods, a solution of a sulfuric peroxide mixture (SPM) or a hydrochloric peroxide mixture (HPM) is used as a cleaning solution. A “SPM” means a mixture of sulfuric acid (H 2 SO 4 ) and hydrogen peroxide (H 2 O 2 ), which may be expressed as a H 2 SO 4 —H 2 O 2  mixture. A “HPM” means a mixture of hydrochloric acid (HCl) and hydrogen peroxide (H 2 O 2 ), which may be expressed as a HC 1 —H 2 O 2  mixture.  
       [0005]FIG. 1 is a flowchart showing the prior-art methods of removing metal contamination existing on a semiconductor substrate.  
       [0006] As seen from FIG. 1, a semiconductor substrate (e.g., a single-crystal silicon (Si) substrate) on which metal contamination exists is rinsed with a SPM or HPM (step S 111 ). Thus, the metal contamination existing on the substrate is removed and as a result, the substrate is cleaned.  
       [0007] Although not shown in FIG. 1, a cleaning process using an ammonium peroxide mixture (APM) may be carried out prior to the step S 111 . An “APM” means a mixture of ammonium hydroxide (NH 4 OH) and hydrogen peroxide (H 2 O 2 ), which may be expressed as a NH 4 OH—H 2 O 2  mixture. However, this process using a APM is to remove “particles” existing on the substrate, not metal contamination existing thereon. In this process using an APM, there is a possibility that metal contamination to be introduced during the fabrication process sequence, such as aluminum (Al) and iron (Fe), is likely to adhere onto the substrate again.  
       [0008] On the other hand, with the conventional fabrication processes of semiconductor devices, there is the following problem.  
       [0009] In recent years, it has become popular to use SiGe layers in the fabrication processes of semiconductor devices. Usually, a silicon dioxide (SiO 2 ) or silicon nitride (SiN x ) layer is formed on or over a single-crystal Si substrate and then, necessary openings are selectively formed in the SiO 2  or SiN x  layer thus formed, selectively exposing the surface of the Si substrate. Thereafter, SiGe layers are epitaxially and selectively grown on the exposed surface of the substrate in the respective openings of the SiO 2  or SiN x  layer.  
       [0010] In this case, it is said that no SiGe layer is grown on the exposed areas of the SiO 2  or SiN x  layer if the epitaxial growth condition for the SiGe layer is well selected or controlled. Actually, the inventor did not observe SiGe in the form of micro-particles on the said exposed areas with the use of a scanning electron microscope (SEM).  
       [0011] However, by way of research, the inventor found that unwanted Ge atoms existed on the exposed areas of the SiO 2  or SiN x  layer even if the epitaxial growth condition for the SiGe layer was well selected, and that the level of the existing Ge atoms as contamination was in the order of 10 11  to 10 12  atoms/cm 2 . Ge contamination existing at this level seems to affect badly the performance of semiconductor devices fabricated by using the said substrate.  
       [0012] In this specification, the word “contamination (including Ge contamination)” means that contamination (including Ge contamination) exists on a semiconductor substrate at a specific level or higher in such a way as to badly affect the performance of a semiconductor device.  
       [0013] If a Si substrate including Ge contamination is used to fabricate semiconductor devices, the Ge contamination will be transferred to other Si substrates used in the same lot and as a result, the other substrates will be contaminated as well. According to the inventor&#39;s research, it was found that if Ge contamination occurs at a high level on a Si substrate, the junction leakage current will increase and/or the reliability of the gate dielectric will deteriorate due to the Ge contamination, thereby degrading the performance of semiconductor devices. Accordingly, to prevent the performance degradation of semiconductor devices fabricated by using a Ge contaminated Si substrate and to avoid the contamination of other Si substrates due to the Ge-contaminated one, it is essential to remove the Ce contamination.  
       [0014] Recently, SiGe layers have been popularly introduced into the semiconductor device fabrication and therefore, to remove Ge contamination on a semiconductor substrate is now an important problem which cannot be bypassed.  
       SUMMARY OF THE INVENTION  
       [0015] Accordingly, an object of the present invention is to provide a method of removing Ge contamination existing on a semiconductor substrate.  
       [0016] Another object of the present invention is to provide a method of removing Ge contamination existing on a semiconductor substrate that is carried out easily.  
       [0017] The above objects together with others not specifically mentioned will become clear to those skilled in the art from the following description.  
       [0018] A method of removing germanium (Ge) contamination on a semiconductor substrate according to the present invention comprises the steps of:  
       [0019] oxidizing a surface of a semiconductor substrate to convert a germanium contaminant existing on the surface of the substrate to an oxide of germanium; and  
       [0020] removing the oxide of germanium from the surface of the substrate with an aqueous solution containing fluorine ions.  
       [0021] With the method of removing Ge contamination on a semiconductor substrate according to the present invention, a surface of a semiconductor substrate is oxidized to convert a Ge contaminant existing on the surface of the substrate to an oxide of Ge and thereafter, the oxide of Ge is removed from the surface of the substrate with an aqueous solution containing fluorine ions. Since the oxide of Ge is soluble in an aqueous solution containing fluorine ions, the oxide of Ge existing on the surface of the substrate will dissolve into the solution and finally, it will be removed from the substrate. As a result, the Ge contamination existing on the surface of the substrate is removed and thus, possible performance degradation of a semiconductor device to be fabricated with the substrate is prevented.  
       [0022] In the present invention, the word “semiconductor substrate” means not only a semiconductor substrate per se but also a semiconductor substrate having at least one dielectric, conductive, or semiconductive layer formed thereon. This is because Ge contamination is typically caused in a selective growth process of an epitaxial SiGe layer in openings of a dielectric layer formed on a semiconductor substrate, where a Ge contaminant or contaminants exist(s) on the exposed areas of the dielectric layer. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0023] In order that the present invention may be readily carried into effect, it will now be described with reference to the accompanying drawings.  
     [0024]FIG. 1 is a flowchart showing the process steps of prior-art methods of removing metal contamination on a semiconductor substrate.  
     [0025]FIG. 2 is a flowchart showing the process steps of a method of removing Ge contamination on a semiconductor substrate according to an embodiment of the invention.  
     [0026]FIG. 3 is a flowchart showing the process steps of a method of removing Ge contamination on a semiconductor substrate according to another embodiment of the invention.  
     [0027]FIG. 4 is a flowchart showing the process steps of a method of removing. Ge contamination on a semiconductor substrate according to a still another embodiment of the invention.  
     [0028]FIG. 5 is a flowchart showing the process steps of a method of removing Ge contamination on a semiconductor substrate according to a further embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
     [0029] The present invention will be described in detail below while referring to the drawings attached.  
     [0030] According to the inventor&#39;s test, the inventor obtained the following technical results.  
     [0031] (i) A SPM (i.e., H 2 SO 4 —H 2 O 2  mixture) and a HPM (i.e., HCl—H 2 O 2  mixture), which have ever been used for removing metal contamination, scarcely have an effect or action to remove a Ge contaminant adhered onto a semiconductor substrate. This is because a Ge contaminant does not dissolve into a SPM and a HPM.  
     [0032] (ii) If a Ge contaminant on the substrate is in the form of oxide of Ge, the Ge contaminant will dissolve into one of HF-system solutions.  
     [0033] (iii) A Ge contaminant generated just after the epitaxial growth of a SiGe layer is not in the form of oxide of Ge. Therefore, even if the Ge contaminant is cleaned with one of HF-system solutions, the said contaminant is unable to be removed.  
     [0034] Based on the knowledge (i), (ii), and (iii), the inventor created the present invention.  
     [0035] As described above, with the method of removing Ge contamination on a semiconductor substrate according to the present invention, a surface of a semiconductor substrate is oxidized to convert a Ge contaminant existing on the surface of the substrate to an oxide of Ge. This step is termed the oxidation step. Subsequently, the oxide of Ge is removed from the surface of the substrate with an aqueous solution containing fluorine (F) ions. This step is termed the removal step.  
     [0036] In the removal step, the oxide of Ge existing on the surface of the substrate is dissolved in the said aqueous solution. This means that the Ge contaminant existing on the surface of the substrate is removed, and that possible performance degradation of a semiconductor device to be fabricated with the substrate is prevented.  
     [0037] On the other hand, with a known cleaning method of removing “native oxide” existing on a surface of a semiconductor substrate, a cleaning process using a diluted hydrofluoric acid (HF), i.e., DHF, is carried out. This cleaning process is conducted after the well-known RCA cleaning process is carried out. A contamination removal process for removing metal contamination is carried out during the RCA cleaning process, which is prior to the process of forming a gate oxide.  
     [0038] The above-described cleaning process using a DHF exhibits some effect of removing metal contamination. However, if the substrate has exposed areas of Si, this process has no effect to remove copper (Cu) contamination. Instead, if the solution of DHF is contaminated by Cu, the Cu contaminant in the solution is likely to adhere to the exposed areas of the Si substrate. Alternately, if the substrate has a contaminated area by Cu, the Cu contaminant on the substrate will dissolve into the solution of DHF and as a result, the Cu contaminant in the solution is likely to adhere to the exposed areas of the Si substrate. Therefore, a cleaning process for removing metal contamination with a strong cleaning effect is usually carried out before the cleaning process using a DHF. After the metal contamination is removed from the substrate, an etching process for selectively removing an oxide layer formed on the substrate is carried out. If a thin chemical oxide layer, which is likely to be formed in the said metal contamination removing process, causes no problem relating to the semiconductor device performance, the metal contamination removing process may be conducted after the etching process for selectively removing an oxide layer formed on the substrate is carried out.  
     [0039] The method of the present invention is to remove Ge contamination, not native oxide. Therefore, the method of the invention is unlike the prior-art cleaning processes explained here.  
     [0040] In a preferred embodiment of the method of the invention, as shown in FIG. 2, a surface of a semiconductor substrate is oxidized with an APM (NH 4 OH—H 2 O 2 ), to convert a Ge contaminant existing on the surface of the substrate to an oxide of Ge (step S 1 ). Subsequently, to remove the oxide of the Ge contaminant, the surface of the substrate is contacted with a DHF as an aqueous solution containing fluorine (F) ions (step S 2 ). Thus, the oxide (i.e., the Ge contaminant) is removed.  
     [0041] In this embodiment of the invention, a process for removing “metal” contamination may be additionally carried out after the removal step (step S 2 ) of contacting the surface of the substrate with a DHF. In the process for removing metal contamination, a HPM (HCl—H 2 O 2 ) or SPM (H 2 SO 4 —H 2 O 2 ) is preferably used as the cleaning solution.  
     [0042] Moreover, it is possible to conduct a process for removing “metal” contamination between the oxidation step of oxidizing the Ge contamination and the removal step of removing the oxide of Ge. In this case, the effect of the invention to remove the Ge contaminant is not lost. However, taking the danger of metal contamination (e.g., Cu contamination) occurring after the removal step using a DHF into consideration, it is preferred that the process (step S 33 ) for removing “metal” contamination is carried out after the removal step (step S 32 ) using an aqueous solution containing F ions (e.g., DHF), as shown in FIG. 5. The step S 31  in FIG. 5 is to oxidize the Ge contaminant on the surface of the substrate.  
     [0043] Furthermore, a fluorine peroxide mixture (FPM) may be used instead of DHF. A “FPM” means a mixture of hydrofluoric acid (HF) and hydrogen peroxide (H 2 O 2 ), which may be expressed as a HF—H 2 O 2  mixture. A mixture of DHF and HCl or HNO 3  may be used instead of DHF. In these two cases, there is an additional advantage that the danger of Cu contamination can be eliminated.  
     [0044] In a preferred embodiment of the method of the invention, at least one selected from the group consisting of DHF, FPM, and BHF is used as the aqueous solution containing F ions. In this embodiment, there is an additional advantage that the oxide of the Ge contamination is soluble more easily.  
     [0045] In a preferred embodiment of the method of the invention, at least one selected from the group consisting of APM, HPM, SPM, and aqua regia is used in the oxidation step. In this embodiment, there is an additional advantage that the Ge contamination is oxidized more easily.  
     [0046] In a preferred embodiment of the method of the invention, an oxygen (O 2 ) plasma is used for oxidizing the Ge contamination existing on the surface of the substrate (step S 11 ), as shown in FIG. 3. The step S 12  in FIG. 3 is the same as the step S 2  in FIG. 2. In this embodiment, there is an additional advantage that the Ge contamination is oxidized more easily.  
     [0047] In a preferred embodiment of the method of the invention, an oxygen (O 2 ) plasma is used for oxidizing the Ge contamination existing on the surface of the substrate (step S 21 ) and thereafter, at least one selected from the group consisting of APM, HPM, SPM, and aqua regia is used for further oxidizing the Ge contamination (step S 22 ), as shown in FIG. 4. In this embodiment, there is an additional advantage that the Ge contamination is oxidized while removing the metal contamination and particles induced by the O 2  plasma.  
     EXAMPLES  
     [0048] Concrete examples of the present invention are explained in detail below.  
     Example 1  
     [0049] A silicon nitride (SiN x ) layer was formed on a single-crystal Si substrate and then, necessary openings were selectively formed in the SiN x  layer thus formed. A SiGe layer was epitaxially and selectively grown in the openings of the SiN x  layer.  
     [0050] Thereafter, the Si substrate with the SiGe layer in the SiN x  layer was immersed into an APM solution. This process was carried out to oxidize a Ge contamination existing on the surface of the SiN x  layer, generating an oxide of Ge. At this time, the composition of the APM solution used was adjusted to NH 4 OH:H 2 O 2 :H 2 O=1:4:20. The temperature of the APM solution was set at 65° C. while the immersion period was set at about 10 minutes.  
     [0051] Subsequently, the substrate thus oxidized was rinsed with a diluted HF (DHF) solution, thereby dissolving the oxide of Ge in the DHF solution. At this time, the composition of the DHF solution used was adjusted to HF:H 2 O=1:100. The temperature of the APM solution was set at room temperature while the immersion period was set at about 10 minutes.  
     [0052] Before the above-described cleaning processes were conducted, the density of a Ge contamination existing on the surface of the SiN x  layer was approximately 2×10 12  atoms/cm 2 . By way of the above-described cleaning processes, the density of the Ge contamination was decreased to approximately 5×10 9  atoms/cm 2 . Thus, it was confirmed that the above-described cleaning processes exhibited a conspicuous cleaning effect.  
     [0053] On the other hand, when only the immersion process into the APM solution was carried out, the density of the Ge contamination was approximately 6×10 11  atoms/cm 2 . When only the immersion process into the DHF solution was carried out, the density of the Ge contamination was approximately 1×10 12  atoms/cm 2 . This means that almost no cleaning effect was observed in these two cases. As a result, it was seen that the Ge contamination existing on the surface of the SiN x  layer was scarcely removed, even if only one of the immersion process into the APM solution and that into the HF solution was carried out. It was seen that a sufficient cleaning effect to remove the Ge contamination existing on the surface of the SiN x  layer was obtainable by the combination of these two processes in this order.  
     [0054] In the above-described immersion process into the APM solution for oxidizing the Ge contamination, the composition (i.e., the ratio of NH 4 OH:H 2 O 2 :H 2 O) of the APM solution, and the temperature of the said solution, and the immersion period were not limited to the above-described values, respectively. These values may be changed or adjusted in such a way that a desired oxidizing operation of the Ge contamination was obtainable.  
     [0055] Similarly, in the above-described immersion process into the DHF solution for dissolving the oxide of the Ge contamination, the composition (i.e., the ratio of HF:H 2 O) of the DHF solution, and the temperature of the said solution, and the immersion period were not limited to the above-described values, respectively. These values may be changed or adjusted in such a way that a desired dissolution operation of the oxide of the Ge contamination was obtainable.  
     [0056] Moreover, instead of the immersion process into the DHF solution, any other aqueous solution such as a FPM solution, a BHF solution, a HF solution containing a surfactant or surface-active agent, may be used if it is capable or dissolution of an oxide of Ge contamination.  
     Example 2  
     [0057] A SiN x  layer was formed on a single-crystal Si substrate and then necessary openings were selectively formed in the SiN x  layer thus formed. A SiGe layer was epitaxially and selectively grown in the openings of the SiN x  layer.  
     [0058] On the other hand, oxygen (O 2 ) gas was passed through a plasma region formed in a plasma chamber. The plasma region was surrounded by an RF (radio frequency) coil for RF plasma excitation. In the plasma region, O 2  gas was excited to ionize the O 2  molecules, resulting in an O 2  plasma in the chamber.  
     [0059] Thereafter, the Si substrate with the SiGe layer in the. SiN x  layer was placed in the plasma chamber to contact the surface of the SiN x  layer at room temperature. This process was carried out to oxidize a Ge contamination existing on the surface of the SiN x  layer, generating an oxide of Ge.  
     [0060] Subsequently, the substrate thus oxidized was rinsed with a DHF solution, thereby dissolving the oxide of Ge in the DHF solution. The composition of the DHF solution used was adjusted to HF:H 2 O=1:100. The temperature of the DHF solution was set at room temperature while the immersion period was set at about 10 minutes. These values were the same as those used in Example 1.  
     [0061] Before the above-described cleaning processes were conducted, the density of a Ge contamination existing on the surface of the SiN x  layer was approximately 2×10 12  atoms/cm 2 . By way of the above-described cleaning processes, the density of the Ge contamination was decreased to the level of 1×10 9  atoms/cm 2  or lower. Thus, it was confirmed that the above-described cleaning processes exhibited a conspicuous cleaning effect.  
     [0062] In the above-described immersion process into the OHF solution for dissolving the oxide of the Ge contamination, the composition (i.e., the ratio of HF:H 2 O) of the DHF solution, and the temperature of the said solution, and the immersion period were not limited to the above-described values, respectively. These values may be changed or adjusted in such a way that a desired dissolution operation of the oxide of the Ge contamination was obtainable.  
     [0063] Moreover, instead of the immersion process into the DHF solution, any other aqueous solution such as a FPM solution, a BHF solution, a HF solution containing a surfactant or surface-active agent, may be used if it is capable of dissolution of an oxide of Ge contamination.  
     [0064] A cleaning process may be additionally carried out between the contact process with a O 2  plasma and the immersion process into a DHF solution. This is to remove contamination such as metal contamination and/or particles, to be adhered onto the exposed areas of the Si substrate in the plasma chamber. In this case, it was found that the cleaning effect to remove the Ge contamination existing on the surface of the SiN x  layer was not lost.  
     Other Examples  
     [0065] It is needless to say that the invention is not limited to the above-described embodiments and examples. Any modification is applicable to them.  
     [0066] While the preferred forms of the present invention have been described, it is to be understood that modifications will be apparent to those skilled in the art without departing from the spirit of the invention. The scope of the present invention, therefore, is to be determined solely by the following claims.