Abstract:
A method of forming a high-quality relaxed SiGe alloy layer on a bulk Si-containing substrate is provided. The method of the present invention includes growing a strained SiGe alloy layer on a Si-containing substrate that has a porous Si-containing layer at or near the surface of the Si-containing substrate. The porous layer is formed by an electrolytic anodization process. The pores create free volume below the strained SiGe layer which can serve to accommodate strain relaxation during SiGe deposition or a subsequent heating step. The subsequent heating step is optional and is performed to further increase the relaxation of the SiGe alloy layer. The buried porous structure allows for a unique relaxation mechanism compared to prior art methods.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to a method of fabricating a semiconductor structure, and more particularly to a method of forming a semiconductor structure in which a high-quality relaxed silicon germanium (SiGe) alloy layer is formed atop a bulk Si-containing substrate.  
       BACKGROUND OF THE INVENTION  
       [0002]     Charge carriers in tensile strained Si layers have a higher mobility compared to carriers in unstrained Si layers. This has resulted in an effort to produce thin strained Si layers for use in future high-performance complementary metal oxide semiconductor (CMOS) devices. The increased mobility of the charge carriers in these materials translates into higher current drive and thus higher operating frequency transistors.  
         [0003]     In most prior art methods, a thin Si layer under tensile strain is formed by growing the Si layer on a relaxed SiGe alloy layer. The ongoing challenge in the development of strained-Si substrates is to achieve high strain relaxation of the SiGe layer while simultaneously minimizing the dislocation defect density. Because the dislocations are ultimately responsible for the strain relaxation of the SiGe layer, most prior art methods have focused on burying the dislocations below the surface once the relaxation is sufficiently high. See, for example, E. A. Fitzgerald, Y. H. Xie, M. L. Green, D. Brasen, A. R. Kortan, J. Michel, Y. J. Mii and B. E. Weir,  Appl. Phys. Lett.,  59, (1991) 811.  
         [0004]     Other prior art methods have addressed the defect issue by growing metastable, defect-free, SiGe layers followed by the creation of a buried damage layer near the SiGe/Si substrate interface and subsequent thermal treatment to relax the SiGe. See, for example, H. Trinkaus, B. Hollander, St. Rongen, S. Mantl, H.-J. Herzog, J. Kuchenbecker and T. Hackbarth,  Appl. Phys. Lett.,  76, (2000) 3552. In this manner, the buried defects act as dislocation nucleation sources and a large number of the dislocation loops remain pinned at the buried interface thereby reducing the number of dislocations that extend to the surface.  
         [0005]     It is possible to create highly relaxed SiGe layers of reasonable crystal quality using either of the two aforementioned approaches. The problem with both prior art methods is twofold 1) the amount of relaxation is directly related to how thick the SiGe layers are and 2) the large processing cost to produce these wafers (by either method) creates a large barrier to industrial acceptance. Items 1) and 2) mentioned above are somewhat related in that if high relaxation is desired (and usually is), much thicker SiGe layers must be grown; adding to the cost of the substrate.  
         [0006]     In view of the above drawbacks with the prior art methods, a new and improved method of forming a highly-relaxed SiGe layer, with a high crystalline quality, in an inexpensive and manufacturable manner, is needed.  
       SUMMARY OF THE INVENTION  
       [0007]     The present invention provides a method for fabricating highly relaxed, low defect, single crystalline SiGe alloy layers. The term “highly relaxed” is used throughout the present application to denote a SiGe alloy layer that has a measured degree of relaxation of about 50% or higher. The term “low defect” is used throughout the present application to denote a SiGe alloy layer that has a defect density of about 10 7  cm −2  or less. The low defect density of the resultant SiGe alloy layer provides a high quality film for forming a strained Si-containing layer thereon.  
         [0008]     In accordance with the method of the present invention, a strained SiGe alloy layer is grown on a Si-containing substrate that has a porous Si-containing layer at or near the surface of the Si-containing substrate. The pores create free volume below the strained SiGe layer which can assist strain relaxation during growth and also during a subsequent heating step. The buried porous structure allows for a unique relaxation mechanism compared to prior art methods.  
         [0009]     In broad terms, the method of the present invention comprises the steps of: 
        forming a porous Si-containing layer at, or near, a surface of a Si-containing substrate; and     growing a relaxed SiGe alloy layer on top of said surface of said Si-containing substrate containing said porous Si-containing layer.        
 
         [0012]     Optionally, a step of thermally treating the Si-containing substrate containing the porous Si-containing layer and the SiGe alloy layer to increase relaxation of the SiGe alloy layer can be performed.  
         [0013]     An optional Si-containing layer or a SiGe/Si layer can be grown on top of the relaxed SiGe alloy layer. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]      FIGS. 1A-1E  are pictorial representations (through cross sectional views) illustrating the basic processing steps that can be employed in the present invention.  
         [0015]      FIG. 2  is a SEM of a structure which includes a substantially relaxed SiGe alloy layer formed atop a bulk Si-containing substrate utilizing the method of the present invention. A 5 minute hydrogen bake step at 1100° C. was performed prior to SiGe alloy layer deposition to form a continuous Si-containing layer over the porous structure. Relaxation of the SiGe layer was performed in a subsequent heating step in an inert ambient.  
         [0016]      FIG. 3  is a XTEM of a structure which includes a substantially relaxed SiGe alloy layer formed atop a bulk Si-containing substrate utilizing the method of the present invention. A 1 minute hydrogen bake step at 1100° C. was performed prior to SiGe alloy layer deposition to form a continuous Si-containing layer over the porous structure. Relaxation of the SiGe layer was performed in a subsequent heating step in an oxidizing ambient. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0017]     The present invention, which provides a method of forming a high-quality relaxed SiGe alloy layer on a bulk Si-containing substrate, will now be described in greater detail by referring to the drawings that accompany the present application.  
         [0018]     Reference is first made to  FIGS. 1A-1E  which illustrate the basic processing steps that are employed in the present invention.  FIG. 1A  shows a structure during the initial stage of the present invention in which a porous Si-containing layer  12  is formed at, or near, a surface layer of a bulk Si-containing substrate  10 .  
         [0019]     The term “Si-containing” is used throughout the present application to denote a semiconductor material that includes at least silicon. Illustrative examples of such Si-containing materials include, but are not limited to: Si, SiGe, SiC, SiGeC, Si/Si, Si/SiC, and Si/SiGeC. The bulk Si-containing substrate  10  used in the present invention may be undoped or it may be an electron-rich or hole-rich Si-containing substrate. The bulk Si-containing substrate  10  can have any crystallographic orientation including, for example, &lt;100&gt;, &lt;110&gt; or &lt;111&gt;.  
         [0020]     The porous Si-containing layer  12  is formed within a surface region of the Si-containing substrate  10  by utilizing an electrolytic anodization process that is capable of forming a porous region within the substrate.  
         [0021]     The anodization process is performed by immersing the Si-containing substrate  10  into an HF-containing solution while an electrical bias is applied to the substrate with respect to an electrode also placed in the HF-containing solution. In such a process, the substrate typically serves as the positive electrode of the electrochemical cell, while another semiconducting material such as Si, or a metal is employed as the negative electrode.  
         [0022]     In general, the HF anodization converts single crystal Si into porous Si. The rate of formation and the nature of the porous Si so-formed (porosity and microstructure) is determined by both the material properties, i.e., doping type and concentration, as well as the reaction conditions of the anodization process itself (current density, bias, illumination and additives in the HF-containing solution).  
         [0023]     Generally, the porous Si-containing layer  12  formed in the present invention has a porosity of about 0.1% or higher. The depth of the porous Si-containing layer  12 , as measured from the uppermost surface of the substrate to the uppermost surface of the porous Si-containing layer  12 , is about 50 nm or less. Hence, within the recited ranges the porous Si-containing layer  12  is formed at, or near, the surface of Si-containing substrate  10 .  
         [0024]     The term “HF-containing solution” includes concentrated HF (49%), a mixture of HF and water, a mixture of HF and a monohydric alcohol such as methanol, ethanol, propanol, etc, or HF mixed with at least one surfactant. The surfactant includes conventional materials well-known in the art. The amount of surfactant that is present in the HF solution is typically from about 1 to about 50%, based on 49% HF.  
         [0025]     The anodization process is performed using a current source that operates at a current density from about 0.05 to about 50 milliamps/cm 2 . A light source may be optionally used to illuminate the sample. More preferably, the anodization process of the present invention is employed using a constant current source operating at a current density from about 0.1 to about 50 milliamps/cm 2 .  
         [0026]     The anodization process is typically performed at room temperature or, alternatively a temperature that is below room temperature may be used. Following the anodization process, the structure is typically rinsed with deionized water and dried.  
         [0027]     After providing the structure shown in  FIG. 1A , a strained SiGe alloy layer  14  is formed atop the structure. The resultant structure including the SiGe alloy layer  14  atop the Si-containing substrate  10  which includes a surface porous Si-containing layer  12  is show, for example, in  FIG. 1B .  
         [0028]     In some embodiments of the present, the structure is subjected to a hydrogen baking step prior to the formation of the SiGe alloy layer. When the hydrogen baking step is employed, the structure is typically heated in a hydrogen-containing ambient at a temperature from about 800° to about 1200° C., with a temperature from about 900° to about 1150° C. being more typical. The hydrogen baking step is performed on the structure for a time period that is about 1 min or greater.  
         [0029]     The term “SiGe alloy layer” denotes a material that includes up to 99.99 atomic percent Ge. Typically, the Ge content in the SiGe alloy is from about 0.1 to about 99.9 atomic percent, with a Ge atomic percent of from about 10 to about 35 atomic percent being even more typical.  
         [0030]     In accordance with the present invention, the SiGe alloy layer  14  is formed atop the upper surface of Si-containing substrate  10  containing the porous Si-containing layer  12  using any conventional epitaxial growth method that is known to those skilled in the art which is capable of growing a strained SiGe layer. It is noted that immediately after growth, the SiGe alloy layer  14  will have some degree of relaxation associated therewith.  
         [0031]     Illustrative examples of such epitaxial growing processes that can be used in the present invention include, but are not limited to: low-pressure chemical vapor deposition (LPCVD), rapid thermal chemical vapor deposition (RTCVD), low-energy plasma deposition (LEPD), ultra-high vacuum chemical vapor deposition (UHVCVD), atmospheric pressure chemical vapor deposition (APCVD), molecular beam epitaxy (MBE) and plasma-enhanced chemical vapor deposition (PECVD).  
         [0032]     The thickness of the SiGe alloy layer  14  formed at this point of the present invention may vary, but typically SiGe alloy layer  14  has a thickness from about 10 to about 500 nm, with a thickness from about 20 to about 200 nm being more highly preferred.  
         [0033]     In an optional embodiment, as shown in  FIG. 1C , an optional cap layer  16  is formed atop the structure including the SiGe alloy layer  14 . The optional cap layer  16  employed in the present invention comprises any Si-containing material including, for example, epitaxial Si (epi-Si), amorphous Si (a: Si), single or polycrystalline Si or any combination thereof. Of the various Si-containing materials listed above, it is preferred that epi-Si be employed as the optional cap layer  16 .  
         [0034]     When present, the optional cap layer  16  has a thickness from about 1 to about 100 nm, with a thickness from about 1 to about 30 nm being more highly preferred. The optional cap layer  16  is formed using known deposition processes including one of the epitaxial growth processes mentioned above.  
         [0035]     The structure including the SiGe alloy  14 , with or without the optional cap layer  16 , (see,  FIG. 1B  or  FIG. 1C ) is then heated, i.e., annealed, at a temperature which permits the further relaxation of the SiGe alloy layer  14  into a substantially relaxed SiGe alloy layer  20 . The heating step is optional, thus it does not need to be performed in all instances. A two-fold or greater improvement in relaxation can be obtained in some preferred embodiments of the present invention when the optional heating step is performed. Although the heating step is optional, it is advantageous to employ the same to increase the relaxation of the SiGe alloy layer. The resultant structure is shown, for example, in  FIG. 1D . That is, the heating step forms a further relaxed single crystal SiGe layer  20  atop the bulk substrate. Note that a surface oxide layer (not shown) can be formed atop layer  20  during the heating step when oxygen-containing ambients are employed. This surface oxide layer is typically, but not always, removed from the structure after the heating step using a conventional wet etch process wherein a chemical etchant such as HF that has a high selectivity for removing oxide as compared to SiGe is employed.  
         [0036]     Specifically, the heating step of the present invention is an annealing step which is performed at a temperature from about 500° to about 1350° C., with a temperature from about 900° C. to at or near the alloy melting point temperature being more highly preferred. Moreover, the heating step of the present invention is carried out in an inert ambient such as He, Ar, N 2 , Xe, Kr, Ne or mixtures thereof, an oxidizing ambient which includes at least one oxygen-containing gas such as O 2 , NO, N 2 O, ozone, air and other like oxygen-containing gases, and mixtures of oxygen-containing gases and inert gases. When such an admixture is employed, the diluted ambient contains from about 0.5 to about 100% of oxygen-containing gas, the remainder, up to 100%, being inert gas.  
         [0037]     The heating step may be carried out for a variable period of time that typically ranges from about 10 to about 1800 minutes, with a time period from about 60 to about 600 minutes being more highly preferred. The heating step may be carried out at a single targeted temperature, or various ramp and soak cycles using various ramp rates and soak times can be employed. The use of rapid thermal annealing (RTA), laser annealing and other energy sources such as electron beams are also contemplated herein as possible alternatives to perform the said heating step.  
         [0038]     In accordance with the present invention, substantially relaxed SiGe layer  20  has a thickness of about 2000 nm or less, with a thickness from about 10 to about 100 nm being more highly preferred. Note that the substantially relaxed SiGe layer  20  formed in the present invention is thinner than prior art SiGe buffer layers and has a defect density of threading dislocations, of less than about 10 7  defects/cm 2 . The substantially relaxed SiGe layer  20  formed in the present invention has a final Ge content from about 0.1 to about 99.9 atomic percent, with an atomic percent of Ge from about 10 to about 35 being more highly preferred. Another characteristic feature of the substantially relaxed SiGe layer  20  is that it has a measured lattice relaxation from about 1 to about 100%, with a measured lattice relaxation from about 50 to about 100% being more highly preferred.  
         [0039]     At this point of the present invention, a Si-containing layer or a SiGe/Si layer can be formed atop the substantially relaxed SiGe alloy layer  20 .  FIG. 1E  shows a structure in which the optional Si-containing layer or SiGe/Si layer is formed atop layer  20 . In the drawing, reference  22  is used to denote the optional layer. Optional layer  22  may be formed utilizing a conventional epitaxial growing method(s) that is well known to those skilled in the art. The optional layer  22  typically has a thickness from about 1 to about 50 nm, with a thickness from about 5 to about 30 nm being more typical.  
         [0040]     In some embodiments, the above processing steps of anodization, formation of a SiGe alloy layer and thermal annealing to relax the SiGe alloy layer may be repeated any number of times.  
         [0041]     It is also contemplated herein that growth of the SiGe alloy layer be performed using Ge and/or Si sources that are isotopically enriched in order to improve the thermal conductivity of the SiGe alloy layer. For example, growing the SiGe alloy layer wherein greater than 37% of the Ge atoms have a mass of 74 amu (atomic mass unit) and greater than 93% of the Si atoms have a mass of 28 amu, is isotopically enriched and will have a higher thermal conductivity than the same alloy with an atomic mass distribution corresponding to the natural abundance. It is preferred that the abovementioned percentages are as close to 100% as possible.  
         [0042]      FIG. 2  is a SEM of a structure that includes a substantially relaxed SiGe alloy layer formed atop a bulk Si-containing substrate utilizing the method of the present invention. A hydrogen bake step for 5 minutes at 1100° C. was performed to create a continuous Si-containing layer over the porous structure and deposition of a 100 nm thick Si 0.8 Ge 0.2  layer was performed thereafter at 650° C. in the same RTCVD chamber. Immediately after growth, the SiGe layer was already about 40% relaxed. A subsequent heating step at 1000° C. in an inert ambient was performed to increase the relaxation to about 85%.  
         [0043]      FIG. 3  is a XTEM of a structure which includes a substantially relaxed SiGe alloy layer formed atop a bulk Si-containing substrate utilizing the method of the present invention. A hydrogen bake step for 1 minute at 1100° C. was performed to create a continuous Si-containing layer over the porous structure and deposition of a 150 nm thick Si 0.8 Ge 0.2  layer was performed thereafter at 650° C. in the same RTCVD chamber. A heating step was then performed in an oxidizing ambient at 1200° C. for 30 minutes. In this XTEM, the grainy region is the porous Si-containing layer  12 , the layer adjacent to the porous Si-containing layer is the highly relaxed, high-quality SiGe alloy layer, and the bright layer adjacent to the relaxed SiGe alloy layer is the surface oxide resulting from the oxidation step.  
         [0044]     In the embodiment described and illustrated above, an unpatterned structure is formed. In another embodiment, a patterned structure is formed by first providing a patterned mask or photoresist (not shown) atop the Si-containing substrate  10 . This providing step occurs prior to formation of the porous region. The patterned mask can be formed by deposition, lithography and optionally etching. Ion implantation of the masked wafer to introduce p-type dopants (e.g., boron) will result in the selective formation of porous regions where implantation occurred during subsequent HF anodization. A patterned porous region is then formed in the portion of the Si-containing substrate that did not include the patterned mask. The various steps of porous layer formation, SiGe alloy formation and thermal treatment include all features and embodiments already described herein above. When patterning is employed, the final structure would like similar to  FIG. 1D , but for the exception that layer  14  and layer  20  do not extend entirely across the surface of the structure.  
         [0045]     In the embodiment described and illustrated above, the Si-containing substrate  10  is typically a Si substrate. In another embodiment, the Si-containing substrate  10  is an existing SiGe (pseudomorphic or partially relaxed) layer atop a Si substrate. Boron, or another p-type dopant can be incorporated into the existing SiGe layer either during growth of the SiGe layer or by a post-growth ion implantation process. The same steps are described above can be carried out to form a porous region which includes a portion or the entirety of the SiGe alloy surface layer of the Si-containing substrate.  
         [0046]     While the present invention has been particularly shown and described with respect to preferred embodiments thereof, it will be understood by those skilled in the art that the foregoing and other changes in forms and details may be made without departing from the scope and spirit of the present invention. It is therefore intended that the present invention not be limited to the exact forms and details described and illustrated, but fall within the scope of the appended claims.