Abstract:
An apparatus that includes a solution bath of a seasoned solution, the seasoned solution containing a mixture of hydrofluoric acid, nitric acid, and acetic acid; and one or more silicon wafers being suspended in a position above the solution bath, wherein at least a portion of the mixture having been used in thinning the one or more silicon wafers.

Description:
BACKGROUND 
       [0001]    The present invention relates to the field of semiconductor device manufacturing. In particular, it relates to a method of etching silicon wafer, and to apparatus and solution used in or by the method. 
         [0002]    State of art semiconductor device manufacturing technologies include, for example, deposition and etching techniques that are most commonly used to add material to or remove material from certain areas of a functional device structure, or a portion thereof, in a process of forming that device, be that material metallic, semiconductor, dielectric, or insulating material. For example, among the various etching techniques, processes using certain types of chemical solutions are widely used. In particular such processes, known as wet etching process or WETS, may be used in thinning semiconductor wafers in a three-dimensional (3-D) semiconductor device integration process. 
         [0003]    Nevertheless, currently available WETS processes commonly used in thinning semiconductor wafers have individually their own shortfalls. For example, some wet etching processes may employ special chemical solutions including, for example, tetramethylammonium hydroxide (TMAH) solution, potassium hydroxide (KOH) solution, and ethylene diamine and pyrocatechol (EDP) solution but these processes generally have the property of anisotropic etching. In other words, their etch profiles depend on wafer crystallographic orientation i.e. (111), (110), etc., which as a result do not suit for wafer scale silicon removal. 
         [0004]    On the other hand, some other wet etching processes that rely on a mixture solutions of for example HF-HNO3-H2504, although being able to provide isotropic etch with high etch rate, have no doping selectivity and thus cannot provide adequate etch stop mechanism that may be required in order to control the etching process. In the meantime, although there are some other traditional wet etching processes but they generally have very low etch rate. 
       SUMMARY 
       [0005]    Embodiments of the present invention provides an apparatus that includes a solution bath of a seasoned solution, the seasoned solution containing a mixture of hydrofluoric acid, nitric acid, and acetic acid; and one or more silicon wafers being suspended in a position above the solution bath, wherein at least a portion of the mixture having been used in thinning the one or more silicon wafers. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]    The invention will be understood and appreciated more fully from the following detailed description of preferred embodiments, taken in conjunction with the accompanying drawings of which: 
           [0007]      FIG. 1  is a demonstrative illustration of a method of wafer etching and an apparatus used therein having a solution re-circulation mechanism according to an embodiment of the present invention; 
           [0008]      FIG. 2  is a sample experimental chart illustrating rapid etch rate change using solution re-circulation according to an embodiment of the present invention; 
           [0009]      FIGS. 3A-3C  are demonstrative illustrations of cross-sectional views of a wafer subjecting to a wafer thinning process according to an embodiment of the present invention; and 
           [0010]      FIG. 4  is a simplified flow-chart illustration of applying a solution re-circulation mechanism in a wafer etching process, according to an embodiment of the present invention. 
       
    
    
       [0011]    It will be appreciated that for the purpose of simplicity and clarity of illustration, elements in the drawings have not necessarily been drawn to scale. For example, dimensions of some of the elements may be exaggerated relative to other elements for clarity purpose. 
       DETAILED DESCRIPTION 
       [0012]    In the following detailed description, numerous specific details are set forth in order to provide a thorough understanding of various embodiments of the invention. However, it is to be understood that embodiments of the invention may be practiced without these specific details. 
         [0013]    In the interest of not obscuring presentation of essences and/or embodiments of the invention, in the following detailed description, some processing steps and/or operations that are known in the art may have been combined together for presentation and/or for illustration purpose and in some instances may have not been described in detail. In other instances, some processing steps and/or operations that are known in the art may not be described at all. In addition, some well-known device processing techniques may have not been described in detail and, in some instances, may be referred to other published articles, patents, and/or patent applications for reference in order not to obscure description of essences and/or embodiments of the invention. It is to be understood that the following descriptions have rather focused on distinctive features and/or elements of various embodiments of the invention. 
         [0014]      FIG. 1  is a demonstrative illustration of a method of semiconductor wafer etching as well as an apparatus used therein that has a solution re-circulation mechanism according to an embodiment of the present invention. More specifically, the apparatus may include at least a solution bath  110 , at the bottom of which there may be a solution re-circulation port  113  and a drain port  119 . During the use of solution bath  110  in etching/thinning semiconductor wafers, drain port  119  may be closed. Solutions collected by solution bath  110  after being used in the thinning of wafers (thus sometimes being referred to as waste solution or used solution) may be channeled  132  via re-circulation port  113  into a chemical cabinet  115  through an input port  114  of the chemical cabinet  115 . Coming out of an output port  116  of the chemical cabinet  115 , the solutions, possibly with newly added chemicals from chemical cabinet  115 , may be re-applied to the semiconductor wafers, such as wafers  121 ,  122 ,  123 , and  124  that are inside solution bath  110  undergoing the thinning process. In some embodiment, new chemicals are periodically added in order to sustain a stable etch rate on silicon. As an example, the new chemical may simply be a fresh solution of what was originally in bath  110 , which is then mixed with the used solution to maintain stable etch rate. When being added, the newly added fresh solution may be, for example, 10 to 20 vol. % of the total solution in solution bath  110 . 
         [0015]    According to an embodiment, during a wafer thinning process chemical solutions (such as one with hydrofluoric acid, nitric acid, and acetic acid known as HNA solution) may be re-circulated through ports  113 ,  114 ,  116  and chemical cabinet  115  to be re-applied to semiconductor wafers  121 ,  122 ,  123 , and  124  through for example a spray nozzle  111  or any other solution outlet port. More specifically, as being demonstratively illustrated in  FIG. 1 , in an embodiment solution  131  coming out of nozzle  111  may be applied to a spinning platform  112 . Spinning platform  112  may subsequently through its spinning motion spray or distribute solution  131 , referred to herein as seasoned solution  131  after solution re-circulation, onto surrounding semiconductor wafers such as wafers  121 ,  122 ,  123 , and  124  as being illustrated in  FIG. 1 . Other solution spraying or applying mechanism may be used as well. 
         [0016]    In an embodiment, semiconductor wafers  121 ,  122 ,  123 , and  124  may be held to suspend in air or certain regulated or controlled environment such that solutions being applied to them may drip away from the wafers and into underneath solution bath  110  and re-collected by re-circulation port  113 . 
         [0017]    According to an embodiment, it is unexpectedly discovered that seasoned HNA solution  131  may contain a high level of concentration of nitride-oxide, NOx (for example NO or NO2), provided uniquely by the wafer thinning process, which helps etch heavily doped semiconductor wafers and in particular heavily boron (B) doped silicon wafers. For example, after starting an etching process with solution re-circulation mechanism, when it reaches to about 10% of volume in the solution mixture coming from re-circulation, it has been observed that etch rate of heavily doped silicon wafer may reach a steady level of approximate 5 μm/min, with the wafer under thinning having a boron doped level of approximate 1×1019 atoms/cm3. This etch rate is confirmed to be more than 6 times faster than the about 0.8 μm/min etch rate being commonly observed in non-circulation (therefore non-seasoned) HNA solution. 
         [0018]      FIG. 2  is an exemplary sample experimental chart illustrating rapid etch rate increase with an HNA solution bath using solution re-circulation mechanism according to an embodiment of the present invention. In the chart illustrated in  FIG. 2 , x-axis denotes seasoning time of the HNA solution bath, expressed in minute. In other word, x-axis represents the time lapsed when HNA solution starts to be sprayed or applied onto heavily boron doped silicon wafers while in the meantime waste solution (or used solution) is being collected by solution bath  110  through re-circulation port  113  ( FIG. 1 ) and re-applied to the silicon wafers. Y-axis denotes silicon wafer etch rate expressed in micrometer (μm) per minute. It is to be noted that the silicon wafer etch rate is only measured on one side of the wafer since the other side is not conditioned, such as heavily boron doped, to be etched. 
         [0019]    In the chart shown in  FIG. 2 , it is clearly observed that the etch rate of silicon wafer increases dramatically within the initial approximate 5 minutes, starting at around 0.8 μm/min, which is the typical etch rate of silicon wafers in a non-seasoned HNA bath, to around 5 μm/min when the solution bath may be considered as being fully seasoned, that is, having at least 10 vol. % of re-circulated solution of the total solution volume in the solution bath. In other words, after approximately 5 minutes, the solution bath may be conditioned to become having sufficiently high level of NOx that in turn aids the etching of wafers that are heavily doped by p-type dopants such as boron. After approximately 5 minutes the etching rate, in the illustrated chart of experiment, tapers down slightly and eventually settles at around a steady level of about 4.2-4.3 μm/min. This tapering may partially be due to the solution in solution bath reaching equilibrium and is considered to be mainly caused by slight lag in reaching uniform mixture through solution recirculation. In a 3-D semiconductor device integration process, the etching or thinning process trims down the thickness of silicon wafer to a level that is desirable for the integration. 
         [0020]      FIGS. 3A-3C  are demonstrative illustrations of cross-sectional views of a semiconductor wafer being subjected to a wafer thinning process according to an embodiment of the present invention. For example, in a 3-D integration process of manufacturing semiconductor devices, embodiments of present invention may be applied in removing a handler substrate. More specifically as being illustrated demonstratively in  FIG. 3A , during manufacturing, a first device layer  312  may be formed on a first substrate  311 , and a second device layer  314  may be formed on a second substrate  316 . Here, the second substrate  316  may be known as a handler substrate and may be heavily doped with boron (B)  320 . Device layer  314  may be formed on top of second substrate  316  or handler substrate via a lightly doped layer  315 . Lightly doped layer  315  may be, for example, epitaxially grown on top of second substrate  316 . In  FIG. 3A , the second device layer  314  may be illustrated upside-down and be bonded together with the first device layer  312  through a bonding layer  313 , which may be for example activated silicon oxide, silicon nitride, metal oxide hybrid bonding layer, polymeric adhesive materials, etc. 
         [0021]    In an embodiment, the heavily doped handler substrate  316  may be doped with a dopant level of at least 1×1019 atoms/cm3, compared with the lightly doped layer  315  which may typically be doped at between about 1×1015 cm−3 and 1×1016 atoms/cm3 in dopant level. In other words, dopant level in handler substrate  316  may be at least 1000 times higher than that in layer  315 . In a 3-D integration process, handler substrate  316  may be removed after integration. In removing handler substrate  316 , according to an embodiment of present invention, a significant portion of handler substrate  316  may first be removed through a grinding or polishing process, which may rapidly reduce the thickness of handler substrate  316  to close to, for example, 10˜12 μm. With a portion of handler substrate  316  (10˜12 μm) still remaining on top of lightly doped layer  315 , seasoned HNA solution may be applied or sprayed onto substrate  316 , as being illustrated in  FIG. 3B , which etches and removes the remaining portion of substrate  316 . This wet etch process may slow down dramatically to stop at lightly doped layer  315  by virtue of the dopant level in layer  315 . For example, layer  315  may be a p-type dopant (such as boron) doped silicon epitaxial layer with a dopant concentration level around approximately 1×1015 to 1×1016 atoms/cm3, as being illustrated in  FIG. 3C . 
         [0022]    According to an embodiment, seasoned HNA solution may be prepared by first creating a mixture of chemical solution having HF: HNO3: CH3COOH in a ratio of approximate 1:3:5 in weight, although embodiment of present invention is not limited in this aspect and certain variation of the ratio of chemical components are acceptable and within the spirits of present invention. For example, ratio variation of above chemicals may range as follows: HF 1: HNO3 3˜6: CH3COOH 3˜5 with HF being as a reference set at 1. It should be noted that other concentration variations outside the suggested range may be used as well, depending upon what the etch rate is desirable. In an embodiment, it is observed that etching and removing of a 12 μm thick substrate  316  took about 2.5 minutes, which is to be compared with the approximate 15 minutes that would otherwise be needed when a conventional, unseasoned HNA solution is used as the best-known method (BKM) process. For clarification, the 2.5 minutes does not include any additional time for wafer handling and rinsing. 
         [0023]      FIG. 4  is a simplified flow-chart illustration of a method of creating a seasoned solution using re-circulation mechanism and applying the seasoned solution in a wafer thinning process, according to an embodiment of present invention. More specifically, an embodiment of a method of present invention may include steps of first creating a HNA solution bath at step  411 , by mixing hydrofluoric acid, nitric acid, and acetic acid in a pre-determined mix ratio such as a ratio of about 1:3:5, although slightly higher or lower content (within for example 5% relative to others) of each acid are fully contemplated by present embodiment as well. In each of the mixing chemical solutions, water is an integral part and concentration of the chemicals may be, for example, HF 49 wt. %, nitric acid 70 wt. % and acetic acid 98 wt. % respectively. After the creation of the solution additional water may be added depending on the intended application, although not necessarily needed, with the effect of dilution where adding water generally will lower the etch rate. Next at step  412 , the prepared mixture of chemical solution may be applied, such as through a spray-on process, onto heavily doped (such as heavily boron doped) silicon wafers and in particular to the side (or sides) of the silicon wafers that are boron doped for the purpose of etching and/or thinning thereof. Solution coming off these wafers, known as waste solution or used solution, may then be collected at step  413  by using for example a solution bath, and subsequently re-circulated back to be applied to the wafers and re-collected by the solution bath at step  414  to create seasoned bath solution. After a certain number of re-circulation, the solution may become seasoned solution to contain a desired level of NOx (such as NO or NO2), that is discovered to be advantageous to the etching of wafers, and the seasoned solution may be re-used and re-applied to the silicon wafer for further thinning the substrate at step  415 . In the seasoned solution, the level of NOx may be proportional to the concentration of HNO3 and in approximate 1:1-2 molar ratio. Once most of the heavily doped portions of silicon is etched away and underneath lightly doped (less than 1×1015 atoms/cm3) portion of wafer is exposed, the etching rate may significantly slow down to close to zero at which point the wafer thinning process may be considered as accomplished, at step  416 . 
         [0024]    While certain features of the invention have been illustrated and described herein, many modifications, substitutions, changes, and equivalents will now occur to those of ordinary skill in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the spirit of the invention.