Abstract:
The invention relates to a method of eliminating fragments of material present on the exposed surface of a first wafer bonded to a second wafer, the method including a step consisting of placing the first wafer in a liquid solution and propagating ultrasonic waves in the solution. The invention also relates to a process for manufacturing a multilayer structure comprising the following successive steps: bonding of a first wafer to a second wafer so as to form a multilayer structure; annealing of the structure; and thinning of the first wafer, including at least one step of chemically etching the first wafer. The process further includes, after the chemical etching step, the elimination of fragments of material present on the exposed surface of the thinned first wafer.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This is a national phase entry under 35 U.S.C. §371 of International Patent Application PCT/FR2011/050238, filed Feb. 7, 2011, published in English as International Patent Publication WO 2011/104461 A2 on Sep. 1, 2011, which claims the benefit under Article 8 of the Patent Cooperation Treaty to French Patent Application Serial No. 1051367, filed Feb. 26, 2010, the disclosure of which is hereby incorporated herein by this reference in its entirety. 
     
    
     TECHNICAL FIELD 
       [0002]    The present invention relates to the field of production of multilayer semiconductor structures or wafers produced by transferring at least one layer onto a final substrate. Such layer transfer is obtained by bonding, for example, direct bonding, a first wafer (or initial substrate) to a second wafer (or final substrate), the first wafer in general being thinned after bonding. The transferred layer may furthermore comprise all or part of a component or a number of microcomponents. 
         [0003]    More precisely, the present invention relates to the problem of material fragments, which appear on the exposed surface of the transferred layer during fabrication of a multilayer structure formed by bonding. This contamination effect has in particular been observed following a chemical etching step carried out on the first wafer of a multilayer structure, for example, during thinning of this first wafer, and particularly when it has not been possible to completely stabilize the bonding interface. 
       BACKGROUND 
       [0004]    This type of contamination has especially been observed during the fabrication of SOS (silicon-on-sapphire (Al 2 O 3 )) heterogeneous multilayer structures. 
         [0005]    The technique commonly used during the fabrication of multilayer structures to clean the surface of a transferred layer after a chemical thinning step consists in a step of rinsing (or cleaning) by means of a pressurized jet. In general, a pressurized jet of water (or any rinsing solution) is manually applied to the surface of the wafer to be cleaned, this technique sometimes being called “shower cleaning.” 
         [0006]    However, the Applicant has observed that the effectiveness of this technique is limited since only a small fraction of the fragments present on the surface of the wafer to be cleaned can be removed thereby. Moreover, it is not currently possible to automate satisfactorily a rinsing step employing a pressurized jet. This technique requires human intervention, which limits industrialization of the rinsing step. 
         [0007]    Moreover, ultrasonic baths are commonly used at the present time to clean wafers during a polishing step. Document EP 1 662 560, for example, describes a process for treating an SOI wafer comprising, on its front side, a peripheral lip bordering a circular recess. This process especially comprises removing this lip by polishing, by means of a machine provided for this purpose. This polishing machine in particular comprises a tray containing a rinsing solution. To clean the wafer after polishing, the treated wafer is submerged in the rinsing solution and ultrasonic waves are propagated in the solution. 
         [0008]    This cleaning technique generally provides satisfactory results when it is a question of removing grit particles remaining after polishing. The Applicant has however, observed that this type of machine does not satisfactorily remove material fragments present on the exposed surface of a multilayer structure following a chemical etching step. 
         [0009]    There is, therefore, at the present time a need to remove, in a simple and effective way, material fragments liable to appear on the surface of a multilayer structure during its fabrication. More particularly, there is a need to effectively clean the first wafer of a multilayer structure, which wafer has been chemically etched. 
       DISCLOSURE 
       [0010]    One of the aims of the invention is to provide a solution that meets the need described above. For this purpose, the present invention provides a process for removing material fragments present on the exposed surface of a first layer that is bonded to a second wafer, the fragments to be removed being larger than 2 μm, the process comprising a step of submerging at least the first layer in a liquid solution, and a step of propagating ultrasonic waves in the solution, the frequency and power of the ultrasonic waves being set to create a cavitation effect in the liquid solution so as to remove the fragments from the exposed surface. 
         [0011]    It will be noted that the size of a fragment can correspond to its length, its width or its diameter. 
         [0012]    Advantageously, the process of the invention allows relatively large material fragments deposited on the surface of a multilayer structure during its fabrication and, in particular, during a chemical etching step, to be removed. 
         [0013]    The process is also advantageous in that the operating parameters (frequency of the ultrasound, power of the ultrasound, etc.) are easily controllable and reproducible. This process thus allows the cleaning of multilayer structures that have been chemically etched, for example, during a thinning step, to be industrialized. 
         [0014]    The material fragments originate, for example, from a prior step of chemically etching the first layer. 
         [0015]    The frequency and power of the ultrasonic waves are preferably set depending on the viscosity of the liquid solution. As described in more detail below, it is thus possible to optimize the effectiveness of the removal process of the invention. 
         [0016]    Furthermore, the liquid solution may be a rinsing solution. Alternatively, the liquid solution may be an etching solution. 
         [0017]    Thus, it is possible to carry out the process of the invention directly in a bath of an etching solution used to chemically etch the first wafer of the multilayer structure. In this case, the etching solution serves as a medium for propagating the ultrasonic waves. In this way, only one tray and one liquid solution are necessary to carry out chemical etching and to remove the material fragments deposited on the surface of the first wafer. 
         [0018]    The liquid solution may furthermore comprise at least one of the following solutions: a TMAH solution, a KOH solution and an H 3 PO 4  solution. 
         [0019]    According to a particular aspect of the invention, at least some of the fragments to be removed are fragments of the first wafer formed during a previous step of chemically etching the first wafer. 
         [0020]    Furthermore, the material fragments to be removed may comprise at least one of the following residues: oxide residues originating from an oxide layer located at least at the bonding interface between the first wafer and the second wafer; silicon residues originating from the peripheral edges of the first wafer; and residues from microcomponents, originating from the peripheral edges of the first wafer. 
         [0021]    The invention also relates to a process for fabricating a multilayer structure comprising the following steps in succession:
       bonding a first wafer to a second wafer so as to form a multilayer structure;   annealing the structure; and   thinning the first wafer, the thinning comprising at least one step of chemically etching the first wafer,       
 
         [0025]    the process being characterized in that it furthermore comprises, after the chemical etching step, removing material fragments present on the exposed surface of the first wafer according to one of the embodiments of the removing process described above. 
         [0026]    The process may furthermore comprise a step of oxidizing the first wafer before the bonding step. 
         [0027]    This additional oxidation step in particular allows an oxidation layer to be placed at the bonding interface between the first wafer and the second wafer so as to make bonding the two wafers easier. 
         [0028]    In a particular embodiment, the chemical etching step is carried out in a bath of an etching solution, and the liquid solution used during the removal of the fragments is the etching solution. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0029]    Other features and advantages of the present invention will become clear from the description given below with reference to the appended drawings, which illustrate a non-limiting exemplary embodiment therefor. In the figures: 
           [0030]      FIGS. 1A to 1D  are schematic views showing production of an SOI multilayer structure; 
           [0031]    FIG. shows, in the form of a flowchart, the main steps of the production process illustrated in  FIGS. 1A to 1D ; and 
           [0032]      FIG. 3  schematically shows a process for removing material fragments, according to the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0033]    The present invention relates, in a general way, to the removal of unwanted material fragments that appear on the exposed surface of a multilayer structure during its fabrication process. 
         [0034]    A multilayer or composite structure is produced by bonding a first wafer to a second wafer that supports the first wafer. 
         [0035]    The wafers forming a multilayer structure are generally circular and may have various diameters, especially diameters of 100 mm, 200 mm or 300 mm. However, they may be any shape, such as rectangular, for example. 
         [0036]    These wafers preferably have a chamfered edge, namely an edge comprising an upper chamfer and a lower chamfer. These chamfers generally have a rounded form. However, the wafers may have chamfers or edge rounding of various forms such as a bevel. 
         [0037]    The role of these chamfers is to make handling the wafers easier and to prevent the edge from fragmenting, which could occur if these edges were sharp, such fragments being sources of particulate contamination of the surfaces of the wafers. 
         [0038]    An exemplary process for fabricating a multilayer structure is now described with reference to  FIGS. 1A and 1B . 
         [0039]    As shown in  FIGS. 1A and 1B , a composite structure  111   a  is formed by joining a first wafer  108  with a second wafer  110 . 
         [0040]    In this example, the first wafer  108  is an SOI structure comprising a buried oxide layer  104  intermediate between two silicon layers (i.e., the upper layer  101  and the lower layer  102 ). The second wafer  110  is here made of sapphire. 
         [0041]    The first and second wafers  108  and  110  here have the same diameter. They could, however, have different diameters. 
         [0042]    Preferably, at least one of the two wafers  108  and  110  has been oxidized before bonding. This oxidation in particular provides an oxide layer intermediate between the two wafers once the bonding has been carried out. This oxidation is obtained by means of a heat treatment in an oxidizing medium. In the example described here, the first wafer  108  is oxidized before bonding, so as to form an oxide layer  106  over the entire surface of the first wafer. Alternatively, it is possible to deposit an oxide layer, called a bonding oxide layer, on the side to be joined of the first wafer  108 , before bonding to the second wafer  110 , the latter possibly also comprising a surface oxide layer. There is thus a bonding oxide layer at the interface between the first wafer  108  and the second wafer  110 , enabling better bonding between these two wafers. 
         [0043]    Moreover, the first wafer  108  has a chamfered edge, namely an edge comprising an upper chamfer  122   a  and a lower chamfer  122   b . The second wafer  110  likewise has an edge comprising an upper chamfer  124   a  and a lower chamfer  124   b.    
         [0044]    In the example described here, the first wafer  108  and the second wafer  110  are joined by means of direct bonding (also called molecular bonding), this technique being well known to those skilled in the art (step E 1 ). 
         [0045]    Other bonding techniques may, however, be used, such as, for example, anodic, metal or adhesive bonding. 
         [0046]    It will be recalled that the principle of direct bonding is based on bringing two surfaces into direct contact, i.e., no specific additional material (adhesive, wax, solder) is used. To implement such an operation, the surfaces to be bonded must be sufficiently smooth, free from particulates or contamination, and brought sufficiently close to each other to allow contact to be initiated—typically a distance smaller than a few nanometers. In this case, the attractive forces between the two surfaces are strong enough to cause direct bonding (bonding induced by all the attractive electron interaction forces (Van der Waals forces) between atoms or molecules in the two surfaces to be bonded). 
         [0047]    It will be noted that the first wafer  108  may comprise microcomponents (not shown in the figures) on the side to be bonded to the second wafer  110 , especially in the case of 3D-integration requiring transfer of one or more layers of microcomponents to a final substrate, or else in the case of circuit transfer, such as, for example, in the fabrication of backlit imagers. 
         [0048]    The composite structure  111   a  is then subjected to a bond-interface-strengthening anneal at a moderate temperature (for example, at 400° C. for 2 hours), this anneal being intended to strengthen the bonding between the first wafer  108  and the second wafer  110  (step E 2 ). 
         [0049]    Once this anneal has been carried out, in general, the first wafer  108  is thinned so as to form a transferred layer having a given thickness (for example, about 10 μm) on the supporting wafer. This thinning operation generally comprises a chemical etching operation. 
         [0050]    Now, the Applicant has observed that unwanted material fragments appear on the exposed surface of the first wafer  108  following a thinning step involving a chemical etching operation. 
         [0051]    In-depth study of these material fragments has allowed their formation mechanism to be understood. The formation mechanism is described in greater detail with regard to  FIGS. 1C and 1D , which illustrate an exemplary step of thinning the first wafer  108 . 
         [0052]    The thinning step in general comprises two separate substeps. The first wafer  108  is first mechanically thinned by a grinder or any other tool able to grind the material of the first wafer (step E 3 ). This first thinning substep removes most of the upper layer  102 , only a residual layer  112  ( FIG. 1C ) remaining. 
         [0053]    Next, in a second thinning substep, the residual layer  112  is chemically etched (step E 4 ). This step consists of placing the composite structure  111   b  in a bath comprising an etching solution  126  ( FIG. 1D ). 
         [0054]    In the example described here, a TMAH solution is used to etch the silicon of the first wafer  108 . Other chemical etching solutions may, however, be envisaged, these solutions being chosen, in particular, depending on the composition of the first wafer to be thinned. For example, a KOH or H 3 PO 4  solution may be used depending on the circumstances. 
         [0055]    The buried oxide layer  104  intermediate between the layers  101  and  102  of the first wafer serves as a stop layer during the chemical etching. Thus, the chemical etching is stopped on the oxide layer  104 . The chemical etching thus removes the residual layer  112  remaining after the mechanical thinning. 
         [0056]    However, the Applicant has observed that, after the chemical etching operation, material fragments  118  were present on the exposed surface of the first wafer  116 . These fragments  118  are typically larger than 2 μm in size. 
         [0057]    Studies have shown that these material fragments are debris originating from the edges of the first wafer. 
         [0058]    More precisely, the chamfered edges of the first and second wafers cause problems with the bonding of these two wafers at their periphery. Despite the moderate temperature bonding-interface-strengthening anneal, an annular portion of the periphery of the first wafer  108 , located in the vicinity of the lower chamfer  122   b , does not bond well (and may even not bond at all) to the second wafer  110 . 
         [0059]    The reduction in the thickness of the first wafer during the steps of mechanical and chemical thinning significantly weakens the edges of the first wafer in the vicinity of the lower chamfer  122   b.    
         [0060]    Lateral etching during the chemical etching operation weakens the unbonded (or weakly bonded) peripheral zone of the first wafer even more. This additional weakness generally leads to uncontrolled fracturing at the periphery of the thinned first wafer. These fractures cause debris or material fragments to form, which are then liable to deposit on the exposed surface of the thinned first wafer  116 . 
         [0061]    Thus, oxide-containing and possibly silicon-containing fragments may contaminate the exposed surface of the thinned first wafer  116  ( FIG. 1D ). 
         [0062]    The fracturing, in particular, occurs during the chemical etching in the course of the thinning, when the remaining thickness of the first wafer does not allow it to support its own weight at the periphery. It appears that once this critical stage has been reached, a peripheral portion of the first wafer in the vicinity of the lower chamfer  122   b  collapses, thus producing the unwanted material fragments  118 . 
         [0063]    The Applicant has moreover observed that these material fragments  118  are generally relatively large. Typically, these fragments are at least 2 μM in size. The large size of these fragments is especially explained by their formation mechanism, the collapse described above. On account of their large size, these fragments cannot be effectively removed by a conventional ultrasonic cleaning process. 
         [0064]    It will also be noted that these fragments  118  may contain circuit residues originating from any microcomponents buried in the first wafer on its side bonded to the second wafer  110 . 
         [0065]    The Applicant has, therefore, developed a process for removing any material fragments that may appear on the surface of a multilayer structure during its fabrication. An exemplary embodiment of the invention is described with reference to  FIGS. 2 and 3 . 
         [0066]    Once the chemical etching has been carried out ( FIG. 1D ), the multilayer structure  111   c  is rinsed and then placed in a tray  128  (or dish) containing a rinsing solution  130 , as illustrated in  FIG. 3 . This rinsing solution may, for example, be deionized water (DIW). However, other rinsing solutions may also be envisaged. 
         [0067]    Ultrasonic waves, i.e., mechanical and elastic waves, transmitted, for example, by a liquid, with a frequency higher than 20 kHz, are then propagated in the rinsing solution in which the composite structure  111   c  is submerged. 
         [0068]    These ultrasonic waves may be produced, for example, by making piezoelectric transducers oscillate at a given frequency and power (using an ultrasonic cleaner, for example). Other ultrasonic transducers may, however, be envisaged in the context of the invention (magnetostrictive transducers, pneumatic generators, etc.). 
         [0069]    The emission of ultrasonic waves under particular conditions leads to what is called an acoustic cavitation effect in the rinsing tray  128 . More specifically, the ultrasonic waves cause substantial pressure drops in the rinsing solution  130 . When these pressure drops reach a critical threshold, they cause bubbles to form in the rinsing solution  130 . These bubbles are commonly called cavitation bubbles. 
         [0070]    Since cavitation bubbles are particularly unstable, they implode when they encounter the exposed surface of the thinned first wafer  116 . When they implode, these bubbles may emit a shock wave that is sufficiently strong to break up, debond and disperse the material fragments  118  present on the exposed surface of the thinned first wafer  116 . 
         [0071]    Once debonded from the exposed surface of the thinned first wafer  116 , the material fragments  118  are removed by the rinsing solution  130 . 
         [0072]    The magnitude of the pressure drops leading to this cavitation effect especially depends on the frequency and power of the ultrasonic waves emitted. The Applicant has observed that to obtain a cavitation effect capable of removing fragments that are at least 2 μm in size, the ultrasonic waves used must have a low frequency. In other words, this frequency must lie in a band located between 20 kHz and 1000 kHz. The closer the frequency is to the lower limit of the band (i.e., 20 kHz), the larger the fragments removed by the process of the invention. In a particular embodiment, the frequency of the ultrasonic waves lies between 20 kHz and 500 kHz and even between 20 kHz and 100 kHz. In a variant, the frequency lies between 700 kHz and 1000 kHz. 
         [0073]    However, the viscosity of the rinsing solution  130  also has an impact on the magnitude of the pressure drops obtained. This is because, the higher the viscosity of the rinsing solution  130 , the more difficult it is to obtain cavitation effect. It is, therefore, recommended to minimize the viscosity of the liquid solution in which the ultrasonic waves propagate. Typically, the viscosity of the liquid solution must be 30 mPa·S (i.e., 30 cP) or less at 25° C. It will be noted that, in this document, the term “viscosity” is understood to mean the dynamic viscosity of a medium. 
         [0074]    The frequency and power of the ultrasonic waves will, therefore, be set depending on the viscosity of the rinsing solution  130 . 
         [0075]    It is also possible to match the temperature of the liquid solution to the situation considered. In particular, the higher the temperature of the solution, the more the viscosity of the latter decreases. It is thus possible to heat the liquid solution in which the ultrasonic waves propagate in order to obtain a viscosity below 30 mPa·S. 
         [0076]    Moreover, changing the power of the ultrasonic waves changes the rate at which the material fragments  118  are removed. Thus, the greater the power of the ultrasonic waves, the higher the removal rate. The power is, for example, set to between 600 W and 1200 W. 
         [0077]    The table below collates the experimental conditions that may typically be applied to obtain a cavitation effect, making it possible to remove the material fragments  118  on the surface of the thinned first wafer  116 : 
         [0000]    
       
         
               
               
             
               
               
             
           
               
                   
                   
               
               
                   
                 Experimental conditions 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                 Tray 
                 Made of stainless steel, quartz or PTFE. 
               
               
                 Rinsing solution 
                 DIW, TMAH, etc. 
               
               
                 used 
                 (Additives may be added to the rinsing solution and 
               
               
                   
                 the rinsing solution may be made to circulate in the 
               
               
                   
                 tray.) 
               
               
                 Temperature 
                 At room temperature or at 40° C. 
               
               
                 Power of the 
                 660 W or 1200 W. 
               
               
                 ultrasound 
               
               
                 Frequency of the 
                 44 kHz or between 700 kHz and 1000 kHz 
               
               
                 ultrasound 
               
               
                 Orientation of the 
                 Horizontal: adequate effectiveness 
               
               
                 composite structures 
                 Vertical: good effectiveness 
               
               
                 in the tray 
               
               
                 Duration 
                 15 min to 20 min 
               
               
                 Composition of the 
                 An oxide layer (from 500 Å to 2 μm in thickness) 
               
               
                 observed material 
                 covered with silicon and/or a circuit multilayer. 
               
               
                 fragments 
               
               
                   
               
             
          
         
       
     
         [0078]    Alternatively, it is possible to carry out the removing process according to the invention, directly in the bath of the etching solution  126 , during the chemical etching operation illustrated in  FIG. 1D . In this case, the etching solution  126  (a TMAH solution, for example) serves as the medium for propagating the ultrasonic waves and the cavitation is concomitant with the etching action of the etching solution  126 . 
         [0079]    Once the process for removing fragments according to the invention has been implemented, it is possible to implement a contouring operation in order to remove an annular portion from the periphery of the thinned first wafer  116 . 
         [0080]    Moreover, the process of the invention is applicable to any type of multilayer structure and, more particularly, to multilayer structures, the wafers of which have chamfered edges (or edge rounding of any shape) and which cannot be heated to high temperatures in order to perfectly stabilize the bonding interface. The invention is, in particular, applicable to SOS structures. 
         [0081]    The removing process according to the invention, therefore, advantageously removes material fragments that deposit, or that are liable to deposit, on the surface of a multilayer structure and, more particularly, on the exposed surface of the transferred layer (i.e., the thinned first layer). 
         [0082]    The process of the invention is particularly suited to removing particulates that are relatively large in size, i.e., typically being more than 2 μm in size. The process thus removes fragments of a few microns, even a few centimeters, in size. 
         [0083]    The process of the invention is also advantageous in that the operating parameters are controllable and reproducible. This technique may thus be optimized and automated for industrial purposes (in contrast to the conventional process of rinsing under a pressurized jet). For example, an ultrasonic bath may advantageously be integrated into a line for producing multilayer structures in order to allow implementation of the process of the invention.