Abstract:
The invention relates to a method for fabricating a locally passivated germanium-on-insulator substrate wherein, in order to achieve good electron mobility, nitridized regions are provided at localised positions. Nitridizing is achieved using a plasma treatment. The resulting substrates also form part of the invention.

Description:
BACKGROUND 
     The invention relates to a method for fabricating a germanium-on-insulator (GeOI) substrate and a germanium-on-insulator substrate. 
     Germanium-on-insulator structures are generally known in the art. EP 1 659 623 A1 discloses a method to fabricate a germanium-on-insulator (GeOI) substrate during which a germanium oxynitride (GeO x N y ) layer is provided as a buried dielectric between a Ge layer and a handle substrate, e.g., a Si wafer. T. Signamarcheix et al in Applied Physics Letters 93, 022109 (2008) describes the advantageous effect of the presence of a GeO x N y  layer underlying the active Ge layer concerning the electron mobility. Thus, this kind of substrate can advantageously be used for electronic devices based on n-MOS technology. It appears, however, that the hole mobility became affected by the presence of the GeO x N y  layer so that concerning p-MOS devices, this kind of substrate is less suited. 
     Accordingly, there is a need for improved germanium-on-insulator substrates and these are now provided by the present invention. 
     SUMMARY OF THE INVENTION 
     The invention relates to a method for fabricating a locally passivated germanium-on-insulator substrate which comprises the steps of: providing a germanium (Ge) substrate or a substrate comprising a Ge containing layer, in particular an epitaxial Ge layer or a silicon germanium layer (SiGe), as a source substrate, and locally treating the source substrate to provide passivated regions, in particular regions of GeO x N y . With the inventive method, a germanium-on-insulator substrate with satisfying electronic properties, in particular high electron mobility in the passivated zones and hole mobility in the non passivated regions, can be obtained. In case of a SiGe layer, the inventive method is particularly advantageous for high Ge contents of more than 50%, in particular more than 70%. 
     Preferably, the passivation of the source substrate, the surface of which comprises a native Ge oxide which needs to be stabilized, is achieved by providing a germanium oxynitride (GeO x N y ) layer with a substantial nitrogen content of 20 to 50%. With this GeO x N y  layer, the desired passivation and the desired improvements concerning electron mobility in the Ge layer directly above the passivated region in the final substrate, can be achieved. 
     Advantageously, the source substrate can be locally treated by providing a patterned mask over the source substrate. With this method, any pattern can be realised on the source substrate so that the regions which should show a high electron mobility according to the desired final structures, can be achieved. As an alternative, maskless patterning methods can also be used. 
     Preferably, the passivating can be achieved by nitridizing. Using this method, advantage is taken of the already present natural Ge oxide to obtain the GeO x N y  layer. 
     Preferably, the source substrate can be locally treated using a plasma, in particular a NH 3 , N 2  or N 2 O plasma. According to a variant, the NH 3 , N 2  or N 2 O plasma can be diluted in 10% to 30% Ar. With this method and in particular at low pressure, in particular with a plasma of less than 40 mTorr, preferably in a range of 1 to 10 mTorr, a significant amount of up to 40% Nitrogen atoms can be incorporated into the substrate. Optimized results have been achieved for a pressure of 5 mTorr. This process is preferably carried out at a temperature of 25° C. up to 600° C. 
     Advantageously, the local treatment of source substrate can further comprise applying an oxygen and/or argon plasma, in particular before applying the NH 3 , N 2  or N 2 O plasma. The oxygen comprising plasma can advantageously be used to improve and/or thicken the germanium oxide layer on top of the source substrate. Providing the nitrogen containing plasma after the oxygen plasma has the advantageous effect of passivating (both chemically and electronically) the surface of the source substrate. 
     According to a preferred embodiment, the method can further comprise a step of providing a predetermined splitting area inside the source substrate before or after the local treatment step. Preferably, this step includes implanting an atomic species into the source substrate to form a zone of weakness at which splitting is intended to occur. 
     The inventive method can further comprise a further step of providing a dielectric layer, in particular an oxide, on the source substrate before or after the implanting step. In case the predetermined splitting area is provided after providing the dielectric layer, the dielectric layer could be at least partially removed to improve surface quality. In all other cases, the dielectric layer is used to provide the desired dielectric properties of the buried layer of the GeOI substrate over the passivated and non passivated regions of the source substrate. The local passivation treatment may create a slight surface topology. Providing the dielectric layer on the locally passivated surface has the advantage that a planarization step (e.g., by CMP) can be carried out. This will also form a surface which can easily be bonded to other substrates. 
     In a further preferred variant, the locally passivated source substrate can be annealed to stabilize the GeO x N y  passivation surface, in particular for 1 hour or more at about 600° C. This can be performed before or after the mask removal. The thermal treatment has the consequence of stabilising the GeO x N y  regions and, depending on the mask type used, also to sublime the mask layer. 
     A further method comprises attaching, preferably by bonding, of the source substrate to a handle substrate, and then detaching the source substrate at the predetermined splitting area, to thereby obtain the locally passivated GeOI substrate. With the stable GeO x N y  dielectric layer on the source substrate it becomes possible to transfer a thin Ge layer together with the GeO x N y  layer in the nitridized regions to achieve the desired locally passivated GeOI substrate. The handle substrate is preferably a silicon wafer. 
     Advantageously, the method can further comprise a step of providing a dielectric layer, in particular an oxide, on the handle substrate before the substrates are bonded together, in particular when there is no dielectric provided on the source substrate. In the case of a silicon wafer, the dielectric can be a thermal oxide and/or a deposited oxide. Thus, bonding can be achieved between the locally passivated surface of the source substrate and the dielectric of the handle substrate, between the dielectric of the source substrate and the surface of the handle substrate without dielectric, or between the dielectric layers of the source and handle substrate. 
     According to an advantageous variant, the method can furthermore comprise a step of activating the handle substrate using a plasma, in particular an oxygen plasma and/or NH 3 , N 2  or N 2 O plasma, in particular mixed with Ar, before the bonding step. Activating of the handle substrate surface provides improved bonding between the substrates. 
     Preferably, bonding can be carried out directly after the plasma treatment step is conducted on the source substrate or the handle substrate without any intervening further process step. By immediately carrying out the bonding step, a deterioration of the GeO x N y  surface can be prevented and superior final products be achieved. Thus, according to the invention, passivation of the Ge material and activating of the surface for bonding are achieved in a single step. Of course, prior to bonding, the patterned mask needs to be removed. 
     Advantageously, the mask used in the mask providing step can be at least one of a shadow mask, in particular a Teflon mask or metal mask or a deposited mask, in particular a photo resist based mask, with a thickness of 1 μm or less or a germanium oxide or GeO 2  mask. It was found that these types of masks are suitable to provide the desired locally passivated regions in the source substrate. Prior to bonding, the mask is removed using, for example, dry or wet etching, in particular a plasma etching using N 2 , or by a thermal treatment as long as the passivation is not negatively affected. 
     Preferably the local treatment step can comprise the steps of: providing a germanium oxide or GeO x  layer, which can be the natural or a deposited one, on the source substrate, providing a photo resist layer, nano-imprinting the photo resist layer, and providing the pattern by plasma etching in particular reactive ion etching (RIE). According to an alternative embodiment, the GeO x  layer can also be patterned using a laser or electron beam treatment. Using the nano-imprinting step, locally passivated structures like islands of a micro- and/or nanometric size are readily achievable. 
     Advantageously, the photoresist layer can be removed before the local passivation such that the step of removing the photoresist layer does not have an impact on the locally passivated regions. 
     According to a preferred embodiment, the method can furthermore comprise a step of providing alignment marks on or in the source substrate. According to the invention, the substrate will present areas of the Ge “top” layer that should receive n-MOS type of devices due to improved electron mobility over the locally passivated regions and other areas away from the locally passivated regions are suited to receive p-MOS type of devices as; in those regions; the hole mobility is better. The alignment marks will support the steps of manufacturing the n-MOS and/or p-MOS devices as they provide a reference point so that the various devices can be formed on their corresponding regions. 
     Advantageously, the method can further comprise a step of providing recesses in the source substrate pattern. The pattern of the recesses is chosen such that it becomes possible to know where the buried locally passivated regions are positioned. It is not necessary to use the same pattern as the one of the passivated regions as long as positioning of preferably all buried locally passivated regions is possible. 
     According to a further preferred embodiment, a dielectric, in particular a SiO 2  layer, can be provided in the recesses. After depositing the SiO 2  layer, a planarization step, e.g., a CMP step, can be carried out to obtain a surface with SiO 2  islands inside a Ge surface. Based on the properties of the dielectric, it becomes possible to identify the passivated regions. Preferably, this dielectric layer is the same as the one described previously. 
     Preferably the dielectric layer can be provided such that the predetermined splitting area is crossing the recesses of the source substrate. This has the advantage that the dielectric is visible on the surface thus the locally passivated regions buried in the substrate can be identified from outside the substrate. Preferably, the recesses can have a depth of 0.5 to 2 μm. 
     The means for alignment can be provided before or after providing the locally passivated regions. Providing them before the passivated regions has the advantage that the fabrication process concerning the means for alignment does not have an impact on the locally passivated regions. 
     The invention also relates to a germanium-on-insulator substrate comprising locally passivated regions, in particular, substrates obtainable according to one or a combination of any of the methods described herein. With such a germanium-on-insulator substrate, the above described advantages can be achieved. 
     Thus, the invention relates to a germanium-on-insulator substrate (GeOI) comprising buried passivated regions. According to a preferred embodiment, the germanium-on-insulator substrate can furthermore comprise alignment marks, in particular SiO 2  islands, configured and arranged to identify the positions of the buried passivated regions. Preferably the alignment marks, extend up to the surface of the GeOI substrate. 
     Another embodiment of the invention is an electronic device comprising nmos and pmos device structures on or in one of the germanium-on-insulator substrates described herein, wherein the nmos structures are provided over the localised, passivated regions, and the pmos structures are provided over other regions of the germanium-on-insulator substrate. Other electronic devices incorporating the novel substrates of the invention can be envsisioned by those persons of ordinary skill in the art. The substrates that include alignment marks are especially useful in forming these electronic devices. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Advantageous and preferred embodiments of the invention will be described in the following text by referring to the drawing figures, wherein: 
         FIGS. 1A-1G  show a first embodiment of the method for fabricating a germanium-on-insulator type wafer according to the invention, 
         FIGS. 2A-2G  show a second embodiment of the method for fabricating a germanium-on-insulator type wafer according to the invention, 
         FIGS. 3A-3G  show a third embodiment of the method for fabricating a germanium-on-insulator type wafer according to the invention, 
         FIGS. 4A-4F  show a fourth embodiment of the method for fabricating a germanium-on-insulator type wafer according to the invention, and 
         FIG. 5  illustrates a fifth embodiment of the invention, namely an electronic device comprising n-MOS and p-MOS structures provided on a substrate fabricated a germanium-on-insulator type wafer according to the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the following specification, the overall method for fabricating a germanium-on-insulator type wafer according to the invention will be described based on the well known SmartCut™ technology. However, other suitable semiconductor on insulator providing manufacturing methods can also be adapted to the invention, for example, a bonding and grind/etch back process. 
       FIG. 1A  illustrates a germanium (Ge) substrate  1  or, as a variant, a substrate with a germanium containing layer, in particular an epitaxial Ge layer or a silicon and germanium comprising layer SiGe, provided on one of its main surfaces. In the case of a SiGe layer, the Ge content is preferably at least 50%, more preferred more than 70%. Prior to further treatment steps, the surface  3  of the source substrate  1  may be cleaned using, for instance, a HF base solution (fluoric acid). 
       FIG. 1B  illustrates a handle substrate  5  which can be, for example, a germanium wafer, a silicon wafer, a silicon carbide wafer, a wafer presenting a silicon germanium front surface or a gallium arsenide wafer. Eventually, also a quartz type wafer could be used. Both the source substrate  1 , as well as the handle substrate  5 , may have any suitable size of form such as, for example 200 mm or 300 mm type wafers. 
       FIG. 1C  illustrates steps b) and c) of the inventive method for fabricating a locally passivated germanium-on-insulator wafer according to claim  1  which consists of first providing a patterned mask  7  over the source substrate  1  and then treating the source substrate  1  via the patterned mask  7  to obtain locally GeO x N y  regions  9  in or on the source substrate  1 . 
     The mask can be a contact mask, thus provided on the source substrate  1 . According to a variant, as illustrated in  FIG. 1C , a non-contact mask positioned above the surface  3  of the source substrate  1  can be provided. In this case, the mask  7  can be a Teflon mask or a metal on the source substrate  1 . 
     The openings  11  in the mask  7  are of micrometric or nanometric size so that the locally passivated regions  9  of the source substrate  1  are also in the micrometric or nanometric range. They can have a regular size or varying size with respect to each other. 
     According to this embodiment of the invention, the treatment step is a nitridizing step using a nitrogen comprising plasma, in particular a NH 3 , N 2  or N 2 O plasma diluted in an Argon plasma, to incorporate nitrogen atoms into substrate  1 . 
     Preferably, the plasma is applied for about 60 seconds at low pressures of less than 40 mTorr, in particular in a range of 1-10 mTorr. The preferred temperature range is 25° C. up to 600° C. Optimized results have been achieved in a pressure range of 5 mTorr and at temperatures of 150° C. It appears that the lower the pressure of the plasma, the more nitrogen can be incorporated. The plasma treatment can be followed by a temperature treatment around 600° C. This is particularly useful in the case where the mask  7  is formed of GeO 2 . The thermal treatment has then the consequence of stabilising the GeO x N y  regions  9  and also to sublime (to render volatile) the GeO 2  mask layer. This treatment is a surface treatment and the thickness of the N rich regions is less than 10 nm, preferably less than 5 nm, more preferably 3 nm. 
     Actually, due to the presence of oxygen as natural germanium oxide on the surface of the germanium substrate  1  or in the environment during the passivation treatment, the nitridizing steps leads to the creation of GeO x N y  type islands in those regions  9 . In the final germanium-on-insulator substrate (see further down), the Ge material of the top layer above those islands shows a high electron mobility. 
     Following the removal of the mask  7 , a predetermined splitting area  13  is provided inside the source substrate  1  as illustrated in  FIG. 1D . The predetermined splitting area  11  is essentially parallel to the main surface  3 , which is provided with the locally passivated germanium oxynitride regions  9 . According to the SmartCut™ technology, the predetermined splitting area is obtained by implanting atomic species  15 , for instance hydrogen or helium ions, with a predetermined energy and dose into the source substrate  1 . According to a variant, this step can also be carried out before the removal of the mask  7 . 
     Afterwards, as illustrated in  FIG. 1E , a dielectric layer  17 , in particular an oxide layer such as a silicon dioxide layer, is provided, e.g., by deposition, on the source substrate  1 . This layer  17  extends over the entire surface of the source substrate  1 , thus also over the passivated regions  9 . Subsequently, a planarization step could be carried out, e.g., by CMP. 
     In a subsequent step, the source substrate  1  with the nitridized regions  9  and the dielectric layers  17  is attached, in particular by bonding, to the handle substrate  5  to form a source-handle compound  19 . Bonding occurs between the surface of the handle substrate  5  and the surface  21  of the dielectric layer  17  (see  FIG. 1F ). 
       FIG. 1G  illustrates the result of the detachment step which occurs at the predetermined splitting area  13 . Typically, a thermal annealing leads to the desired detachment, however other energy providing means may also be suitable in replacement or in complement of the anneal. Actually, during annealing, a weakening of the predetermined splitting area  13  until complete detachment between the remainder  23  and the newly formed germanium-on-insulator substrate  25  with locally passivated regions  9  occurs. The inventive germanium-on-insulator substrate  25  with locally passivated regions  9 , comprises the handle substrate  5 , the dielectric layer  17 , the passivated regions  9  and a transferred germanium layer  27 . Like already mentioned, the substrate  25  shows improved electron mobility in the passivated regions and in the non passivated regions the hole mobility is not negatively affected by the passivation. 
     The remainder  23  of the source substrate  1  can then be reused as a source substrate  1  in subsequent locally passivated germanium-on-insulator manufacturing processes. 
     According to variants of the first embodiment, the sequence of the various steps can be changed. Thus, according to one variant, the ion implanting step illustrated in  FIG. 1D  is carried out before creating the nitridized regions  9 , thus before the step illustrated in  FIG. 1C . Furthermore, according to a second variant, the dielectric layer  17  is provided on the nitridized regions  9  before ion implantation. Thus, the step illustrated in  FIG. 1E  can be carried out before the step illustrated in  FIG. 1D . Eventually, in this variant, a surface region of the dielectric layer  17  is removed after ion implantation to improve the surface quality. According to a third variant of the first embodiment, prior to applying the nitrogen containing plasma, an oxygen and/or argon containing plasma can be applied to improve and/or thicken the germanium oxide layer which is already present on top of the source substrate  1 . The role of the nitrogen containing plasma is then to activate the surface of the source substrate  1  in the nitridized regions  9 . 
     The final product  25  can furthermore receive additional treatments, such as a polish and/or a heat treatment, to stabilize the structure. 
       FIGS. 2A-2G  illustrate a second embodiment of the inventive method. 
     The steps illustrated in  FIGS. 2A-2D  correspond to the steps illustrated in  FIGS. 1A-1D . Their description is therefore not repeated again, but incorporated herewith by reference to the description of  FIGS. 1A-1D . The difference between the first and second embodiments is that, instead of providing a dielectric layer  17  onto the source substrate  1  following the nitridizing step, a dielectric layer  31 , e.g., silicon dioxide, is provided, for example by deposition or a thermal treatment, on the handle substrate  5 , like illustrated in  FIG. 2E . 
     The surface  33  of the dielectric layer  31  undergoes an activation using a plasma, in particular an oxygen plasma and/or nitrogen containing plasma e.g., NH 3 , N 2  or N 2 O plasma. The plasma treatment of layer  31  is carried out under conditions compared to the plasma treatment conditions of the source substrate described in detail with respect to  FIG. 1C . 
       FIG. 2F  then illustrates the step of attaching, in particular by molecular bonding, the source substrate  1  to the handle substrate  5  to form the source-handle compound structure  33 , In this case, bonding occurs at surface  33  of the dielectric layer  31  and the surface  3  of the source substrate  1  with the passivated regions  9 . 
     Just like in the first embodiment, the next step, illustrated in  FIG. 2G , consists in detaching the germanium-on-insulator type substrate  37  from the remainder of the source substrate  1  (not shown). The germanium-on-insulator substrate  37  in this embodiment comprises the handle substrate  5 , the dielectric layer  31 , the locally passivated regions  9  and the transferred layer  27 . 
     According to a variant, the steps illustrated in  FIGS. 2C and 2D  can be exchanged such that the passivated regions  9  are achieved after providing the predetermined splitting area  13 . In this case, attachment can be carried out immediately after the plasma treatment on the source substrate  1  and the plasma treatment on the dielectric layer  31  and an improved stability of the stoichiometry of the GeO x N y  regions  9  is observed. 
     Of course, the methods according to embodiment  1  and embodiment  2  can be combined in which case the attachment is achieved between dielectric layer  17  and dielectric layer  31 . 
       FIGS. 3A-3G  illustrate a third embodiment of providing a locally passivated germanium-on-insulator substrate according to the invention.  FIGS. 3A-3G  illustrate in detail one possibility to provide a patterned mask and, using this mask, to treat the source substrate  1  to obtain locally passivated regions  9  on the source substrate  1 . The mask in this embodiment is in contact with the surface  3  of the source substrate  1 . 
       FIG. 3A  corresponds to  FIGS. 1A and 2A , thus represents the source substrate  1 , a germanium wafer or a wafer with a germanium layer on its surface  3 .  FIG. 3B  illustrates the next step of providing a germanium dioxide GeO 2  layer  41  on the surface  3  of the source substrate  1 . The germanium dioxide layer  41  corresponds to the natural oxide layer or can be a deposited one. 
     The next step consists in providing a photoresist layer  43  on the germanium dioxide layer  41 . Subsequently, using a nano-imprint process known in the art, the photoresist layer  43  is patterned. Via the patterned photoresist layer  45 , a patterned mask  47  is created inside the germanium dioxide layer  43  to play the role of mask  7 , as illustrated in  FIG. 1C . Patterning of layer  43  is e.g., achieved by using a reactive ion etching process. As an alternative, instead of providing the photoresist layer, also a laser ablation, ion beam or electron beam sputtering process can be used to create the mask  47  in the germanium dioxide layer  41 . Preferably, the photoresist layer  45  is removed before the passivation step such that the step of removing the photoresist layer does not have an impact (removal) on the passivated surface layer. Via the mask  47 , a nitridizing treatment, as described with respect to  FIG. 1C , is then applied to form the nitridized regions  9  in the source substrate  1 . The result of this step is illustrated in  FIG. 3F . 
     Finally, the mask  47  is removed, e.g., using an N 2  plasma or a thermal treatment at about 600° C. for one hour or more. 
       FIGS. 4A-4F  illustrate a fourth embodiment for fabricating a locally passivated germanium-on-insulator substrate according to the invention. As already illustrated in  FIGS. 1A ,  2 A and  3 A, the step illustrated in  FIG. 4A  consists in providing a germanium substrate  1  or a substrate comprising an epitaxial germanium layer, such that the surface  3  of substrate  1  is a germanium layer. Process steps already previously described are not repeated again in detail, but their description is incorporated herewith by reference. 
     Subsequently, using a mask  51 , recesses  53  are provided in the source substrate  1 . These recesses  53  have a depth d 1  and are, for example, obtained using an etching process such as reactive ion etching. The recesses  53  have a micrometric depth of, for example, 1 micron (see  FIG. 4B ). 
     Subsequently, the process step as illustrated in  FIG. 1C  is carried out ( FIG. 4C ), namely the source substrate  1  with the recesses  53  is treated via mask  7  to obtain locally passivated regions  55  comparable to the ones  9  illustrated in  FIG. 1C . Actually, the only difference between the locally passivated regions  55  and the ones  9  illustrated in  FIG. 1C  is that, now, these locally passivated regions  55  are provided inside the recesses  53 . 
     As a next step ( FIG. 4D ), a dielectric layer  57 , e.g., SiO 2 , is provided over the source substrate  1  which covers the locally passivated regions  55  and the recesses  53 . To smoothen the surface of the dielectric layer  57 , a polishing step, such as CMP, is carried out. 
     Then, as illustrated in  FIGS. 1D and 4E , a predetermined splitting area  13  is created inside the source substrate  1 . According to the invention, the predetermined splitting area  13  is created at a depth d 2  being smaller than the distance d 1  of the surface of the germanium substrate down to the bottom  59  of the recess  53 . 
     Subsequently, the source substrate  1  with dielectric layer  57  is attached, preferably by bonding, to a handle substrate  5  (like described with respect to  FIGS. 1B and 1F ) and detachment occurs at the predetermined splitting area  13  as illustrated in  FIG. 1G . After detachment, the locally passivated germanium-on-insulator substrate  61 , as illustrated in  FIG. 4F , is obtained. As the alignment marks  63  are on the surface of substrate  61 , it becomes easy to identify the position of the germanium islands  65  overlaying the locally passivated regions  55  based on the fixed positional relationship between alignment marks  63  and passivated regions (based on their respective masks  7  and  51 ). 
     Embodiments 1-4 can be freely combined to achieve further variants of the invention. 
     The locally passivated germanium-on-insulator substrates  25 ,  37  and  61 , as well as the various variants thereof, find their application in devices having n-MOS and p-MOS structures fabricated on a single substrate. In particular, advantage is taken of the high electron mobility in the passivated regions and the good hole mobility in the other regions. 
       FIG. 5  illustrates the substrate  61  and further electronic structures. As illustrated in  FIG. 5 , n-MOS devices  71 ,  73  are positioned above the passivated regions  55  (because of higher electron mobility in this area) and p-MOS devices  75 ,  77  are fabricated over the non passivated areas (because of higher hole mobility). This allows to have a wafer on which both type of devices can have improved performance. The alignment marks  63  on the surface are advantageously used to precisely position each respective device  71 ,  73 ,  75 ,  77  over the specific area of the substrate  61 . Thus superior devices based on germanium-on-insulator substrates can be fabricated.