Patent Publication Number: US-11387100-B2

Title: Method for manufacturing a mixed substrate

Description:
TECHNICAL FIELD 
     The present invention relates to the field of microelectronics, and especially that of substrates for microelectronics. 
     The present invention particularly relates to a method for manufacturing a mixed or hybrid substrate for co-integrating regions or stacks having different natures. 
     The present invention relates, according to one particular embodiment, to a method for manufacturing a mixed substrate provided with so-called Sal (Silicon on Insulator) regions lying on an SOI Trap Rich layer and with regions made of a one-piece or bulk material. 
     The present invention especially relates to a method for manufacturing a mixed substrate for co-integrating logic, analogue, and radio frequency structures. 
     STATE OF PRIOR ART 
     The interest for mixed or hybrid substrates no longer needs to be proven. 
     Indeed, the latter generally provided with so-called bulk zones, and with SOI type zones make it possible to make components having different natures on one and a single substrate. 
     In this respect, a method for forming a mixed (or hybrid) substrate known from the state of the art is illustrated in  FIGS. 1A-1E . 
     This method especially comprises the following steps consisting in: 
     i) providing an SOI substrate  1  which comprises a stack comprising from its front face  1 A to its back face  1   b  a useful layer  2 , made of silicon, a dielectric layer  3 , made of silicon dioxide, and a support substrate  4  made of single crystal silicon; 
     ii) forming a mask  5  provided with apertures  5   a  exposing first regions of the stack, and for protecting second regions of the stack using parts  5   b;    
     iii) forming cavities  8  by fully etching the useful layer  2  and the dielectric layer  3  at the apertures  5   a ; and 
     iv) filling the cavities  8  with single crystal silicon  7  by epitaxial growth. 
     Forming the mask when performing step ii) may involve forming a layer  6   a  of dielectric material overlaying the front face, and defining apertures  5   a  by means of a resin mask  6   b  made according to one photolithographic method. 
     At the end of step iv), the first regions of the mixed substrate are made of bulk silicon, whereas the second regions comprise an SOI stack. 
     Step iv) can also be followed by a step v) of forming Shallow Trench Isolations (STI)  9 , for isolating the first regions from the second regions. 
     This method known from the art and performed on an SOI substrate restricted to one useful layer and one dielectric layer is relatively satisfactory. 
     However, since contemplated applications involve forming radiofrequency components, a trap rich layer is interposed between the dielectric layer and support substrate. 
     This trap rich layer especially comprises polycrystalline silicon likely to disturb the execution of step iv). Particularly, the trap rich layer exposed at the walls of the cavities disturbs the execution of step iv), and consequently results in an erratic growth and in creating defects. 
     One purpose of the present invention is therefore to provide a method for manufacturing a mixed substrate for limiting the number of defects when performing the epitaxial growth step. 
     Another purpose of the present invention is to provide a method for manufacturing a mixed substrate provided with first bulk regions and with second regions comprising an SOI type stack lying on a trap rich layer. 
     DISCLOSURE OF THE INVENTION 
     The present invention therefore provides a method for manufacturing a mixed substrate from a starting substrate. 
     The starting substrate comprises, from a front face to a back face, an intermediate layer lying on a support substrate, especially a substrate of single crystal silicon. 
     The intermediate layer, according to the terms of the present invention, comprises a material likely to be used as a seed for epitaxially growing a first material. 
     By “be used as a seed”, it is meant a layer at which seeds for growing said first material can be observed during an epitaxial growth step. By way of example, the intermediate layer can comprise a polycrystalline material, and more particularly polycrystalline silicon. 
     The invention therefore comprises the formation of a cavity in the starting substrate from the front face and which extends into the support substrate. The walls of the cavity are then lined with a protective layer, made of an amorphous material so as to mask sections of the intermediate layer which are uncovered at said walls. 
     Finally, the first material is epitaxially deposited from the bottom of the cavity in order to form a first region. 
     The support substrate is, in this respect, configured to promote single crystal growth of the first material. 
     Implementing the protective layer makes it possible to better control the growth of the first material, and particularly to limit creation of defects by seeding the first material at the walls of the cavity. 
     The purposes of the present invention are therefore, at least partly, achieved by a method for manufacturing a mixed substrate which comprises, on a main face of a support substrate, a first region, comprising a first single crystal material, and a second region, contiguous to the first region, and comprising a second material, the method comprising the following steps of: 
     a) providing a starting substrate which comprises a stack, formed by at least one intermediate layer, and overlaying the main face of the support substrate, the intermediate layer comprising the second material in a single crystal or polycrystalline form; 
     b) forming a mask on the stack, said mask comprising an aperture delimiting the location of the first region; 
     c) etching the stack at the aperture so as to form a cavity delimited by a bottom and walls, the bottom being located on or in the support substrate; and 
     d) forming the at least one first region by epitaxially growing the first material in a single crystal form in the at least one cavity, 
     the method being characterised in that it comprises a step c1), performed before step d), of forming a protective layer, made of an amorphous material, overlaying the walls of the at least one cavity and leaves the bottom of said cavity exposed to external environment. 
     According to one advantageous embodiment, step d) also comprises etching the first material likely to be formed in a polycrystalline form on the walls of the at least one cavity. 
     According to one advantageous embodiment, step c1) comprises forming a layer of material overlaying the stack, bottom and walls of the cavities, step c1) further comprising removing said layer of material at the bottom of the cavities. 
     Removing the layer of material can comprise anisotropic dry etching. 
     According to one implementation, the protective layer comprises at least one of the materials selected from: silicon dioxide, silicon nitride, SiCO, and SiBCN 
     According to one implementation, step d) comprises at least one sequence comprising the steps of 
     d1) epitaxially growing a layer made of the first material overlaying the bottom and walls of the at least one cavity; and 
     d2) selectively etching the layer made of the first material deposited on the walls of the at least one cavity, 
     The sequence is then repeated as many times as necessary for the at least one cavity to be filled with the first single crystal material. 
     This at least one sequence is advantageously implemented when the at least one cavity has an aspect ratio greater than one quarter. 
     According to another implementation, step d) comprises an epitaxial growth step for growing the first material in a single crystal way from the bottom of the cavity, and in a polycrystalline way on the protective layer, the growth step being performed in the presence of a chemical agent for selectively etching the first material in a polycrystalline form in view of its single crystal form. 
     For example, step d) implements a precursor comprising SiH 4  or dichlorosilane (DCS), and a chemical agent comprising HCl. 
     This implementation advantageously makes it possible to consider cavities having a small aspect ratio and especially lower than one quarter. 
     According to one implementation, the at least one first region and at least one second region each have a surface exposed to external environment flush with each other. 
     Advantageously, the stack can further comprise a useful layer and a dielectric layer disposed between said useful layer and the intermediate layer, the useful layer and dielectric layer being also etched at the at least one aperture when performing step c). 
     For example, the useful layer can comprise silicon, the dielectric layer can comprise silicon dioxide, and the intermediate layer can comprise polycrystalline silicon. 
     According to one implementation, step b) comprises forming a hard mask, the hard mask especially comprises a dielectric material. 
     According to one implementation, step c) comprises dry etching, especially plasma etching. 
     According to one implementation, the method further comprises a step e), performed after step d), of forming a trench isolation filled with a dielectric material for electrically insulating the at least one first region from the at least one second region. 
     According to one implementation, the trench isolation extends into the support substrate or stops in the intermediate layer. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further characteristics and advantages will appear in the following description of a method for manufacturing a mixed substrate, given by way of non-limiting examples, with reference to the appended drawings in which: 
         FIGS. 1A, 1B, 1C, 1D, and 1E  are schematic representations of the different steps implemented for making a mixed substrate known from the state of the art, these figures are especially views along a section plane perpendicular to the front face of the SDI substrate; 
         FIG. 2  is a schematic representation of a mixed substrate likely to be obtained by the manufacturing method according to the present invention, this figure is represented along a section plane perpendicular to a main face of the support substrate; 
         FIGS. 3A, 3B, 3C, 3D, and 3E  are schematic representations of the different steps implemented for making a known mixed substrate according to the present invention, these figures are especially views along a section plane perpendicular to a main face of the support substrate; 
         FIGS. 4A, 4B, and 4C  are schematic representations illustrating forming the protective layer by thermal oxidation; 
         FIGS. 5A and 5B  schematically illustrate forming trench isolations according to the first alternative of the present invention; 
         FIGS. 6A and 6B  schematically illustrate forming trench isolations according to the second alternative of the present invention; and 
         FIG. 7  is a schematic representation of a mixed substrate obtained by the manufacturing method according to the present invention and on which microelectronic devices are formed. 
     
    
    
     DETAILED DISCLOSURE OF PARTICULAR EMBODIMENTS 
     The present invention relates to a method for manufacturing a mixed (or hybrid) substrate from a starting substrate. 
     As represented in  FIG. 2 , the mixed substrate  10  comprises one or more first regions  20 , one or more second regions  30 , and a support substrate  40 . Especially, the one or more first regions  20  as well as the one or more second regions  30  lie on a main face  40   a  of the support substrate  40 . 
     The first region  20  comprises a first single crystal material  20   a  and the crystallographic orientation of which is dictated by the crystallographic orientation of the support substrate  40 . It will clearly appear in the following of the description that the support substrate  40 , since it dictates the crystal orientation of the first material  20   a , is also in a single crystal form. The first material can comprise Si or SiGe. 
     The second region  30  comprises at least one intermediate layer  31  made of a second material  31   a.    
     The intermediate layer  31  is intended to ensure a trapping function, and more particularly a function of trapping charges for example resulting from electrical resistivity of the second material  31   a.    
     This charge trapping effect enables crosstalk between adjacent microelectronic devices to be limited. 
     The second material  31   a  can, in this respect, comprise a polycrystalline material, and more particularly polycrystalline silicon. 
     According to one particularly advantageous embodiment, the second region  30  can also comprise a dielectric layer  32  and a useful layer  33 , lying on the second material and in this order. 
     By “useful layer”, it is meant a layer on or in which one or more devices, for example microelectronic devices, is or are to be formed. The useful layer can especially comprise a region for forming a transistor channel. 
     The mixed substrate also comprises spacers  50  electrically insulating the first regions  20  from the second regions  30 . 
     The method for manufacturing the mixed substrate is described, in the following of the presentation, in connection with  FIGS. 3A to 3E . 
     The method comprises a step a) consisting in providing a starting substrate  60 . 
     The starting substrate  60  is formed by a stack  15  of layers, especially comprising an intermediate layer  31  which lies on the main face  40   a  of the support substrate  40  ( FIG. 3A ). 
     “A stack”, according to the terms of the present invention comprises a layer or a stack of layers. A stack direction is then defined along an axis orthogonal to the main face  40   a  of the support substrate  40  and passing through it. A stack sense is also defined along the stack direction XX′ extending from the main face  40   a  of the support  40  to the last stacked layer. 
     According to one advantageous embodiment, the stack  15  can comprise, along the stack sense the intermediate layer  31 , a dielectric layer  32 , and a useful layer  33 . 
     The detailed description of the method for manufacturing a mixed substrate according to the present invention is limited to implementing a starting substrate  60  provided with the support substrate  40  and on a main face  40   a  of which, the intermediate layer  31 , the dielectric layer  32  and the useful layer  33  lie in this order (in other words, along the stack sense XX′). 
     Those skilled in the art with their general knowledge and the following description, will be able to easily adapt the invention to a starting substrate free of either or both of the dielectric layer  32  and useful layer  33 . 
     According to one particularly advantageous embodiment, the starting substrate  60  is a substrate of silicon on insulator integrating a so-called  501  trap Rich intermediate layer. 
     By way of example, the intermediate layer  31  is made of the second material  31   a  of polycrystalline silicon (“polysilicon”) with a thickness between 10 nm and 2000 nm, the dielectric layer  32  can comprise silicon dioxide and have a thickness between 10 nm and 145 nm (for example equal to 25 nm), and the useful layer  33  can comprise silicon and have a thickness between 3 nm and 30 nm (for example equal to 6 nm). 
     The method according to the present invention comprises a step b) of forming a mask  70  on the last layer of the stack  15  ( FIG. 3B ). 
     The mask  70  especially comprises one or more apertures  71  delimiting the location of the first region  20 . 
     The mask  70  can comprise a hard mask. 
     Forming the hard mask can involve the following sequence:
         depositing one or more masking layers  70   a , for example a layer of silicon nitride and/or silicon dioxide;   depositing a layer  70   b  of photolithographic resin overlaying the one or more masking layers  70   a;      a photolithography step for printing the pattern relating to the aperture  71 ; and   a step of etching the one or more masking layers  70   a  through the pattern in order to form the aperture  71 .       

     Step b) is followed by a step c) which consists in etching the useful layer  33 , dielectric layer  32  and intermediate layer  31  at the aperture  71  so as to form a cavity  72  in the starting substrate  60  ( FIG. 3C ). 
     The cavity  72  is delimited by a bottom  72   a  surmounted by walls  72   b  which extend from the useful layer  33  to at least the interface  40   b  between the intermediate layer  31  and support substrate  40 . In other words, the support substrate  40  has an exposed cross-section forming the bottom  72   a  of the cavity  72 . 
     According to one first alternative, the etching step c) can be stopped at the interface  40   b  formed between the intermediate layer  31  and support substrate  40 , or according to one second alternative it can be stopped in the bulk of the support substrate  40 . It is understood that implementing the second alternative enables an intact part of the support substrate  40  to be kept so as to form the bottom  72   a  of the cavity  72 . 
     The etching step c) can comprise dry etching, and especially plasma etching. For example, the dry etching step can be performed by means of a fluorocarbon plasma in a reactive ion etching equipment. 
     The method according to the present invention also comprises a step c1) of forming a protective layer  80 . According to one example of embodiment, the protective layer  80  is made of an amorphous material. As illustrated in  FIG. 3D , at the end of step c1), the protective layer  80  is located on the walls  72   b  delimiting the cavity  72  ( FIG. 3D ). 
     More particularly, the protective layer  80  is formed overlaying the walls  72   b  and leaves the bottom  72   a  of the cavity  72  exposed to external environment. More particularly, and advantageously, the protective layer  80 , at the end of step c1), exclusively, and advantageously totally, overlays the walls  72   b  of the cavity, and leaves the bottom  72   a  exposed to the external environment. 
     Step c1) can especially comprise forming a material, especially layer  80   a  of dielectric material, overlaying the bottom  72   a  and walls  72   b  of the cavity  72 , followed by removing said layer  80   a  of material at the bottom of the cavity  72  ( FIGS. 3C and 3D ). 
     According to one alternative, forming the layer  80   a  of material can comprise a step of depositing an electrically insulating material, such as a dielectric For example, depositing can be performed by an atomic layer deposition technique which enables conformal deposition. In this respect the layer  80   a  of material can comprise one of the materials selected from: Si3N4, SiO2, and SiCO. 
     The thickness of the protective layer  80 , subsequent to deposition can be between 5 nm and 15 nm, 
       FIGS. 4A and 4B  are schematic representations of another alternative implementation of step c1). More particularly,  FIG. 4A  schematically illustrates forming the layer  80   a  of material, whereas  FIG. 4B  relates to a removing step hereafter discussed. According to this other alternative, forming the layer  80   a  of material can be made by thermal oxidation ( FIG. 4A ). As the starting substrate  60  comprises an intermediate layer made of silicon, and more particularly polycrystalline silicon, the layer  80   a  of material comprises silicon dioxide. 
     Whatever the alternative considered for forming the protective layer  80 , removing the layer  80   a  of material at the bottom of the cavity  72  can be performed, as illustrated in  FIGS. 3D and 4B , by an anisotropic etching step. Anisotropic etching is known to those skilled in the art and is similar to that implemented for forming spacers on side faces of a transistor gate. 
     Step c1) of forming the protective layer  80  is followed by a step d) of forming the first region  20  by epitaxially and selectively growing the first material  20   a  in a single crystal form in the cavity  72  ( FIG. 3E ). Step d) can especially be performed for the first material  20   a  to entirely fill the cavity  72 . 
     It is understood that, insofar as the walls  72   b  of the cavity  72  are covered with the protective layer  80 , the growth, and especially crystal orientation, of the first material  20   a  is determined by the nature and crystal orientation of the support substrate  40  at the bottom  72   a.    
     The protective layer  80  (in an amorphous material form) limits occurrence of crystal defects during the step of epitaxially growing the first material. 
     According to one first alternative, the selective growth step can comprise a sequence comprising the steps consisting in: 
     d1) epitaxially growing a layer made of the first material  20   a  overlaying the bottom  72   a  and walls  72   b  of the cavity  72 ; and 
     d2) selectively etching the layer made of the first material  20   a  deposited on the walls  72   b  of the cavity  72  ( FIGS. 3E and 4C ). 
     The sequence comprising steps d1) and d2) can then be repeated as many times as necessary for the cavity  72  to be filled with the first single crystal material  20   a.    
     According to this first alternative, the aspect ratio of the cavity  72  is advantageously greater than one quarter. 
     By “aspect ratio”, it is meant the ratio of a characteristic dimension of the aperture of the cavity to the depth thereof. The width, according to the present invention, is measured from one wall of the cavity to the other along an axis orthogonal to the stack direction. Particularly, this characteristic dimension can be the side of a square, the length of a rectangle or even the diameter of a circle. However the invention is not limited to these shapes only, and those skilled in the art, in consideration of the given definition thereof, will be able to determine the dimension characteristic of other shapes. 
     According to one second alternative, the growth step d) comprises an epitaxial growth step for growing the first material  20   a  in a single crystal way from the bottom  72   a  of the cavity  72 , and in a polycrystalline or defective way from the protective layer  80 . 
     The growth step is additionally performed in the presence of a chemical agent for selectively etching the first material in a polycrystalline form in view of its single crystal form. 
     By way of example, forming a first region  20  made of single crystal silicon can entail a silane (SiH 4 ) or dichlorosilane (SiH 2 Cl 2 ) type precursor in the presence of a chemical agent which comprises hydrogen chloride (HCl). 
     This second alternative enables aspect ratios for the cavity  72  lower than one quarter to be considered. 
     Whatever the alternative considered, the first region  20  and second region  30  each have a surface exposed to external environment flush with each other. The method according to the present invention can also comprise a step e) of forming a trench isolation  90  filled with a dielectric material  91  for electrically isolating the at least one first region  20  from the at least one second region  30 . 
     Step e) particularly comprises forming a trench  90  around or along the first region  20 . 
     Especially,  FIGS. 5A and 6A  illustrate forming the trench  90  according to the first alternative and second alternative, respectively. 
     Forming the trench  90  can comprise the following steps of: 
     e1) forming a hard mask  100  of silicon dioxide and/or silicon nitride; and 
     e2) dry and anisotropic etching. 
     The second alternative ( FIG. 6A ) reveals a remaining amount  80   a  of the protective layer  80  at the end of step e). 
     The trench  90  is subsequently filled with a dielectric material  91 , for example silicon dioxide. 
     In this respect,  FIGS. 5B and 6B  illustrate filling the trench  90  according to the first alternative and the second alternative, respectively. 
     Filling can particularly comprise conformally depositing said dielectric material  91  followed by a chemical mechanical polishing step so as to keep the dielectric material only in the trench. 
     The hard mask can also be removed according to one selective etching step. 
     Finally, the method according to the present invention can also comprise a step of forming microelectronic devices  110  and  120 , on the first and second regions, respectively ( FIG. 7 ). 
     The mixed substrate obtained at the end of step d) is free of microelectronic component, and especially of transistor.