Patent Publication Number: US-2023154802-A1

Title: Finfet epi channels having different heights on a stepped substrate

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation of U.S. Pat. Application No. 16/696,327, filed on Nov. 26, 2019, which is a divisional of U.S. Pat. Application No. 15/332,055 (now U.S. Pat. No. 10,510,618, issued Dec. 17, 2019), entitled “FinFET EPI Channels Having Different Heights on a Stepped Substrate,” filed on Oct. 24, 2016, which applications are incorporated herein by reference. 
    
    
     BACKGROUND 
     With the increasing down-scaling of integrated circuits and the increasingly demanding requirements to the speed of integrated circuits, transistors need to have higher drive currents with increasingly smaller dimensions. Fin Field-Effect Transistors (FinFET) were thus developed. The FinFETs include vertical semiconductor fins above a substrate. The semiconductor fins are used to form source and drain regions, and to form channel regions between the source and drain regions. Shallow Trench Isolation (STI) regions are formed to define the semiconductor fins. The FinFETs also include gate stacks, which are formed on the sidewalls and the top surfaces of the semiconductor fins. Although existing FinFET devices and methods of fabricating FinFET devices have been generally adequate for their intended purposes, they have not been entirely satisfactory in all respects. For example, a more flexible integration for forming fin and isolation structures is desired. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Aspects of the present disclosure are best understood from the following detailed description when read with the accompanying figures. It is noted that, in accordance with the standard practice in the industry, various features are not drawn to scale. In fact, the dimensions of the various features may be arbitrarily increased or reduced for clarity of discussion. 
         FIGS.  1 - 8    illustrate cross-sectional views of intermediate stages in the manufacturing of fins of a FinFET, in accordance with some embodiments. 
         FIGS.  9 - 11    illustrate alternative arrangements of certain stages in the manufacturing of fins of a FinFET, in accordance with some embodiments. 
     
    
    
     DETAILED DESCRIPTION 
     The following disclosure provides many different embodiments, or examples, for implementing different features of the invention. Specific examples of components and arrangements are described below to simplify the present disclosure. These are, of course, merely examples and are not intended to be limiting. For example, the formation of a first feature over or on a second feature in the description that follows may include embodiments in which the first and second features are formed in direct contact, and may also include embodiments in which additional features may be formed between the first and second features, such that the first and second features may not be in direct contact. In addition, the present disclosure may repeat reference numerals and/or letters in the various examples. This repetition is for the purpose of simplicity and clarity and does not in itself dictate a relationship between the various embodiments and/or configurations discussed. 
     Further, spatially relative terms, such as “beneath,” “below,” “lower,” “above,” “upper” and the like, may be used herein for ease of description to describe one element or feature’s relationship to another element(s) or feature(s) as illustrated in the figures. The spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. The apparatus may be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein may likewise be interpreted accordingly. 
     Fin Field-Effect Transistors (FinFETs) and methods of forming the same are provided in accordance with various example embodiments. The intermediate stages of forming the fins of a FinFET in accordance with some embodiments are illustrated. Variations of the embodiments are discussed. Throughout the various views and illustrative embodiments, like reference numbers are used to designate like elements. 
     FinFETs that have been epitaxially grown can be complex because different device types can require different fin materials. For example, a p-type fin can call for a growth of silicon germanium (SiGe), while an n-type fin can call for a growth of another semiconductor-based material or less of silicon germanium than the p-type fin. The differences may be due to the requirements of a punch through stop region at the base of the fin, where in a p-type fin subthreshold leakage current would tend to present a greater problem than in an n-type fin. Epitaxy of different materials can result in different growth rates. Or even if the fins were made of the same materials over a crystalline substrate, varying the height of the epitaxy can be difficult for different fin types or different fin applications. 
     Embodiments of the present disclosure allow for two or more fins of a FinFET device to be epitaxially grown on a substrate using the same materials in each fin, but producing a different lattice constant in one fin verses the other and different strain profiles in each fin. This allows one fin to be used as an n-type fin in an n-type semiconductor area of a semiconductor device and another fin to be used as a p-type fin in a p-type semiconductor area of a semiconductor device. A FinFET including n-type fins can be used to manufacture a p-channel metal oxide semiconductor (PMOS) using a p-type doped substrate. A FinFET including p-type fins can be used to manufacture an n-channel metal oxide semiconductor (NMOS) using an n-type doped substrate. The two types of fins (the NMOS and PMOS fins) can be used together to manufacture a complimentary metal oxide semiconductor (CMOS) FinFET device. Further, although the two fins have different effective heights, their top most surfaces are aligned, thereby allowing for formation of other components of a FinFET as if the fins were the same height. 
       FIG.  1    illustrates a FinFET semiconductor structure at an early stage of production. Semiconductor substrate  101  can be part of a semiconductor wafer or a semiconductor device. In accordance with some embodiments of the present disclosure, semiconductor substrate  101  includes crystalline silicon. Other materials that can be used in the substrate  101  include carbon, germanium, gallium, boron, arsenic, nitrogen, indium, and/or phosphorus, and the like. Semiconductor substrate  101  may also include other semiconductor materials such as III-V compound semiconductor materials. Semiconductor substrate  101  can be a bulk substrate or a Semiconductor-on-Insulator (SOI) substrate. Furthermore, the substrate  101  may include other features. For example, the substrate may include various doped regions depending on design requirements (e.g., p-type substrate or n-type substrate). For example, the doped regions may be doped with p-type dopants, such as boron or BF 2 , n-type dopants, such as phosphorus or arsenic, and/or combinations thereof. The doped regions may be configured for an n-type FinFET, or alternatively configured for a p-type FinFET. 
     Element  105  of  FIG.  1    illustrates the addition of a photoresist layer or silicon oxide resist layer  105  that is deposited over substrate  101  and then patterned, thereby forming openings in the resist layer  105 . Generally, photolithography techniques utilize a photoresist material (such as photo resist  105 ) that is deposited, irradiated (exposed), and developed to remove a portion of the photoresist material. The remaining photoresist material protects the underlying material, such as the substrate  101  in this example, from subsequent processing steps, such as etching. In this example, the photoresist material is patterned to define an area of the substrate  101  that will be etched and, conversely, an area of the substrate  101  that will be protected from etchant. 
     Referring to  FIG.  2   , the silicon substrate  101  is etched to form a stepped substrate  103 . Stepped substrate  103  can have a lower step  103   a  and an upper step  103   b . The lower step  103   a  has a height less than the height of the upper step  103   b . The difference in the height of  103   a  and  103   b  is h1. The height h1 will vary by application. In some embodiments h1 is between about 15 nm and 100 nm, for example about 20 nm. In other embodiments the step height difference h1 can be up to and including about 1 µm. The step height h1 can be optimized based on different fin height requirements. The etchant used to etch opening  109  in substrate  101  to form stepped substrate  103  can be a dry etch, a wet etch, a plasma etch and so forth. In some embodiments, the silicon etch can be chlorine gas (Cl 2 ) or hydrogen bromide (HBr). Although an anisotropic etch is desirable, which can more closely result in the stepped substrate illustrated in  FIG.  2   , in some embodiments, the step can be angled toward or away from the upper step  103   b . In other words the angle θ can be about 90 degrees for an anisotropic etch, as illustrated, or can be more or less than 90 degrees in some embodiments. For example, the angle θ can be in a range of about 75 degrees to about 130 degrees, depending on the etchant used and step height h1. 
     Following the etching of the substrate  101  to form the stepped substrate  103 , photoresist mask layer  105  can be removed. Various techniques can be used to remove photoresist mask layer  105 , such as by a Chemical Mechanical Polish (CMP) process, photoresist solvent, or etch. 
     Turning to  FIG.  3   , a hard mask  113  can be disposed over the stepped substrate  103 . The hard mask can be made of any suitable material that has a high selectivity to etchants, such as silicon oxide (SiO), silicon nitride (SiN), or silicon oxycarbide (SiOC). Other suitable materials for hard mask  113  can include silicon carbide (SiC), Tantalum Nitride (TaN), Titanium Nitride (TiN), a low-k dielectric material (such as SiO2, SiOCH, borophosphosilicate glass (BPSG), TEOS, spin-on glass (SOG), undoped silicate glass (USG), fluorinated silicate glass (FSG), high-density plasma (HDP) oxide, plasma-enhanced TEOS (PETEOS), fluorine-doped silicon oxide, carbon-doped silicon oxide, porous silicon oxide, porous carbon-doped silicon oxide, organic polymers, or silicone based polymers), or spin-on carbon (SOC). In some embodiments, after deposition or formation of hard mask  113 , a top surface of the hard mask  113  can be planarized, for example, by a CMP process. 
     Hard mask  113  can be applied through a single step or multi-step process. Such process(es) can include deposition, such as Sub Atomic Chemical Vapor Deposition (SACVD), Flowable Chemical Vapor Deposition (FCVD), Chemical Vapor Deposition (CVD), Atomic Layer Deposition (ALD), and the like. In some embodiments, hard mask  115  can be applied as a liquid and then cured, heated, or annealed. For example, in embodiments where hard mask  113  is SOC, the SOC material may be dispensed as a liquid, and may be formed from raw materials that include elements such as C, H, O, N, F, Br, and S. The SOC material may be between about 50% and about 95% carbon. 
     In some embodiments, hard mask  113  can be deposited in a first step to fill the opening  109  at a thickness of about h1 and in a second step to a thickness of about h2. The thickness h2 can be equal to or slightly more than a desired minimum fin height, where the minimum fin height is measured from a top surface of the upper step  103   b  of stepped substrate  103  to a top surface of a completed fin. 
     A second resist layer  117  of  FIG.  3    can be deposited over hard mask  113 . Second resist layer  117  can be a photoresist layer that is deposited over hard mask  113  and then patterned, thereby forming openings  121   a  and  121   b  in the resist layer  117 . The remaining photoresist material protects the underlying material, such as the hard mask  113 , from subsequent processing steps, such as etching. The photoresist material is patterned to define openings  121   a  and  121   b , which correspond to where fins will be formed over the stepped substrate  103 . 
     Referring to  FIG.  4   a   , using resist layer  117 , recesses  125   a  and  125   b  can be formed in hard mask  113 . In some embodiments, such as illustrated in  FIG.  4   a   , recess  125   a  can be vertically aligned with the rise of the step of stepped substrate  103 , such that one side of recess  125   a  comprises the rise of the step. In other embodiments, such as illustrated in  FIG.  4   b   , recess  125   b  can be vertically aligned with the rise of the step of stepped substrate  103 , such that one side of recess  125   b  coincides with the top edge of the upper step  103   b . In other embodiments, such as illustrated in  FIG.  4   c   , recesses  125   a  and  125   b  can be situated on either side of a portion of hard mask  113  such that neither is coincident with the step rise of stepped substrate  103 . In such embodiments, the portion of hard mask  113  that lies between recess  125   a  and  125   b  can overlap the step rise. 
     As illustrated in  FIG.  5   , a material  129   a  and  129   b  can be formed within the recess  125   a  and  125   b . In some embodiments, the material can be formed by epitaxial growth from the bottom of the recess, i.e., at an uppermost surface of the stepped substrate portion  113   a  and an uppermost surface of the stepped substrate portion  113   b . The material used in the formation of epitaxy  129   a  and  129   b  is the same in both recesses. Because the material formed in each recess is the same, the cost and complexity of growing the material  129   a  and  129   b  can be reduced. Also, because the material is the same, the growth rates of  129   a  and  129   b  will be similar, subject to normal variations. This results in the height of material  129   a  and  129   b  being approximately the same when measured from the stepped substrate to an uppermost surface of each of the respective materials  129   a  and  129   b . As the material is epitaxially grown, the resulting material is crystalline. Because the stepped substrate  103  is comprised of crystalline silicon, it can help seed the epitaxy materials  129   a  and  129   b . 
     The height and width of the epitaxy materials  129   a  and  129   b  can depend on the application and fin technology. For example, the epitaxy materials  129   a  and  129   b  can be grown to be about 25 nm to about 2 µm tall and about 5 nm to about 1 µm wide. A ratio of height to width can be in a range of about 2:1 to about 10:1. For example, in some embodiments the height of the epitaxy materials  129   a  and  129   b  can be about 30 nm and the width about 8 nm. 
     In some embodiments, the material in  129   a  and  129   b  is silicon germanium (SiGe). Other materials that can be used depending on application are pure germanium (Ge) or elements taken from groups III-V or combinations and compositions thereof, and so forth. 
     In embodiments consistent with the structure illustrated in  FIG.  4   a   , the additional growth surface provided by the step rise of substrate  103  can provide a larger lattice constant due to strain relaxation in the material  129   a  grown in recess  125   a  than the material  129   b  grown in recess  125   b . 
     Where the material  129   a  has a larger lattice constant than the material  129   b , the fins can be used differently. For example, in one embodiment, the fin comprising material  129   a  can be used for an n-type transistor and the fin comprising material  129   b  can be used for a p-type transistor. The material  129   a  can be considered a virtual substrate and a strain supplier for the material deposited thereon. 
     In other embodiments, the fins can be used in other configurations according to a desired application. 
     Because the epitaxial growth of materials  129   a  and  129   b  is constrained by hard mask  113  on either side of recess  125   a  and  125   b , as materials  129   a  and  129   b  epitaxially grow, outward lateral pressure (strain) can be exhibited against hard mask  113 . This outward pressure can cause or exacerbate an increased density of the hard mask  113  proximate to the growth area. In some embodiments, for example in  FIG.  5   , the left side of hard mask portion  113   a  will be affected to form higher density portions of  113   a  proximate to the material  129   a . Similarly, the right side of hard mask portion  113   c  will be affected to form higher density portions of  113   c  proximate to the material  129   b . Where the hard mask material  113   b  is between both material  129   a  and  129   b , the density can be even greater as pressure is exerted from both materials  129   a  and  129   b  on hard mask  113   b . 
     Referring to  FIG.  6   , next semiconductor-based epitaxy  133   a  and  133   b  can be epitaxially grown over materials  129   a  and  129   b . Because the recess  125   a  and  125   b  of  FIG.  5    have different depths, the semiconductor-based epitaxy is overgrown in the recesses to form faceted portions  134   a  and  134   b . The semiconductor-based epitaxy can be referred to as a semiconductor epitaxy and can include materials such as silicon (Si), silicon geranium (SiGe), germanium (Ge), germanium alloys such as GeSn, and other group III-IV compounds. The epitaxy in  129   a  and  133   a  are made of different materials. Using different materials with different lattice constants will induce tensile/compressive stress in the channel region. The width of the semiconductor-based epitaxy  133   a  and  133   b  will follow the profile of recess  125   a  and  125   b  and the height of the semiconductor-based epitaxy  133   a  and  133   b  can be greater than the height of the depth of the recess  125   a  and  125   b  after the epitaxial growth of material  129   a  and  129   b , such as illustrated in  FIG.  5   . In some embodiments, the semiconductor-based epitaxy  133   a  may not overgrow the recess  125   a . 
     As illustrated in  FIG.  7   , the top surface of the hard mask layer  113  can be planarized to make a top surface of the semiconductor epitaxy  133   a  and  133   b  coplanar with each other and coplanar with hard mask layer  113 . Planarization can be accomplished, for example by a CMP process. Planarization can remove none or some of hard mask layer  113 . In some embodiments, after planarization, the height h3 can be less than the height h2 of  FIG.  3   . In other embodiments, after planarization, the height h3 can be substantially equal to the height h2 of  FIG.  3   . 
     The epitaxy material  129   a  and  129   b , such as SiGe, applies compressive stress to the adjacent area of the recess  125   a  and  125   b  with positive strain values. The epitaxy material  129   a  and  129   b  also induces tensile stress to the upper area (the semiconductor-based epitaxy  133   a  and  133   b ) with negative strain values. The induced tensile stress can result in mobility in the upper area of the channel  141   a . In the channel  141   b  the semiconductor-based epitaxy  133   b  is much smaller in volume/height than the semiconductor-based epitaxy  133   a  in the channel  141   a , resulting in a strain differential between the strained semiconductor-based epitaxy  133   a  for an n-type fin (such as an NMOS fin) and the epitaxy material channel  129   b  (e.g., SiGe) for a p-type fin (such as a PMOS fin). The semiconductor-based epitaxy cap  133   b  remaining over the epitaxy material  129   b  can provide a layer for increased resiliency to high temperatures that can be subjected to the FinFET structure in subsequent steps, such as annealing. 
     Following planarization of the structure, as illustrated in  FIG.  8   , the hard mask material  113  can be removed to reveal fins  141   a  and  141   b . The hard mask material  113  can be removed using an etchant. The etchant can include a wet etchant or a dry etchant. In some embodiments, the silicon etch can be an oxygen (O 2 ) plasma, N 2 H 2  plasma, or other suitable etchant. In some embodiments, the etchant can remove most of the hard mask material  113 , however, some hard mask material  137  can remain behind. For example, in some embodiments, where the density of the hard mask material  113  was increased due to the epitaxy material  129   a  and  129   b  of  FIG.  5   , some high density hard mask material  137  can remain behind. In some embodiments, all of the hard mask material  113  can be removed. 
     In embodiments where high density hard mask  137  remains behind after etching hard mask  113 , the high density hard mask can act as an isolation region  137  with similar properties to a shallow trench isolation (STI) region. In embodiments where all the hard mask material is removed, an isolation material  137  can be deposited or formed between fins  141   a  and  141   b . 
     In some embodiments, additional isolation material can be deposited (not shown), for example to the right side of fin  141   a  and to the left side of fin  141   b . 
     Although isolation region  137  is shown as having a flat top surface which is coincidental to a transition between epitaxy material  129   a  and semiconductor epitaxy  133   a , in some embodiments the isolation region  137  can have a curved top surface, for example a u-shaped top in cross-section view. In some embodiments, the isolation region  137  can have a height h5 with a top surface that is higher or lower than the transition between epitaxy material  129   a  and semiconductor epitaxy  133   a . In some embodiments, where the top surface of the isolation region  137  is lower than the transition between epitaxy material  129   a  and semiconductor epitaxy  133   a , additional isolation material can be added over isolation region  137 . 
     The resulting structure of  FIG.  8    includes a first fin (channel)  141   a  and a second fin (channel)  141   b . The fins have top surfaces that are aligned at a same height h4 from the lower step substrate  103   a  top surface. The overall height of the first fin  141   a  is a height of h4. The overall height of the second fin  141   b  is a height of h3. Each fin comprises a same material  129   a  and  129   b , however  129   a  and  129   b  can have different lattice constants. Each fin also comprises a semiconductor-based epitaxy  133   a  and  133   b . An isolation region  137  remains between the two fins. In some embodiments, the fin  141   a  can form an NMOS strained silicon (Si) channel. The fin  141   b  can form a PMOS silicon germanium (SiGe) channel (or other epitaxy material channel, depending on design and application). 
     By differentiating fin heights in different device regions, the junction window is increased, which means that the fin heights of a FinFET in different device regions are no longer tied together. With the FinFET having different fin heights in different device regions, tuning the performance of devices in different device regions can be more easily accomplished. For example, in one embodiment fin  141   a  is an n-type fin in a device region coincident with  103   a  of the FinFET and fin  141   b  is a p-type fin in a device region coincident with  103   b  of the FinFET. The effective fin height h4 of n-type fin  141   a  is greater than the fin height h3 of p-type fin  141   b . Accordingly, n-type fin  141   a  and p-type fin  141   b  may be used in a same logic area. For example, n-type fin  141   a  may be a pull-up transistor, and p-type fin  141   b  may be a pull-down transistor. The greater fin height of n-type fin  141   a  may compensate for the lower hole mobility compared to the higher electron mobility of p-type fin  141   b . The performance of p-type fin  141   b  and the performance of n-type fin  141   a  may thus be balanced. 
     In another example, in accordance with some embodiments, fin  141   a  and fin  141   b  can be used to manufacture a complementary metal oxide semiconductor (CMOS) FinFET device. However, because each fin can comprise the same materials, the complexity of manufacturing the CMOS device can be reduced. The CMOS device manufactured from the structure in  FIG.  8    can have a semiconductor n-region defined by the lower step  103   a  of the stepped substrate  103  and a semiconductor p-region defined by the upper step  103   b  of the stepped substrate  103 . A boundary between the n-region and p-region can be defined by the step rise. 
     In some embodiments, a CMOS device can be made from the structure in  FIG.  8    by adding a gate insulator and gate over the fins and source/drain regions. Because the tops of the fins  141   a  and  141   b  are aligned, the addition of other component parts of the FinFET can be accomplished more easily than if the tops were not aligned. 
     It should be noted that while  FIG.  8    shows the FinFET structure as including two fins (e.g., fins  141   a  and  141   b ), this is merely an example. One skilled in the art will recognize there may be many modifications, alternatives and variations. For example, the semiconductor device of  FIG.  8    may accommodate any number of fins depending on different applications and design needs. 
       FIGS.  9 - 11    illustrate various other embodiments consistent with the description illustrating additional FinFET arrangements, in accordance with some embodiments. 
       FIG.  9    illustrates a double CMOS structure according to some embodiments. The structure of  FIG.  9    can be created in the same way as previously described. The structure of  FIG.  9    includes a multi-stepped substrate  103 . The substrate  103  can be made from a bulk crystalline substrate or other suitable substrate, as described above. A photoresist can be disposed over the substrate above the  103   b  portions and the substrate etched to produce a recess over the  103   a  portion of the stepped substrate. The photoresist can be removed and a hard mask disposed over the stepped substrate. The hard mask can be formed over the substrate in a manner and of a material consist with that discussed above with respect to  FIG.  3   . A resist layer can be disposed over the hard mask layer and patterned to form openings corresponding to the fins  141   a  and  141   b  of  FIG.  9   . 
     The hard mask can be recessed down to the surfaces of the stepped substrate in a process consistent with that discussed above with respect to  FIG.  4   . A first epitaxy material  129   a  and  129   b  of  FIG.  9    can be grown in each of the recesses in a process consistent with that discussed above with respect to  FIG.  5   . The epitaxy material  129   a  can have a higher lattice constant than the epitaxy material  129   b . A semiconductor-based epitaxy  133   a  and  133   b  can respectively be grown over  129   a  and  129   b  in a process consistent with that described above with respect to  FIG.  6   . The semiconductor-based epitaxy  133   a  and  133   b  can overgrow the hard mask, forming a faceted structure above the hard mask. The structure can be planarized to remove the overgrowth and align a top surface of  133   a  and a top surface of  133   b . 
     The hard mask can then be removed. A high dense portion  147  can remain behind between the fins  141   a  and  141   b , such as described above with respect to  FIG.  8   . The region defined by  103   a  of  FIG.  9    has a width  w   1 . The width  w   1  can be wider or narrower than the perspective depicted. For example,  FIG.  10    illustrates an alternative embodiment with a width  w   2  which is much smaller than  w   1 . Otherwise, the structure depicted in  FIG.  10    can be manufactured in a manner consistent with that of  FIG.  9   . 
       FIG.  11    illustrates a double CMOS structure according to some embodiments. The structure of  FIG.  11    can be created in the same way as previously described above, for example, with respect to  FIG.  9   . The structure is similar to  FIG.  9   ’s structure except the etched substrate portion  103   a  appears on the outsides of the FinFET structure. The structure of  FIG.  11    includes a multi-stepped substrate  103  and four FinFET fins manufactured in a way similar to that described above with respect to  FIG.  9   . The width in cross section of the substrate upper step  103   b  can be more or less than the perspective illustrated, similar to that described above with respect to the width of the lower step  103   a  in  FIG.  10   . 
     In some embodiments, embodiments consistent with the  FIG.  9    and  FIG.  11    can be combined to form additional step substrate combinations. 
     Advantages of the present disclosure include providing a FinFET structure capable of having different strain values in each fin, while the fins can be made from the same materials and in the same process steps. Further, in some embodiments, the lattice constant in one fin can be different than the lattice constant of the same material in another fin. The FinFET can be used to provide an n-type fin channel to provide an NMOS and a p-type fin channel to provide a PMOS, which can be used to provide a CMOS FinFET device. The resulting fin structure can have different fin heights with different strain values and tensile strengths. The fin structure can be used to create a semiconductor device by adding a gate dielectric/insulator and gate material perpendicular to and over the fins. Source/drain regions can be added. In some embodiments the source or drain regions of the fins can be coupled together depending on application. 
     In accordance with some embodiments, a structure can include a stepped crystalline substrate that includes an upper step, a lower step, and a step rise. A first fin can include a crystalline structure having a first lattice constant. The first fin can be formed over the lower step. A second fin can include a crystalline structure having a second lattice constant, the second lattice constant being different than the first lattice constant. The second fin can be formed over the upper step apart from the first fin. 
     In accordance with some embodiments, a semiconductor structure can include a first fin and a second fin. The first fin can have a first height as measured from a substrate. The second fin can have a second height as measured from the substrate. The first height can be greater than the second height. Or, in other words, the first fin can be taller than the second fin. The uppermost surface of the first fin can be aligned with the uppermost surface of the second fin. 
     In accordance with some embodiments, a method includes etching a substrate to form a stepped substrate having an upper step and a lower step. A hard mask can be deposited over the stepped substrate. A first and second recess can be formed in the hard mask, with the first recess being over the lower step and the second recess being over the upper step. A first epitaxy material can be epitaxially grown in the first and second recess. The hard mask can be removed. 
     In accordance with some embodiments, a structure includes a first fin of a crystalline structure having a first lattice constant, the first fin formed over a lower step of a stepped substrate. The structure also includes a second fin of a crystalline structure having a second lattice constant, the second lattice constant being different than the first lattice constant, the second fin formed over an upper step of the stepped substrate, apart from the first fin. 
     In accordance with some embodiments, a semiconductor structure includes a first fin having a first height as measured from a first upper surface of a stepped substrate. The semiconductor structure also includes a second fin having a second height as measured from a second upper surface of the stepped substrate, where the first height is greater than the second height, and an uppermost surface of the first fin is aligned with an uppermost surface of the second fin. 
     In accordance with some embodiments, a device includes a first fin disposed on a lower surface of a stepped substrate, the first fin including a first epitaxial material and a second epitaxial material over the first epitaxial material, the first epitaxial material having a first thickness, the second epitaxial material having a second thickness. The device also includes a second fin disposed on an upper surface of a stepped substrate, the second fin adjacent the first fin, the second fin including a third epitaxial material, the third epitaxial material having a third thickness, the first epitaxial material being the same as the third epitaxial material, the first thickness being the same as the third thickness, where an upper surface of the first epitaxial material is offset an upper surface of the third epitaxial material. 
     In accordance with some embodiments, a structure includes a first fin formed over a lower step of a stepped substrate. The structure also includes a second fin formed over the upper step of the stepped substrate, a sidewall of the second fin aligned with a sidewall of a step rise to an upper step of the stepped substrate apart from the first fin. 
     In accordance with some embodiments, a semiconductor structure includes a first fin having a first height over a stepped substrate and a second fin having a second height greater than the first height over the stepped substrate, an uppermost surface of the first fin being aligned with an uppermost surface of the second fin. The semiconductor structure also includes a first insulating material disposed between the first fin and the second fin and a second insulating material disposed surrounding the first fin and the second fin, the first insulating material having a higher density than the second insulating material. 
     In accordance with some embodiments, a device includes a first fin disposed on a lower surface of a stepped substrate, the first fin including a first epitaxial material, the first epitaxial material having a first thickness. The device also includes a second fin disposed on an upper surface of a stepped substrate, the second fin including the first epitaxial material at the first thickness, an upper surface of the first epitaxial material of the first fin being offset an upper surface of the first epitaxial material of the second fin. 
     The foregoing outlines features of several embodiments so that those skilled in the art may better understand the aspects of the present disclosure. Those skilled in the art should appreciate that they may readily use the present disclosure as a basis for designing or modifying other processes and structures for carrying out the same purposes and/or achieving the same advantages of the embodiments introduced herein. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the present disclosure, and that they may make various changes, substitutions, and alterations herein without departing from the spirit and scope of the present disclosure.