# Patent

## Title
Inverted trapezoidal recess for epitaxial growth

## Abstract

A semiconductor device having an epitaxial layer a method of manufacture thereof is provided. The semiconductor device has a substrate with a trench formed therein and a recess formed below the trench. The recess has sidewalls with a (111) crystal orientation. The depth of the trench is such that the depth is greater than or equal to one-half a length of sidewalls of the recess. An epitaxial layer is formed in the recess and the trench. The depth of the trench is sufficient to cause dislocations formed between the interface of the semiconductor substrate and the epitaxial layer to terminate along sidewalls of the trench.

## Background
This application is a continuation of U.S. patent application Ser. No. 12/847,877, entitled “Inverted Trapezoidal Recess for Epitaxial Growth,” filed on, Jul. 30, 2010, which application is incorporated herein by reference.


TECHNICAL FIELD
This disclosure relates generally to semiconductor devices and, more specifically, to the use of an inverted trapezoidal recess for epitaxial growth.
BACKGROUND
Performance of semiconductor devices may often be increased by epitaxially growing other materials, such as group III-V materials, on a semiconductor substrate. The difference in the lattice structure between the epitaxial material and the semiconductor substrate causes a stress in the epitaxial layer. This stress in the epitaxial layer may improve the speed and performance of integrated circuits. For example, to further enhance transistor performance, transistors have been fabricated using strained channel regions located in portions of a semiconductor substrate. Strained channel regions allow enhanced carrier mobility to be realized, thereby resulting in increased performance when used for n-channel or for p-channel devices. Generally, it is desirable to induce a tensile strain in the channel region of an n-channel transistor in the source-to-drain direction to increase electron mobility and to induce a compressive strain in the channel region of a p-channel transistor in the source-to-drain direction to increase hole mobility.
During the epitaxial growth process, however, dislocations form in the interface between the epitaxial layer and the semiconductor material due to the difference in the lattice structures of the different materials. These dislocations extend from the interface through the epitaxial layer. In some instances, the dislocations may extend to a surface of the epitaxial layer. In situations such as these in which the dislocations extend to or near the surface, the dislocations may adversely affect the performance of the devices formed therein.



BRIEF DESCRIPTION OF THE DRAWINGS
For a more complete understanding of the embodiments, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:
FIGS. 1-4 illustrate intermediate stages of forming a semiconductor device having an inverted trapezoidal recess in accordance with an embodiment;
FIG. 5 illustrates geometries of a sidewall of a recess in accordance with an embodiment; and
FIG. 6 illustrates another embodiment of a semiconductor device having an inverted trapezoidal recess.



DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS
The making and using of the embodiments are discussed in detail below. It should be appreciated, however, that the present disclosure provides many applicable inventive concepts that can be embodied in a wide variety of specific contexts. The specific embodiments discussed are merely illustrative of specific ways to make and use embodiments, and do not limit the scope of this disclosure.
FIG. 1 illustrates a substrate 102 having isolation regions 104 formed therein in accordance with an embodiment. The substrate 102 may comprise bulk silicon, doped or undoped, or an active layer of a semiconductor-on-insulator (SOI) substrate. Generally, an SOI comprises a layer of a semiconductor material, such as silicon, formed on an insulator layer. The insulator layer may be, for example, a buried oxide (BOX) layer or a silicon oxide layer. The insulator layer is provided on a substrate, typically a silicon or glass substrate. Other substrates, such as a multi-layered or gradient substrate may also be used.
The isolation regions 104 may be formed by first forming trenches and then filling the trenches with a dielectric material. In an embodiment, a patterned mask (not shown), such as a photoresist mask and/or a hard mask, is formed on the substrate 102 using deposition and photolithography techniques. Thereafter, an etching process, such as a reactive ion etch (RIE) or other dry etch, an anisotropic wet etch, or any other suitable anisotropic etch or patterning process, is performed to form trenches in the substrate 102.
Once formed, the trenches are filled with a dielectric material thereby forming the isolation regions 104 as illustrated in FIG. 1. The dielectric material may comprise, for example, a thermal oxidation, a chemical vapor deposition (CVD) silicon oxide, or the like. It may also comprise a combination of materials, such as silicon nitride, silicon oxy-nitride, high-k dielectrics, low-k dielectrics, CVD poly-silicon, or other dielectrics. A planarization process, such as a chemical mechanical polish (CMP) or etch back step, may be performed to planarize an upper surface of the dielectric material and an upper surface of the substrate 102 as illustrated in FIG. 1.
FIG. 2 illustrates a trench 206 formed in the substrate 102 between the isolation regions 104. The trench 206 may be formed using, for example, an isotropic dry etch process. As illustrated in FIG. 2, the isotropic dry etch process removes the substrate 102 from between the isolation regions 104 for a depth A. As will be discussed in greater detail below with reference to FIG. 3, the depth A is controlled such that a ratio of the depth A to a length of a sidewall surface of a subsequently formed recess in the substrate 102 of greater than or equal to 0.5 is maintained.
FIG. 3 illustrates a second etch process to form an inverted trapezoidal recess 310 along the bottom of the trench 206 in accordance with an embodiment. As will be discussed in greater detail below, the recess 310 is formed in the substrate 102 such that the substrate 102 along sidewalls of the recess 310 has a {111} surface orientation. To achieve this {111} surface orientation for the recess sidewall, it may be desirable for the substrate 102 to have a (001) surface orientation. Thus, by using a substrate having a (001) crystal orientation and etching to expose the (111) plane of the substrate, the direction and propagation of the dislocations may be controlled to better provide an epitaxial layer with fewer dislocations on the surface.
The second etch process for the trench 206 may be performed using a crystal surface selective anisotropic wet etch process using, for example, tetra-methyl ammonium hydroxide (TMAH) solution in a volume concentration range of 1% to 10% and a temperature range of 15° C. to 50° C. In alternative embodiments, other crystal surface selective wet etching solutions, such as ammonium hydroxide (NH3OH), potassium hydroxide (KOH) or amine-based etching solution may also be used. The selective wet etch results in the {111} surfaces of silicon substrate 102 to be exposed along sidewalls of the trench 206. As illustrated in FIG. 3, this process results in an inverted trapezoidal recess.
FIG. 4 illustrates an epitaxial growth of a group III-V material 408 in the recess. As illustrated in FIG. 3, threading dislocations (illustrated generally by lines 410) extend in a direction normal to the {111} surface of the sidewalls. In an embodiment in which the group III-V epitaxial layer comprises GaN having a hexagonal crystal structure is grown on a (111) surface of the substrate 102, the threading dislocations 410 of the epitaxial material 408 extends in a (0001) direction of the GaN. When the threading dislocations intersect the (1-101) plane, however, the threading dislocations tend to change direction to the (1-100) direction, which is generally parallel to the {111} surface of the sidewalls of the recess.
Accordingly, the depth A of the trench 206 is chosen to allow the threading dislocations to terminate along sidewalls of the isolation regions 104, thereby providing a surface of the epitaxial material substantially free of threading dislocations. In order to achieve this structure, the depth A has a height that is greater than or equal to one-half the length of the sidewall (distance Y in FIG. 4) of the recess 310.
FIG. 5 illustrates the derivation of maximum depth dmax (the maximum theoretical value of depth A) in accordance with an embodiment, wherein like reference numerals refer to like elements. Plane A represents the plane (1-101) of the GaN epitaxial layer and plane B represents the plane (0001) of the GaN epitaxial layer. The angle between these two planes is known to be about 62°. Thus, the relationship between dmax and the distance X may be determined by the following equation:




  
    
      
        
          d
          max
        
        =
        
          X
          
            cos
            ⁡
            
              (
              
                54.7
                ⁢
                °
              
              )
            
          
        
      
    
    
      
        (
        
          Eq
          .
          
            
          
          ⁢
          1
        
        )
      
    
  




Additionally,

X=tan(62°)*0.5*Y  (Eq. 2)

Thus, combining Equations 1 and 2:





  
    
      
        
          d
          max
        
        =
        
          
            
              0.5
              *
              Y
              *
              
                tan
                ⁡
                
                  (
                  
                    62
                    ⁢
                    °
                  
                  )
                
              
            
            
              cos
              ⁡
              
                (
                
                  54.7
                  ⁢
                  °
                
                )
              
            
          
          =
          
            1.627
            *
            Y
          
        
      
    
    
      
        (
        
          Eq
          .
          
            
          
          ⁢
          3
        
        )
      
    
  




While the theoretical maximum value for the depth A is calculated above, it has been found the growth conditions may be optimized to result in smaller values for depth A. For example, it has been found that by optimizing the etching and growth conditions, values of depth A/Y of between 0.75 and 0.5 may be achieved. Accordingly, in an embodiment, the trench 206 has a depth A such that depth A/Y is greater than or equal to 0.5 with the threading dislocations (410) terminates along sidewalls of the trench 206. In an embodiment, the depth of the recess 310 is controlled to be smaller than about 200 nm by controlling the dry etching depth (etching time) and KOH or TMAH anisotropic etching depth (etching time, solution concentration and temperature). A buffer layer of AlN may be grown in the silicon trench before, for example, a GaN growth at growth temperature of about 1100° C., a pressure of about 100 mbar by using a Group-V source (e.g., NH3) and a Group-III precursor (e.g., trimethylaluminium), maintaining a low V/III ratio of about 650. A GaN may then be grown on the AlN buffer layer at growth temperature of about  ̃1120° C. and a pressure of about 200 mbar by using a Group-V source (e.g., NH3) and a Group-III precursor (e.g., trimethygalium), maintaining a relatively high V/III ratio of about 1500.
FIG. 6 illustrates another example of an inverted trapezoidal recess. In this example, the depth A extends for the complete thickness of the isolation regions 104. As a result, this embodiment allows for the trench 206, and hence the epitaxial material 408, to extend under the isolation regions 104. The thickness of the isolation regions 104 is adjusted to maintain the ratio described above.
It should be appreciated that the length Y1 of the recess sidewalls determines how propagation of the dislocations 410 toward the surface of the epitaxial layer. Accordingly, the greater the length Y1, the greater the thickness A of the isolation regions 104 to allow a sufficient depth to for the dislocations to terminate at the isolation regions rather than at an upper surface of the epitaxial material 408.
In an embodiment, a semiconductor device having an inverted trapezoidal recess is provided. A trench is formed in the semiconductor substrate and a recess is formed along the bottom of the trench such that sidewalls of the recess have a (111) crystal orientation. Sidewalls of the trenches may be formed of a different material than the semiconductor substrate, such as a dielectric material. A depth of the trench is such that a ratio of the depth of the trench to a length of the sidewalls of the inverted trapezoidal recess is equal to or greater than 0.5.
In another embodiment, a method of forming a semiconductor device having an inverted trapezoidal recess is provided. The method includes performing a first etch process to recess the substrate a first distance, thereby forming a trench. A second etch process is performed to recess the bottom of the trench such that sidewalls of the recess have (111) crystal orientations. A depth of the trench is at least one-half a length of the sidewalls of the recess.
In yet another embodiment, a semiconductor device is provided. The semiconductor device comprises a semiconductor substrate and isolation trenches extending into the semiconductor substrate, wherein a recess in the semiconductor substrate extends between the isolation trenches, the recess having a bottom surface and sidewalls, the recess having sidewalls extending at an obtuse angle relative to the bottom surface of the recess, surfaces of the sidewalls having a (111) crystal orientation. A compound semiconductor layer is formed in the recess.
In yet still another embodiment, a method of forming a semiconductor device is provided. The method includes forming trenches in a substrate and filling the trenches with a dielectric material. One or more etch processes are performed to form an inverted trapezoidal recess in the substrate between the trenches, the inverted trapezoidal recess having a bottom surface and sidewalls extending from the bottom surface to the dielectric material, the sidewalls of the inverted trapezoidal recess having (111) facet planes of the substrate, an angle between the bottom surface and the sidewalls being an obtuse angle. A compound semiconductor material is epitaxially grown in the recess.
In yet still another embodiment, a method of forming a semiconductor device is provided. The method includes performing a first etch process to form a first recess in a substrate, the first recess having first sidewalls, and performing a second etch process to extend the first recess into the substrate, the second etch process forming a bottom surface and second sidewalls extending from the bottom surface, the second sidewalls of the recess having a (111) crystal orientation, the first etch process being a different etch process from the second etch process. A compound semiconductor material is epitaxially grown on the substrate in the recess, the compound semiconductor material having dislocations extending from the second sidewalls of the recess and terminating along the first sidewalls.
Other embodiments are disclosed.
Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, and composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure of the present invention, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed, that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present invention. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps.

## Claims

1. A semiconductor device comprising:
a semiconductor substrate;
isolation trenches extending into the semiconductor substrate, wherein a recess in the semiconductor substrate extends between the isolation trenches, the recess having a bottom surface and sidewalls, the recess having sidewalls extending at an obtuse angle relative to the bottom surface of the recess, surfaces of the sidewalls having a (111) crystal orientation; and
a compound semiconductor layer in the recess, the compound semiconductor layer extending continuously between the isolation trenches and contacting each of the isolation trenches.

2. The semiconductor device of claim 1, wherein the semiconductor substrate has a (001) crystal orientation.

3. The semiconductor device of claim 1, wherein the compound semiconductor layer is a group III-V semiconductor layer.

4. The semiconductor device of claim 3, wherein the group III-V semiconductor layer comprises GaN.

5. The semiconductor device of claim 1, wherein the compound semiconductor layer comprises threading dislocations that extend from the sidewalls of the recess.

6. The semiconductor device of claim 5, wherein the threading dislocations terminate at sidewalls of the isolation trenches.

7. The semiconductor device of claim 1, wherein the sidewalls of the recess extend to a bottom of the isolation trenches.

8. A method of forming a semiconductor device, the method comprising:
forming trenches in a substrate;
filling the trenches with a dielectric material;
performing one or more etch processes to form an inverted trapezoidal recess in the substrate between the trenches, the inverted trapezoidal recess having a bottom surface and sidewalls extending from the bottom surface to the dielectric material, the sidewalls of the inverted trapezoidal recess having (111) facet planes of the substrate, an angle between the bottom surface and the sidewalls being an obtuse angle; and
epitaxially growing a compound semiconductor material in the inverted trapezoidal recess, the compound semiconductor material extending continuously and contacting the dielectric material in each of the trenches.

9. The method of claim 8, wherein the substrate comprises silicon.

10. The method of claim 8, wherein the substrate comprises a semiconductor-on-insulator.

11. The method of claim 8, wherein an upper surface of the substrate has a (001) crystal orientation.

12. The method of claim 8, wherein the compound semiconductor material is a III-V semiconductor.

13. The method of claim 12, wherein the III-V semiconductor comprises GaN.

14. The method of claim 8, wherein the one or more etch processes is performed at least in part by etching using a solution of ammonium hydroxide (NH3OH) or tetra-methyl ammonium hydroxide (TMAH).

15. A method of forming a semiconductor device, the method comprising:
performing a first etch process to form a first recess in a substrate, the first recess having first sidewalls;
performing a second etch process to extend the first recess into the substrate to form a second recess, the second recess having a bottom surface and second sidewalls extending from the bottom surface to the first sidewalls, the second sidewalls of the second recess having a (111) crystal orientation, the first etch process being a different etch process from the second etch process; and
epitaxially growing a compound semiconductor material on the substrate in the second recess, the compound semiconductor material having dislocations extending from the second sidewalls of the second recess and terminating along the first sidewalls.

16. The method of claim 15, wherein the first etch process comprises an isotropic etch process.

17. The method of claim 16, wherein the second etch process comprises using a solution of ammonium hydroxide (NH3OH) or tetra-methyl ammonium hydroxide (TMAH).

18. The method of claim 15, wherein the first etch process and the second etch process is performed between dielectric features.

19. The method of claim 18, wherein the second etch process extends the second recess under the dielectric features.

20. The method of claim 18, wherein the dislocations terminate at the dielectric features.