Patent Publication Number: US-2022238656-A1

Title: In-Situ Straining Epitaxial Process

Description:
PRIORITY INFORMATION 
     This application is a continuation of U.S. patent application Ser. No. 16/866,279 filed May 4, 2020 and entitled “In-situ Straining Epitaxial Process,” which is a divisional of U.S. patent application Ser. No. 14/174,588 filed Feb. 6, 2014 and entitled “In-situ Straining Epitaxial Process,” the disclosures of which are hereby incorporated by reference in the entirety. 
    
    
     BACKGROUND 
     The semiconductor integrated circuit (IC) industry has experienced rapid growth. In the course of IC evolution, functional density (i.e., the number of interconnected devices per chip area) has generally increased while geometry size (i.e., the smallest component (or line) that can be created using a fabrication process) has decreased. This scaling down process generally provides benefits by increasing production efficiency and lowering associated costs. Such scaling down has also increased the complexity of processing and manufacturing ICs and, for these advances to be realized, similar developments in IC manufacturing are needed. 
     For example, as semiconductor devices, such as metal-oxide-semiconductor field-effect transistors (MOSFETs), are scaled down through various technology nodes, strained source/drain features (e.g., stressor regions) have been implemented to enhance carrier mobility and improve device performance. For example, in an N-type metal oxide semiconductor (NMOS) device, the channel between the source and drain region can have increased carrier mobility through tensile strain. 
     Although existing approaches to forming stressor regions for IC devices have been generally adequate for their intended purposes, they have not been entirely satisfactory in all respects. Accordingly, it is desirable to have processes that create tensile stressed channels in an efficient manner. 
    
    
     
       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. 1A-1B  are diagrams showing an illustrative fabrication process for forming an in-situ straining defect during an epitaxial process, in accordance with some embodiments. 
         FIG. 2  is a diagram showing a cross-sectional view of a region formed by an epitaxial process, in accordance with some embodiments. 
         FIG. 3  is a diagram showing a relationship between pressure and the nature of the defects within the epitaxial region, in accordance with some embodiments. 
         FIG. 4  is a diagram showing a relationship between temperature and the nature of the defects within the epitaxial region, in accordance with some embodiments. 
         FIG. 5  is a flowchart showing an illustrative method for performing an in-situ straining epitaxial process, in accordance with some embodiments. 
     
    
    
     DETAILED DESCRIPTION 
     The following disclosure provides many different embodiments, or examples, for implementing different features of the provided subject matter. 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&#39;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. 
       FIGS. 1A-1B  are diagrams showing an illustrative fabrication process for forming an in-situ straining defect during an epitaxial process. The process begins with a substrate  102 . In one embodiment, the substrate includes silicon, although other materials, including combination of materials and/or layers may be used. 
     Structures  104  are provided over the substrate  102 . The structures may be made of a conductive material such as a metal or polysilicon material, or a stack of materials. In the present embodiment, the structures  104  are gates structures, include a gate dielectric, and are part of transistor devices. A transistor can be used as a switching device. Based on the voltage being applied to the gate  104 , electric current will be either allowed to flow between the source and drain region on both sides of the gate  104 , or it will be inhibited from flowing between the source and drain regions on either side of the gate. 
       FIG. 1A  illustrates the formation of a recess  106 . Continuing with the transistor embodiment discussed above, the recess  106  is formed where a source/drain region is to be formed. According to the present example, the recess  106  is formed within the substrate  102 , adjacent and between the two gate structures  104 . 
     The areas beneath the gates  104  where electric current flows between the source and drain regions are referred to as the channels  107 . Transistor devices may be p-type devices or n-type devices. In general, it is desirable to make the channels  107  more amenable to the type of carrier for a particular type of transistor device. For example, for n-type devices, the majority carrier is electrons and the minority carrier is electron holes. Thus, it is desirable to increase the carrier mobility for electrons in a channel  107  of an n-type device. 
     One way to increase the carrier mobility for electrons is to provide a tensile strain on the channels  107 . A tensile strain is a stretching force. To do this, the source or drain region adjacent to the gate may be formed as a straining region. 
       FIG. 1B  is a diagram showing an illustrative formation of the straining region  108  within the recess  106 . According to the present example, the straining region  108  is formed using an epitaxial process. The straining region  108  also includes a defect that is formed in-situ with the epitaxial process. As will be described in further detail below, the straining region  108  causes a tensile strain on the channel region  107  underneath the gate  104 . 
     According to the present example, the straining region  108  is formed using an epitaxial growth process. Such a process involves growing a crystal structure on top of a pre-existing crystal structure. Specifically, epitaxy refers to the deposition of a crystalline overlayer on a crystalline substrate. Additionally, the crystal structure being formed registers with the crystal structure of the substrate on which the epitaxial process is performed. Specifically, the deposited film may lock into one or more crystallographic orientations with respect to the substrate crystal. 
     The straining region  108  may be formed of a material such as silicon carbide (SiC). When silicon is mixed with carbon, it produces a material with a smaller lattice constant. A lattice constant is the distance between corresponding points of adjacent cells within the crystal structure. Because carbon is a smaller element than silicon, the lattice constant of a crystal structure of silicon carbide is smaller than the lattice constant of pure silicon. 
     The straining region  108  may also be doped with an n-type dopant such as phosphorous. The n-type dopant creates an n-type semiconductor which is used for an NMOS device. NMOS devices are typically used to complement P-type metal oxide semiconductor (PMOS) devices. 
     The epitaxial growth process can be designed so that when the straining region  108  is being formed, a defect is formed as well. The defect within the straining region  108  helps cause tensile stress on the channel region  107 . A defect within a crystal structure is an interruption in the pattern of the crystal. For example, a dislocation defect occurs when the pattern of the crystal gets shifted at a specific point. 
     In the present example, the defect includes a first dislocation  110  and a second dislocation. The first dislocation  110  and the second dislocation cross at a specific point, which will be referred to as the cross-point. The dislocations  110 ,  111  are formed in-situ. Thus, the dislocation does not exist within the substrate  102 . Rather, the dislocations  110 ,  111  start at the epitaxially grown straining region  108 . 
     In one example, the defects are caused during formation of the straining region  108  by tuning the epitaxial growth process. For example, the epitaxial growth process may be applied differently at different angles. For example, the epitaxial growth process may have a faster rate of application on the sides  116  of the recess than at the bottom  114  of the recess. This will cause the dislocations  110 ,  111  to form at the desired positions. Other differences between the epitaxial growth applied to the sides  116  and the epitaxial growth applied to the bottom  114  can cause the dislocation as well. 
     Additionally, various parameters of the epitaxial growth process can be used to tune the precise position of the dislocation and the nature of the dislocation. For example, variations in temperature, pressure, doping concentration, and precursor type may have an effect on the angle of the dislocation as well as the position of the cross-point. Variations in the dislocation can be used to apply a desired amount of tensile stress to the channel region  107 . 
     According to the present example, a silicon layer  112  is formed on top of the straining region  108 . The silicon layer  112  is also doped with the same type of dopant as the underlying straining region  108 , thus making it conductive. The silicon layer  112  is also formed using an epitaxial growth process. Thus, the silicon layer  112  is grown on the crystal of the straining region  108 . 
     The silicon layer  112  may be used to provide better contact to a later-applied conductive material. For example, typically, the gate device, and source/drain regions are connected to interconnect layers above through conductive vias. These vias are made of an electrically conductive material such as metal. A better connection to the source/drain region, which in this case is the straining region  108 , can be provided through use of the silicon layer  112 . 
       FIG. 2  is a diagram showing a cross-sectional view of a straining region  200  formed by an epitaxial process, such as the straining region  108  of  FIG. 1B . According to the present example, the recess is formed at a specified depth  202  and a specified width  204 . Thus straining region  200  formed within that recess thus shares that depth  202  and width  204 . 
     The straining region  200  includes a defect of two dislocations  216 ,  217 . The dislocations cross at a specific point, which will be referred to as the cross-point  206 . The distance between the cross-point  206  and the bottom of the straining region is indicated by dotted line  208 . The distance between the cross-point  206  and the top of the straining region  200  is indicated by dotted line  210 . The distance between the cross-point  206  and the sides of the straining region  200  are indicated by dotted lines  212  and  214 . 
     The location of the cross-point  206  may be adjusted in a variety of ways. For example, the length of dotted lines  212  and  214  may be adjusted by adjusting the width  204  of the recess, or the depth of the cross-point  206 . The depth of the cross-point  206  may be adjusted by adjusting the angle of the dislocations  216 ,  217 . 
     In general, the dislocations  216 ,  217  may be at an angle such that the distance between the cross-point  206  and either side is equal. In other words, the x-shape made by the dislocations is symmetrical. But, in some cases, the angles of the dislocation may not be exactly similar, which will cause lengths  212  and  214  to be of slightly different sizes. 
     In some examples, it may be desirable to have the cross-point  206  at a relatively low depth. Thus, instead of the cross-point  206  being closer to a top of the straining region  200 , the cross-point  206  may be closer to the bottom of the straining region  200 . Accordingly, the length of dotted line  208  is less than the length of dotted line  210 . The depth of the cross-point  206  may be adjusted to better provide the desired tensile strain on the channel region (e.g.  107 ,  FIG. 1 ). 
     The location of the cross-point  206  may be tuned by adjusting various parameters of the epitaxial growth process. For example, various parameters such as temperature, pressure, doping concentration, materials used, and precursor type may affect the angle at which the dislocations are formed in-situ. Thus, the location of the cross-point  206  can be adjusted as desired based on changing such parameters. 
     The location of the cross-point  206  can be adjusted by changing the doping concentration of the straining region  200 . The straining region  200  is part of the source/drain region and should thus be doped accordingly. Various dopants can be added to the epitaxial growth process. Specifically, n-type dopants may be used, such as phosphorous. Other n-type dopants are contemplated. 
     The location of the cross-point  206  can also be adjusted by changing the ratio of silicon to an element with a smaller crystal lattice. For example, as mentioned above, the straining region may be made of a material such as silicon carbide. Because carbon is a smaller element, the crystal structure of silicon carbide has a smaller lattice constant. This helps cause the desired tensile stress on the channel region. The ratio of silicon to carbon in such a compound can affect the nature of the dislocations and thus the location of the cross-point  206 . 
     The location of the cross-point  206  may also be affected by the type of precursor. The precursor is typically a gas that provides the silicon atoms to be deposited onto the substrate during the epitaxial growth process. Various precursors include silane, dichlorosilane, trichlorosilane, and silicon tetrachloride. Different precursors may affect the angle of dislocation and thus the location of the cross-point  206 . 
       FIG. 3  is a diagram showing a relationship between pressure and the nature of the defects within the epitaxial straining region. In this example, the vertical axis  302  represents length. The horizontal axis  304  represents pressure. The smaller dotted line  306  represents the distance (e.g.  210 ,  FIG. 2 ) from the cross-point to the top of the straining region. The larger dotted line  310  represents the distance (e.g.  208 ,  FIG. 2 ) between the cross-point and the bottom of the straining region. The solid line  308  represents the distance (e.g.  212 ,  214 ,  FIG. 2 ) between the cross-point  206  and the sides of the straining region. 
     Pressure is an important component of an epitaxial growth process. The pressure under which the epitaxial growth process is applied will affect the nature of the growth process itself. For example, the pressure may range between 0 and 700 torr. Other pressure ranges are contemplated. 
     As the pressure increases, the angle of the dislocations becomes shallower. Thus, the depth of the cross-point increases. Thus, the length represented by  306  increases. Additionally, the length represented by  310  decreases. Because the cross-point gets lower, the distance between the cross-point and the sides, represented by line  308  also gets smaller due to the shape of the recess. 
       FIG. 4  is a diagram showing a relationship between temperature and the nature of the defects within the epitaxial region. Specifically,  FIG. 4  is a graph  400  showing the relationship between temperature and the position of the cross-point. In this example, the vertical axis  402  represents length. The horizontal axis  404  represents temperature. The smaller dotted line  406  represents the distance (e.g.  210 ,  FIG. 2 ) from the cross-point to the top of the straining region. The larger dotted line  410  represents the distance (e.g.  208 ,  FIG. 2 ) between the cross-point and the bottom of the straining region. The solid line  408  represents the distance (e.g.  212 ,  214 ,  FIG. 2 ) between the cross-point and the sides of the straining region. 
     Temperature is also an important component of the epitaxial growth process. Specifically, the temperature at which the process is applied can affect the nature of the dislocation. In one example, the temperature at which the process is applied may range from about 600 degrees Celsius to about 800 degrees Celsius. Other temperature ranges are contemplated. 
     As the temperature of the epitaxial process increases, the location of the cross-point gets closer to the top of the straining region. Thus, the line  406 , representing the distance between the cross-point and the top of the straining region decreases. Accordingly, the line  410  representing the distance between the cross-point and the bottom of the straining region increases. Because the cross-point is rising, the distance between the cross-point and the sides of the straining region, represented by line  408 , also increases due to the shape of the straining region. 
       FIG. 5  is a flowchart showing an illustrative method for performing an in-situ straining epitaxial process. According to the present example, the method  500  includes a step  502  for forming a recess in a semiconductor substrate, the recess being adjacent to a gate stack. The gate stack may also be placed above a channel region. The recess may be formed through various fabrication processes such as etching. 
     According to the present example, the method  500  further includes a step  504  for performing an epitaxial growth process within the recess to form a straining region. The straining region may be doped with an n-type dopant. Additionally, the straining region may be made of a semiconductor material such as silicon carbide. 
     According to the present example, the method  500  further includes a step  506  forming a defect within the straining region in-situ with the epitaxial growth process. The defect may include two dislocations that cross to form an x-shape. The cross-point of the x-shape may be positioned as desired based on tuning various parameters of the epitaxial growth process. For example, various parameters such as pressure, temperature, doping concentration, and precursor type may tuned in order to provide the desired characteristics of the defect. 
     According to one embodiment, a method includes forming a recess in a semiconductor substrate, the recess being adjacent to a gate stack, performing an epitaxial growth process within the recess to form a straining region, forming a defect within the straining region in-situ with the epitaxial growth process. 
     According to one embodiment, a method includes, forming a recess in a semiconductor substrate, the recess being adjacent to a gate stack, the gate stack positioned above a channel. The method further includes performing an epitaxial growth process within the recess to form a straining region to cause a tensile strain on the channel, forming a defect within the straining region in-situ with the epitaxial growth process, the defect comprising at least two dislocations that form a cross-point within the straining region. The position of the cross-point is tuned by adjusting parameters of the epitaxial growth process. 
     According to one embodiment, a semiconductor device includes an epitaxial straining region formed within a semiconductor substrate, the straining region being positioned adjacent to a gate stack, the gate stack being positioned above a channel. The straining region comprises a defect comprising two crossing dislocations such that a cross-point of the dislocations is closer to a bottom of the straining region than to a top of the straining region. The straining region comprises an element with a smaller lattice constant than a material forming the substrate. 
     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.