Patent Publication Number: US-2023135392-A1

Title: Isolation structures for semiconductor devices

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of U.S. Provisional Patent Application No. 63/275,689, titled “isolation Structures for Semiconductor Devices,” filed Nov. 4, 2021, the disclosure of which is incorporated by reference in its entirety. 
    
    
     BACKGROUND 
     With advances in semiconductor technology, there has been increasing demand for higher storage capacity, faster processing systems, higher performance, and lower costs. To meet these demands, the semiconductor industry continues to scale down the dimensions of semiconductor devices, such as metal oxide semiconductor field effect transistors (MOSFETs), including planar MOSFETs and fin field effect transistors (finFETs). Such scaling down has increased the complexity of semiconductor manufacturing processes. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Aspects of the present disclosure are best understood from the following detailed description when read with the accompanying figures. 
         FIG.  1    illustrates an isometric view of a semiconductor device having an isolation structure with a protection layer, in accordance with some embodiments. 
         FIG.  2    illustrates a schematic plan view of a semiconductor device having an isolation structure with a protection layer, in accordance with some embodiments. 
         FIGS.  3  and  4    illustrate cross-sectional views of a semiconductor device having an isolation structure with a protection layer, in accordance with some embodiments. 
         FIG.  5    is a flow diagram of a method for fabricating a semiconductor device having an isolation structure with a protection layer, in accordance with some embodiments. 
         FIGS.  6 - 12    illustrate isometric and cross-sectional views of a semiconductor device having an isolation structure with a protection layer at various stages of its fabrication, in accordance with some embodiments. 
     
    
    
     Illustrative embodiments will now be described with reference to the accompanying drawings. In the drawings, like reference numerals generally indicate identical, functionally similar, and/or structurally similar elements. 
     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 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. As used herein, the formation of a first feature on a second feature means the first feature is formed in direct contact with the second feature. In addition, the present disclosure may repeat reference numerals and/or letters in the various examples. This repetition 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. 
     It is noted that references in the specification to “one embodiment,” “an embodiment,” “an example embodiment,” “exemplary,” etc., indicate that the embodiment described may include a particular feature, structure, or characteristic, but every embodiment may not necessarily include the particular feature, structure, or characteristic. Moreover, such phrases do not necessarily refer to the same embodiment. Further, when a particular feature, structure or characteristic is described in connection with an embodiment, it would be within the knowledge of one skilled in the art to effect such feature, structure or characteristic in connection with other embodiments whether or not explicitly described. 
     It is to be understood that the phraseology or terminology herein is for the purpose of description and not of limitation, such that the terminology or phraseology of the present specification is to be interpreted by those skilled in relevant art(s) in light of the teachings herein. 
     In some embodiments, the terms “about” and “substantially” can indicate a value of a given quantity that varies within  20 % of the value (e.g., ±1%, ±2%, ±3%, ±4%, ±5%, ±10%, ±20% of the value). These values are merely examples and are not intended to be limiting. The terms “about” and “substantially” can refer to a percentage of the values as interpreted by those skilled in relevant art(s) in light of the teachings herein. 
     With continuous scaling down of the dimensions, semiconductor devices are placed in increasingly closer proximity and higher density, resulting in increased coupling effect between adjacent semiconductor devices in abutted active regions. The abutted active regions can be abutted well regions or abutted standard cells. The increased coupling effect results in significant noise increases, signal delays, logic errors, and even integrated circuit (IC) malfunctions. Isolation of the adjacent semiconductor devices in abutted active regions can help prevent the coupling effect, thereby improving IC performance. 
     The adjacent semiconductor devices in abutted active regions can be isolated by an isolation structure aligned with a middle line between the abutted active regions to reduce the coupling effect. In some embodiments, a continuous poly on oxide definition edge (CPODE) pattern can be used to form the isolation structure. The term “oxide definition” can define an active region located adjacent to the isolation structure. In some embodiments, the isolation structure can include a dielectric structure to isolate the abutted active regions. In some embodiments, the isolation structure can include a metal gate structure on a dielectric structure to improve polishing uniformity of metal gate structures in subsequent polishing processes, However, the isolation structure can have its challenges. The metal gate structure of the isolation structure is located close to source/drain (S/D) structures of adjacent semiconductor devices, The metal gate structure can extrude into the S/D structures due to an over etch during the formation of S/D structures. The over etch can be worse with an increase of metal gate structure dimensions and/or an overlay shift of the metal gate structure. 
     Various embodiments in the present disclosure provide example methods for forming a protection layer in an isolation structure between adjacent field effect transistors (FET) devices (e.g., finFETs, gate-all-around FETs, and MOSFETs) and/or other semiconductor devices in an IC. The example methods in the present disclosure can form an isolation structure on a substrate between adjacent semiconductor devices in abutted active regions. The isolation structure can include a dielectric structure, a gate structure, and a protection layer between the dielectric structure and the gate structure. The protection layer can be disposed between the gate structure and S/D structures of adjacent semiconductor devices to prevent the extrusion of the gate structure due to the over etch during the formation of the S/D structures, The dimensions of the protection layer and the dielectric structures can be controlled to minimize parasitic capacitances between the gate structure and the S/D structures, In some embodiments, the protection layer can reduce about 75% to about 95% extrusions of the gate structure in the isolation structure. 
       FIG.  1    illustrates an isometric view of a semiconductor device  100  having an isolation structure  103  with a protection layer  107 , in accordance with some embodiments. Semiconductor device  100  can have finFETs  102 A and  102 E separated by isolation structure  103 .  FIG.  2    illustrates a schematic plan view of semiconductor device  100  having isolation structure  103 , in accordance with some embodiments.  FIG.  3    illustrates a cross-sectional view of semiconductor device  100  having isolation structure  103  along line A-A in  FIG.  1   , in accordance with some embodiments.  FIG.  4    illustrates a cross-sectional view of semiconductor device  100  having isolation structure  103  along line B-B in  FIG.  1   , in accordance with some embodiments, Referring to  FIGS.  1 - 4   , semiconductor device  100  having finFETs  102 A and  102 B separated by isolation structure  103  can be formed on a substrate  104  and can include fin structures  108 . shallow trench isolation (STI) regions  106 , S/D structures  110 , gate structures  112 A- 112 C, fin sidewall spacers  109 , gate spacers  114 , etch stop layer (ESL)  116 , interlayer dielectric (ILD) layer  118 , fill structures  120 , and hard mask layer  122 . 
     As shown in  FIG.  3   , finFETs  102 A and  102 B can be formed on abutted active regions  111 A and  111 B, respectively. In some embodiments, active regions  111 A and  111 B can be abutted well regions or abutted standard cells. In some embodiments, finFETs  102 A and  102 B can be both n-type finFETs (NFETs). In some embodiments, finFET  102 A can be an NFET and have n-type S/D structures  110 . FinFET  102 B can be a p-type finFET (PFET) and have p-type S/D structures  110 . In some embodiments, finFETs  102 A and  102 B can be both PFETs. Though  FIGS.  1 - 4    show two finFETs  102 A and  102 B on abutted active regions  111 A and  111 B, semiconductor device  100  can have any number of finFETs in active regions  111 A and  111 B. In addition, semiconductor device  100  can be incorporated into an IC through the use of other structural components, such as contact structures, conductive vias, conductive lines, dielectric layers, passivation layers, and interconnects, which are not shown for simplicity. ESL  116  and ILD layer  118  are not shown in  FIG.  2    and ILD layer  118  is not shown in  FIG.  3    for simplicity. The discussion of elements of finFETs  102 A and  102 B with the same annotations applies to each other, unless mentioned otherwise. And like reference numerals generally indicate identical, functionally similar, and/or structurally similar elements. 
     Referring to  FIGS.  1 - 4   , substrate  104  can include a semiconductor material, such as silicon. In some embodiments, substrate  104  includes a crystalline silicon substrate (e,g., wafer). In some embodiments, substrate  104  includes (i) an elementary semiconductor, such as germanium; (ii) a compound semiconductor including silicon carbide, gallium arsenide, gallium phosphide, indium phosphide, indium arsenide, and/or indium antimonide; (iii) an alloy semiconductor including silicon germanium carbide, silicon germanium, gallium arsenic phosphide, gallium indium phosphide, gallium indium arsenide, gallium indium arsenic phosphide, aluminum indium arsenide, and/or aluminum gallium arsenide; or (iv) a combination thereof. Further, substrate  104  can be doped depending on design requirements p-type substrate or n-type substrate). In some embodiments, substrate  104  can be doped with p-type dopants (e.g., boron, indium, aluminum, or gallium) or n-type dopants (e.g,, phosphorus or arsenic). 
     STI regions  106  can provide electrical isolation between finFET  102 A and  102 B from each other and from neighboring finFETs (not shown) on substrate  104  and/or neighboring active and passive elements (not shown) integrated with or deposited on substrate  104 . STI regions  106  can be made of a dielectric material. In some embodiments, STI regions  106  can include silicon oxide, silicon nitride, silicon oxynitride, fluorine-doped silicate glass (FSG), a low-k dielectric material, and/or other suitable insulating materials. In some embodiments, STI regions  106  can include a multi-layered structure. In some embodiments, STI regions  106  can have a height  106 h along a Z-axis ranging from about 40 nm to about 80 nm. 
     Referring to  FIGS.  1 - 3   , fin structures  108  can be formed from patterned portions of substrate  104 . Embodiments of the fin structures disclosed herein may be patterned by any suitable method. For example, the fin structures may be patterned using one or more photolithography processes, including double-patterning or multi-patterning processes. Double-patterning or multi-patterning processes can combine photolithography and self-aligned processes, forming patterns that have, for example, pitches smaller than what is otherwise obtainable using a single, direct photolithography process. For example, a sacrificial layer is formed over a substrate and patterned using a photolithography process. Spacers can be formed alongside the patterned sacrificial layer using a self-aligned process. The sacrificial layer is then removed, and the remaining spacers can then be used to pattern the fin structures. 
     to  FIGS.  1 - 3   , fin structures  108  can extend along an X-axis and through finFETs  102 A and  102 B. Fin structures  108  can include active region  111 A and active region  111 B disposed on substrate  104 , as shown in  FIGS.  2  and  3   . In some embodiments, the formation of active regions  111 A and  111 B can include an implantation process. Active regions  111 A and  111 B can include the same type of dopant or different types of dopant. In some embodiments, fin structures  108  can include a semiconductor material similar to or different from substrate  104 . In some embodiments, fin structures  108  can include silicon. In some embodiments, fin structures  108  can include silicon germanium. In  FIGS.  1 - 3   , fin structures  108  under gate structures  112 A- 112 B can form channel regions of semiconductor device  100  and represent current carrying structures of semiconductor device  100 . 
     S/D structures  110  can be disposed on opposing sides of gate structures  112 A- 112 B and function as S/D regions of semiconductor device  100 . Referring to  FIGS.  1 - 3   , S/D structures  110  can be disposed on fin structures  110  on opposing sides of gate structures  112 A- 112 B. In some embodiments, S/D structures  110  can have any geometric shape, such as a polygon, an ellipsis, and a circle. In some embodiments, S/D structures  110  can include an epitaxially-grown semiconductor material the same as the material of fin structures  108 . In some embodiments, the epitaxially-grown semiconductor material can include a material different from the material of fin structures  108  and imparts a strain on the channel regions under gate structures  112 A- 112 B. Since the lattice constant of such epitaxially-grown semiconductor material is different from the material of fin structures  108 , the channel regions are strained to advantageously increase carrier mobility in the channel regions of semiconductor device  100 . The epitaxially-grown semiconductor material can include: (i) a semiconductor material, such as germanium and silicon; (ii) a compound semiconductor material, such as gallium arsenide and aluminum gallium arsenide; or (iii) a semiconductor alloy, such as silicon germanium and gallium arsenide phosphide. 
     In some embodiments, S/D structures  110  can include silicon and can be in-situ doped during an epitaxial growth process using n-type dopants, such as phosphorus and arsenic. In some embodiments, S/D structures  110  can include silicon, silicon germanium, germanium, or III-V materials (e.g., indium antimonide, gallium antimonide, or indium gallium antimonide) and can be in-situ doped during an epitaxial growth process using p-type dopants, such as boron, indium, and gallium. In some embodiments, S/D structures  110  can include one or more epitaxial layers and each epitaxial layer can have different compositions. 
     Referring to  FIGS.  1 - 4   , gate structures  112 A- 112 C can be multi-layered structures and can be disposed on fin structures  108  and protection layer  107 , respectively. Each of gate structures  112 A- 112 C can include an interfacial layer, a high-k gate dielectric layer, a work-function layer, and a gate electrode. The term “high-k” can refer to a high dielectric constant. In the field of semiconductor device structures and manufacturing processes, high-k can refer to a dielectric constant that is greater than the dielectric constant of silicon oxide (e.g., greater than about 3.9). In some embodiments, the interfacial layer can be disposed on fin structures  108  and can include silicon oxide. In some embodiments, the high-k gate dielectric layer can be disposed on the interfacial layer and can include hafnium oxide (HfO 2 ), zirconium oxide (ZrO 2 ), and other suitable high-k dielectric materials. The work-function layer can be disposed on the high-k gate dielectric layer and can include work-function metals to tune threshold voltage (V t ) of finFETs  102 A and  102 B. In some embodiments, the work-function layer can include titanium nitride, ruthenium, titanium aluminum, titanium aluminum carbon, tantalum aluminum, tantalum aluminum carbon, or other suitable work-function metals. In some embodiments, the work-function layer can include a single metal layer or a stack of metal layers. The stack of metal layers can include work-function metals having work-function values equal to or different from each other. The gate electrode can include titanium, tantalum, aluminum, cobalt, tungsten, nickel, ruthenium, and other suitable conductive materials. 
     Isolation structure  103  can be disposed on substrate  104  and can include a dielectric structure  105 , protection layer  107 , and gate structure  112 C. In some embodiments, isolation structure  103  can be aligned with a middle line between abutted active regions  111 A and  111 B to reduce the coupling effect between adjacent finFETs  102 A and  102 B, as shown in  FIGS.  2  and  3   . In some embodiments, dielectric structure  105  can extend into substrate  104  by a distance  105   d  along a Z-axis ranging from about 100 nm to about 150 nm to isolate abutted active regions  111 A and  111 B. If distance  105   d  is less than about 100 nm, dielectric structure  105  may not isolate abutted active regions  111 A and  111 B. If distance  105   d  is greater than about 150 nm, the isolation between abutted active regions  111 A and  111 B may not be further improved and the manufacturing cost may increase. 
     In some embodiments, dielectric structure  105  and STI regions  106  can be formed in the same process and can include the same dielectric material. In some embodiments, a recess  105   r  along a Z-axis between top surfaces of dielectric structure  105  and STI regions  106  can range from about 10 nm to about 30 nm. With recess  105   r , protection layer  107  can fully separate gate structure  112 C and S/D structures  110 . If recess  105   r  is less than about 10 nm, protection layer  107  may not prevent extrusion of gate structure  112 C into S/D structures  110  and fin structures  108 . If recess  105   r  is greater than about 30 nm, a dimension of protection layer  107  between gate structure  112 C and S/D structures  110  may increase and the parasitic capacitance between gate structure  112 C and S/D structures  110  may increase. In some embodiments, dielectric structure  105  can have sloped sidewall surfaces after actual processes, as shown in  FIG.  2   . A top portion of dielectric structure  105  can have a width  105   w  greater than that of a bottom portion of dielectric structure  105 . In some embodiments, width  105   w  of dielectric structure  105  can range from about 5 nm to about 20 nm. 
     Protection layer  107  can be disposed on dielectric structure  105  and sidewall surfaces of fin structures  108 . As shown in  FIGS.  1 - 4   , protection layer  107  can surround gate structure  112 C and prevent extrusions of gate structure  112 C into S/D structures  110  and fin structures  108 . In some embodiments, protection layer  107  can have a thickness  107   t  ranging from about 2 nm to about 10 nm. If thickness  107   t  is less than about 2 nm, protection layer  107  may not prevent extrusions of gate structure  112 C. If thickness  107   t  is greater than about 10 nm, a dimension of protection layer  107  between gate structure  1120  and S/D structures  110  may increase and the parasitic capacitance between gate structure  112 C and S/D structures  110  may ,  increase. 
     Referring to  FIGS.  1 - 4   , gate structure  112 C can be disposed on protection layer  107 . In some embodiments, gate structures  112 A- 112 C can be formed in the same process. In some embodiments, gate structure  112 C can have a width  112   w  along an X-axis greater than width  105   w  of dielectric structure  105  to have additional tolerance for gate structure overlay shift. In some embodiments, width  112   w  can range from about 15 nm to about 35 nm. In some embodiments, a difference between width  112   w  and width  105   w  can range from about 1 nm to about 10 nm to prevent gate structure overlay shift. In some embodiments, gate structure  1120  can include the interfacial layer, the high-k gate dielectric layer, the work-function layer, and the gate electrode. Gate structure  112 C can be separated from dielectric structure  105  and S/D structures  110  by protection layer  107 . Protection layer  107  can prevent extrusions of gate structure  112 C due to the over etch during the formation of S/D structures  110 . In some embodiments, protection layer  107  can reduce about 75% to about 95% extrusions of gate structure  112 C into isolation structure  103 . 
     Referring to  FIGS.  1  and  3   , gate spacers  114  can be disposed on sidewalls of gate structures  112 A- 112 C and fin sidewall spacers  109  can be disposed on sidewalk of fin structures  108 . Gate spacers  114  and fin sidewall spacers  109  can include insulating materials, such as silicon oxide, silicon nitride, silicon oxynitride, silicon carbonitride, silicon oxycarbide, silicon oxycarbonitride, a low-k material, and a combination thereof. Gate spacers  114  and fin sidewall spacers  109  can include a single layer or a stack of insulating layers. Gate spacers  114  and fin sidewall spacers  109  can have a low-k material with a dielectric constant less than about 3.9 (e.g., about 3.5, about 3.0, or about 2.8). 
     ESL  116  can be disposed on STI regions  106 , S/D structures  110 , and sidewalk of gate spacers  114  and fin sidewall spacers  109 . ESL  116  can be configured to protect STI regions  106 , S/D structures  110 , and gate structures  112 A- 112 C during the formation of S/D contact structures on S/D structures  110 . In some embodiments, ESL  116  can include, for example, silicon oxide, silicon nitride, silicon oxynitride, silicon carbide, silicon carbonitride, boron nitride, silicon boron nitride, silicon carbon boron nitride, or a combination thereof. 
     ILD layer  118  can be disposed on ESL  116  over S/D structures  110  and STI regions  106 . ILD layer  118  can include a dielectric material deposited using a deposition method suitable for flowable dielectric materials. For example, flowable silicon oxide can be deposited using flowable chemical vapor deposition (FCVD). In some embodiments, the dielectric material can include silicon oxide. 
     Fill structures  120  can be disposed in gate structures  112 A- 112 B to separate gate structures into shorter portions, as shown in  FIGS.  1  and  2   . In some embodiments, fill structures  120  can include a nitrogen-based dielectric material. Hard mask layer  122  can be disposed on gate structures  112 A- 112 C to protect gate structures  112 A- 112 C during formation of S/D contact structures  110  and other processes. In some embodiments, hard mask layer  122  can include a nitrogen-based dielectric material. 
       FIG.  5    is a flow diagram of a method  500  for fabricating semiconductor device  100  having isolation structure  103  with protection layer  107 , in accordance with some embodiments. Method  500  may not be limited to finFET devices and can be applicable to devices that would benefit from protection layers in isolation structures, such as planar FETs, finFETs, gate-all-around FETs, and other semiconductor devices. Additional fabrication operations may be performed between various operations of method  500  and may be omitted merely for clarity and ease of description. Additional processes can be provided before, during, and/or after method  500 ; one or more of these additional processes are briefly described herein. Moreover, not all operations may be needed to perform the disclosure provided herein. Additionally, some of the operations may be performed simultaneously or in a different order than shown in  FIG.  5   . In some embodiments, one or more other operations may be performed in addition to or in place of the presently described operations. 
     For illustrative purposes, the operations illustrated in  FIG.  5    will be described with reference to the example fabrication process for fabricating semiconductor device  100  as illustrated in  FIGS.  6 - 12   .  FIGS.  6 - 12    illustrate isometric and cross-sectional views of semiconductor device  100  having isolation structure  103  with protection layer  107  at various stages of its fabrication, in accordance with some embodiments. Elements in  FIGS.  6 - 12    with the same annotations as elements in  FIGS.  1 - 4    are described above. 
     In referring to  FIG.  5   , method  500  begins with operation  510  and the process of forming a fin structure on a substrate. For example, as shown in  FIG.  6   , fin structures  108  can be formed on substrate  104 . Fin structures  108  can be formed from patterned portions of substrate  104  and can include the same material as substrate  104 , In some embodiments, fin structures  108  and substrate  104  can include silicon. :In some embodiments, fin structures  108  and substrate  104  can include silicon germanium. 
     Referring to  FIG.  5   , in operation  520 , a trench is formed in the fin structure. For example, as shown in  FIG.  6   , trench  603  can be formed in fin structures  108 . In some embodiments, trench  603  can be formed by a patterning process followed by an etch process to separate active regions  111 A and  111 B. In some embodiments, trench  603  can extend into substrate  104  to fully separate active regions  111 A and  111 B, as shown in  FIG.  7   . In some embodiments, trench  603  can extend into substrate by distance  105   d  ranging from about 100 nm to about 150 nm. 
     Referring to  FIG.  5   , in operation  530 , a dielectric structure is formed in the trench. For example, as shown in  FIGS.  6  and  7   , dielectric structure  105  can be formed in trench  603 . In some embodiments, dielectric structure  105  and STI regions  106  can be formed by the same process. The formation of dielectric structure  105  and STI regions  106  can include (i) depositing a layer of insulating material on substrate  104  and fin structures  108 , (ii) annealing the layer of insulating material, (iii) chemical mechanical polishing (CMP) the annealed layer of insulating material, and (iv) etching back the polished structure to expose fin structures  108  and form dielectric structure  105  and STI regions  106  in  FIGS.  6  and  7   . In some embodiments, the layer of insulating material can include silicon oxide, silicon nitride, silicon oxynitride, FSG, a low-k dielectric material, and/or other suitable insulating materials deposited by chemical vapor deposition (CVD) or other suitable deposition methods. In sonic embodiments, dielectric structure  105  can extend into substrate  104  by distance  105   d  ranging from about 100 nm to about 150 nm. In some embodiments, STI regions  106  can have height  106   h  along a Z-axis ranging from about 40 nm to about 80 nm. 
     Referring to  FIG.  5   , in operation  540 , a portion of the dielectric structure is removed. For example, as shown in  FIG.  8   , a top portion of dielectric structure  105  can be removed and dielectric structure  105  can be recessed by recess  105   r  ranging from about 10 nm to about 30 nm. With recess  105   r , protection layer  107  can better prevent extrusions of the gate structure formed on dielectric structure  105  in subsequent processes, In some embodiments, dielectric structure  105  can be recessed by a plasma dry etch at a temperature from about 30° C. to about 70° C. under a pressure from about 0.1 mtorr to about 30 mtorr. The plasma dry etch can include etchants, such as carbon tetrafluoride (CF 4 ), difluoromethane (CH 2 F 2 ), sulfur hexafluoride (SF 6 ), hydrogen bromide (HBr), and oxygen (O 2 ). The etchants can be carried by carrier gases, such as argon (Ar), helium (He), and nitrogen (N 2 ). In some embodiments, a plasma power of the plasma dry etch process can range from about 0.1 V to about 1000 V. In some embodiments, a flow rate of the etchants can range from about 0.1 standard cubic centimeter per minute (sccm) to about 450 sccm. The parameters for the plasma dry etch can he controlled within these ranges to adjust recess  105   r  from about 10 nm to about 30 nm. 
     Referring to  FIG.  5   , in operation  550 , a protection layer is formed on a top surface of the dielectric structure and on sidewall surfaces of the trench. For example, as shown in  FIGS.  9 - 11   , protection layer  107  can be formed on the top surface of dielectric structure  105  and on sidewall surfaces of trench  603 . The formation of protection layer  107  can include depositing a layer  107 * of dielectric material and remove a portion of layer  107 * of dielectric material outside trench  603 . In some embodiments, the dielectric material can include a nitrogen-based dielectric material, such as silicon nitride, silicon carhonitride, and silicon oxycarbonitride. In some embodiments, layer  107 * of dielectric material can be blanket deposited on fin structures  108  and in trench  603  by CVD, plasma enhanced CVD (PECVD), atomic layer deposition (ALD), and other suitable deposition methods. Deposition of layer  107 * of dielectric material can include precursors, such as dichlorosilane (DCS) and ammonia (NH 3 ). In some embodiments, layer  107 * of dielectric material can be deposited at a temperature from about 500° C. to about 650° C. under a pressure from about 1 mtorr to about 10 mtorr. 
     The deposition of layer  107 * of dielectric material can be followed by removing a portion of layer  107 * of dielectric material outside trench  603 . For example, as shown in  FIG.  10   , a photomask structure  1026  can be formed to fill trench  603 . The formation of photomask structure  1026  can include blanket depositing a photomask layer on layer  107 * of dielectric material and patterning and removing a portion of the photomask layer outside trench  603  to form photomask structure  1026  in trench  603 . A portion of layer  107 * of dielectric material can be removed from fin structures  108  outside of trench  603  to form protection layer  107 , as shown in  FIG.  11   . In some embodiments, the portion of layer  107 * of dielectric material can be removed by a wet etching process or a dry etching process, After formation of dielectric layer  107 , photomask structure  1026  can be removed from trench  603 . 
     Referring to  FIG.  5   , in operation  560 , a gate structure is formed on the protection layer. For example, as shown in  FIG.  12   , gate structure  112 * can be patterned and formed on protection layer  107 . in some embodiments, gate structure  112 * can be formed by a blanket deposition of polysilicon, followed by photolithography and etching of the deposited polysilicon. The deposition process can include CVD, ALD, physical vapor deposition (PVD), other suitable deposition methods, or a combination thereof. The photolithography can include photoresist coating (e.g., spin-on coating), soft baking, mask aligning, exposure, post-exposure baking, developing the photoresist, rinsing, drying (e.g., hard baking), other suitable processes, or a combination thereof. The etching process can include dry etching, wet etching, and/or other etching methods (e.g., reactive ion etching). In some embodiments, a hard mask layer (not shown) can be patterned on gate structure  112 * to protect gate structure  112 * in subsequent processing steps. 
     The formation of gate structure  112 * can be followed by formation of S/D structures  110  on fin structures  108 , as shown in  FIGS.  1 - 4   . S/D structures  110  can be epitaxially grown on fin structures  108 . Protection layer  107  can protect gate structure  112 * during the etching process prior to the deposition of S/D structures  110 . The formation of S/D structures  110  can be followed by replacing polysilicon gate structures  112 * with metal gate structure  1120 , as shown in  FIGS.  1 - 4   . Protection layer  107  can prevent the extrusion of metal gate structure  1120  into S/D structures  110  and fin structures  108  during the formation of metal gate structure  112 C. In some embodiments, protection layer  107  can reduce about 75% to about 95% extrusions of gate structure  112 C in isolation structure  103 . 
     Various embodiments in the present disclosure provide example methods for forming, protection layer  107  in isolation structure  103  between adjacent finFETs  102 A and  102 B. The example methods in the present disclosure can form isolation structure  103  on substrate  104  to isolate adjacent finFETs  102 A and  102 B in abutted active regions  111 A and  111 B. As shown in  FIGS.  1 - 4   , isolation structure  103  can include dielectric structure  105  on substrate  104 , gate structure  1120  over dielectric structure  105 , and protection layer  107  between dielectric structure  105  and the gate structure  1120 . Protection layer  107  can separate gate structure  112 C and S/D structures  110  of adjacent finFETs  102 A and  102 B to prevent the extrusion of gate structure  112 C into S/D structures  110  and fin structures  108  due to the over etch during the formation of S/D structures  110 . The dimensions of protection layer  107  and dielectric structure  105  can be controlled to minimize parasitic capacitances between gate structure  112 C and S/D structures  110 . 
     In some embodiments, a semiconductor device includes a substrate, a transistor with a source/drain (S/D) structure on the substrate, and an isolation structure on the substrate and adjacent to the transistor. The isolation structure includes a dielectric structure on the substrate, a protection layer on the dielectric structure, and a gate structure on the protection layer. The protection layer is disposed between the gate structure and the S/D structure. 
     In some embodiments, a semiconductor device includes a substrate including a first active region and a second active region, a first transistor on the first active region including a first source/drain (S/D) structure, a second transistor on the second active region including a second S/D structure, and an isolation structure on the substrate and between the first and second transistors. The isolation structures includes a dielectric structure on the substrate, a protection layer on the dielectric structure, and a gate structure on the protection layer. The protection layer is disposed between the gate structure and the first S/D structure and between the gate structure and the second S/D structure. 
     In some embodiments, a method includes forming a fin structure on a substrate forming a trench in the fin structure, forming a dielectric structure in the trench, removing a portion of the dielectric structure, forming a protection layer on a top surface of the dielectric structure and on sidewall surfaces of the trench, and forming a gate structure on the protection layer. The trench extends into the substrate. 
     It is to be appreciated that the Detailed Description section, and not the Abstract of the Disclosure section, is intended to be used to interpret the claims. The Abstract of the Disclosure section may set forth one or more but not all possible embodiments of the present disclosure as contemplated by the inventor(s), and thus, are not intended to limit the subjoined claims in any way. 
     The foregoing disclosure 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 will 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 will 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.