Patent Publication Number: US-2022238716-A1

Title: Fin field effect transistor (finfet) device structure and method for forming the same

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a Continuation of pending U.S. patent application Ser. No. 16/396,852, filed Apr. 29, 2019 and entitled “FIN FIELD EFFECT TRANSISTOR (FINFET) DEVICE STRUCTURE AND METHOD FOR FORMING THE SAME”, which is a Divisional of U.S. patent application Ser. No. 15/692,085, filed on Aug. 31, 2017 (now U.S. Pat. No. 10,276,720, issued on Apr. 30, 2019), the entire of which is incorporated by reference herein. 
    
    
     BACKGROUND 
     Semiconductor devices are used in a variety of electronic applications, such as personal computers, cell phones, digital cameras, and other electronic equipment. Semiconductor devices are typically fabricated by sequentially depositing insulating or dielectric layers, conductive layers, and semiconductive layers of material over a semiconductor substrate, and patterning the various material layers using lithography to form circuit components and elements thereon. Many integrated circuits are typically manufactured on a single semiconductor wafer, and individual dies on the wafer are singulated by sawing between the integrated circuits along a scribe line. The individual dies are typically packaged separately, in multi-chip modules, for example, or in other types of packaging. 
     As the semiconductor industry has progressed into nanometer technology process nodes in pursuit of higher device density, higher performance, and lower costs, challenges from both fabrication and design issues have resulted in the development of three-dimensional designs, such as the fin field effect transistor (FinFET). FinFETs are fabricated with a thin vertical “fin” (or fin structure) extending from a substrate. The channel of the FinFET is formed in this vertical fin. A gate is provided over the fin. The advantages of a FinFET may include reducing the short channel effect and providing a higher current flow. 
     Although existing FinFET devices and methods of fabricating FinFET devices have generally been adequate for their intended purposes, they have not been entirely satisfactory in all respects. 
    
    
     
       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 should be 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-1I  show perspective representations of various stages of forming a FinFET device structure, in accordance with some embodiments of the disclosure. 
         FIGS. 2A-2I  show cross-sectional representations of various stages of forming the FinFET device structure shown in  FIGS. 1A-1I . 
         FIGS. 3A-3H  show perspective representations of various stages of forming a FinFET device structure, in accordance with some embodiments of the disclosure. 
         FIGS. 4A-4H  show cross-sectional representations of various stages of forming the FinFET device structure shown in  FIGS. 3A-3H . 
         FIGS. 5A-5E  show perspective representations of various stages of forming a FinFET device structure, in accordance with some embodiments of the disclosure. 
         FIGS. 6A-6E  show cross-sectional representations of various stages of forming the FinFET device structure shown in  FIGS. 5A-5E . 
         FIGS. 7A-7D  show perspective representations of various stages of forming a FinFET device structure, in accordance with some embodiments of the disclosure. 
         FIGS. 8A-8D  show cross-sectional representations of various stages of forming the FinFET device structure shown in  FIGS. 7A-7D . 
     
    
    
     DETAILED DESCRIPTION 
     The following disclosure provides many different embodiments, or examples, for implementing different features of the subject matter provided. 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. 
     Some variations of the embodiments are described. Throughout the various views and illustrative embodiments, like reference numbers are used to designate like elements. It should be understood that additional operations can be provided before, during, and after the method, and some of the operations described can be replaced or eliminated for other embodiments of the method. 
     The fins may be patterned by any suitable method. For example, the fins may be patterned using one or more photolithography processes, including double-patterning or multi-patterning processes. Generally, double-patterning or multi-patterning processes combine photolithography and self-aligned processes, allowing patterns to be created that have, for example, pitches smaller than what is otherwise obtainable using a single, direct photolithography process. For example, in one embodiment, a sacrificial layer is formed over a substrate and patterned using a photolithography process. Spacers are formed alongside the patterned sacrificial layer using a self-alignment process. The sacrificial layer is then removed, and the remaining spacers may then be used to pattern the fins. 
     Embodiments for forming a fin field effect transistor (FinFET) device structure are provided.  FIGS. 1A-1I  show perspective representations of various stages of forming a FinFET device structure  100 , in accordance with some embodiments of the disclosure.  FIGS. 2A-2I  show cross-sectional representations of various stages of forming the FinFET device structure  100  shown in  FIGS. 1A-1I .  FIGS. 2A-2I  show cross-sectional representations taken along line aa′ in  FIGS. 1A-1I . 
     Referring to  FIG. 1A , a substrate  102  is provided. The substrate  102  includes a first region  11  and a second region  12 . The substrate  102  may be made of silicon or other semiconductor materials. In some embodiments, the substrate  102  is a silicon wafer. Alternatively or additionally, the substrate  102  may include other elementary semiconductor materials such as germanium. In some embodiments, the substrate  102  is made of a compound semiconductor such as silicon carbide, gallium arsenic, indium arsenide, or indium phosphide. In some embodiments, the substrate  102  is made of an alloy semiconductor such as silicon germanium, silicon germanium carbide, gallium arsenic phosphide, or gallium indium phosphide. In some embodiments, the substrate  102  includes an epitaxial layer. For example, the substrate  102  has an epitaxial layer overlying a bulk semiconductor. 
     Afterwards, a dielectric layer  104  and a mask layer  106  are formed over the substrate  102 , and a photoresist layer  108  is formed over the mask layer  106 . The photoresist layer  108  is patterned by a patterning process. The patterning process includes a photolithography process and an etching process. The photolithography process includes photoresist coating (e.g., spin-on coating), soft baking, mask aligning, exposure, post-exposure baking, developing the photoresist, rinsing and drying (e.g., hard baking). The etching process may include a dry etching process or a wet etching process. 
     The dielectric layer  104  is a buffer layer between the substrate  102  and the mask layer  106 . In addition, the dielectric layer  104  is used as a stop layer when the mask layer  106  is removed. The dielectric layer  104  may be made of silicon oxide. The mask layer  106  may be made of silicon oxide, silicon nitride, silicon oxynitride, or another applicable material. In some other embodiments, more than one mask layer  106  is formed over the dielectric layer  104 . 
     The dielectric layer  104  and the mask layer  106  are formed by deposition processes, such as a chemical vapor deposition (CVD) process, a high-density plasma chemical vapor deposition (HDPCVD) process, a spin-on process, a sputtering process, or another applicable process. 
     As shown in  FIGS. 1B and 2B , after the photoresist layer  108  is patterned, the dielectric layer  104  and the mask layer  106  are patterned by using the patterned photoresist layer  108  as a mask, in accordance with some embodiments. As a result, a patterned dielectric layer  104  and a patterned mask layer  106  are obtained. Afterwards, the patterned photoresist layer  108  is removed. 
     Afterwards, an etching process is performed on the substrate  102  to form a number of fin structures  110   a ,  110   b  by using the patterned dielectric layer  104  and the patterned mask layer  106  as a mask. The fin structures  110   a ,  110   b  include first fin structures  110   a  in the first region  11  and second fin structures  110   b  in the second region  12 . In some embodiments, each of the fin structures  110   a ,  110   b  has a width that gradually increases from the top portion to the bottom portion. In other words, each of the fin structures  110   a ,  110   b  has a tapered fin width which is gradually tapered from the bottom portion to the top portion. 
     The etching process may be a dry etching process or a wet etching process. In some embodiments, the substrate  102  is etched using a dry etching process. The dry etching process includes using a fluorine-based etchant gas, such as SF 6 , C x F y , NF 3  or a combination thereof. The etching process may be a time-controlled process, and continue until the fin structures  110   a ,  110   b  reach a predetermined height. 
     Before forming the isolation layer  112  (shown in  FIG. 3C ), a thermal oxidation process may be optionally performed on the fin structures  110   a ,  110   b  to repair any damage to the sidewall surfaces of the fin structures  110   a ,  110   b  during the etching process. It should be noted that although only four fin structures  110   a ,  110   b  shown in  FIGS. 1A-1I  and  FIGS. 2A-2I , the number of the fin structures  110   a ,  110   b  may be changed according to actual application. 
     As shown in  FIGS. 1C and 2C , after the fin structures  110   a ,  110   b  are formed, the isolation layer  112  is formed to cover the fin structures  110   a ,  110   b  over the substrate  102 , in accordance with some embodiments. 
     In some embodiments, the isolation layer  112  is made of silicon oxide, silicon nitride, silicon oxynitride, fluoride-doped silicate glass (FSG), or another low-k dielectric material. The isolation layer  112  may be deposited by a deposition process, such as a chemical vapor deposition (CVD) process, a spin-on-glass process, or another applicable process. 
     Afterwards, as shown in  FIGS. 1D and 2D , the isolation layer  112  is thinned or planarized to expose the top surface of the patterned mask layer  106 . In some embodiments, the insulating layer  112  is thinned by a chemical mechanical polishing (CMP) process. 
     Afterwards, as shown in  FIGS. 1E and 2E , a portion of the insulating layer  112  is removed to form a number of isolation structures  114   a ,  114   b , in accordance with some embodiments. In addition, the patterned dielectric layer  104  and the patterned mask layer  106  are removed. The isolation structures  114   a ,  114   b  include first isolation structures  114   a  over the first region  11  and second isolation structures  114   b  over the second region  12 . The isolation structures  114   a ,  114   b  may be a shallow trench isolation (STI) structure surrounding the fin structures  110   a ,  110   b.    
     A lower portion of each of the fin structures  110   a ,  110   b  is surrounded by each of the isolation structures  114   a ,  114   b , and an upper portion of each of the fin structures  110   a ,  110   b  protrudes from each of the isolation structures  114   a ,  114   b . In other words, a portion of each of the fin structures  110   a ,  110   b  is embedded in each of the isolation structures  114   a ,  114   b . The isolation structures  114   a ,  114   b  prevent electrical interference or crosstalk. 
     Afterwards, as shown in  FIGS. 1F and 2F , a mask layer  210  is formed over the first fin structures  110   a  and the first isolation structures  114   a , and a portion of the second isolation structures  114   b  is removed, in accordance with some embodiments. In some embodiments, an etching process  212  is performed on the second isolation structures  114   b  to remove the portion of the second isolation structures  114   b.    
     The mask layer  210  covers the first isolation structures  114   a , but the second isolation structures  114   b  are exposed. In some embodiments, the mask layer  210  is made of photoresist materials, such as polybenzoxazole (PBO), SU-8-photo-sensitive epoxy, film type polymer materials and/or the like. 
     Next, as shown in  FIGS. 1G and 2G , after the etching process  212 , the mask layer  210  is removed, in accordance with some embodiments. As a result, a top surface of each of the first isolation structures  114   a  is higher than a top surface of each of the second isolation structures  114   b . In other words, the top surface of each of the second isolation structures  114   b  is lower than the top surface of each of the first isolation structures  114   a.    
     Each of the first fin structures  110   a  has a first fin height H 1  measured from a top surface of each of the first fin structures  110   a  to a top surface of each of the first isolation structures  114   a . Each of the second fin structures  110   b  has a second fin height H 2  measured from a top surface of each of the second fin structures  110   b  to a top surface of each of the second isolation structures  110   b . The first fin height H 1  is smaller than the second fin height H 2 . In some embodiments, the first fin height H 1  is in a range from about 40 nm to about 60 nm. In some embodiments, the second fin height H 2  is in a range from about 40.1 nm to about 80 nm. The higher difference ΔH between the first fin height H 1  and the second fin height H 2  is in a range from about 0.1 nm to about 20 nm. 
     Each of the first isolation structures  114   a  has a first depth D 1 , and each of the second isolation structures  114   b  has a second depth D 2 . In some embodiments, the first depth D 1  of each of the first isolation structures  114   a  is greater than the second depth D 2  of each of the second isolation structures  114   b.    
     In the first region  11 , a first pitch P 1  is between two adjacent first fin structures  110   a , and a second pitch P 2  is between two adjacent second fin structures  110   b . In some embodiments, the first pitch P 1  is in a range from about 20 nm to about 60 nm. In some embodiments, the second pitch P 2  is in n a range from about 20 nm to about 60 nm. 
     As shown in  FIG. 2G , a first angle θ 1  between the sidewall surface of each of the fin structures  110   a ,  110   b  and the top surface of each of the isolation structures  114   a ,  114   b . In some embodiments, the first angle θ 1  is in a range from about 95 degrees to about 120 degrees. Each of the first structures  110   a  has a top width W 1  and a bottom width W 2 , and the top width W 1  is smaller than the bottom width W 2 . 
     Afterwards, as shown in  FIGS. 1H and 2H , a gate dielectric layer  116  is formed on the fin structures  110   a ,  110   b , the isolation structures  114   a ,  114   b , and a gate electrode layer  118  is formed on the gate dielectric layer  116 , in accordance with some embodiments. A gate structure  120  is constructed by the gate dielectric layer  116  and the gate electrode layer  118 . In some embodiments, the gate dielectric layer  116  is a dummy gate dielectric layer and the gate electrode layer  118  is a dummy gate electrode layer. The dummy gate dielectric layer and the dummy gate electrode layer may be replaced by the following steps to form a real gate structure with a high-k dielectric layer and a metal gate electrode layer. 
     In some embodiments, the gate dielectric layer  116  may include dielectric materials, such as silicon oxide, silicon nitride, silicon oxynitride, dielectric material(s) with high dielectric constant (high-k), or combinations thereof. In some embodiments, the gate electrode layer  118  is made of polysilicon. The gate dielectric layer  116  and the gate electrode layer  118  are independently formed by a deposition process, such as chemical vapor deposition (CVD), physical vapor deposition (PVD), atomic layer deposition (ALD), high density plasma CVD (HDPCVD), metal organic CVD (MOCVD), or plasma enhanced CVD (PECVD). 
     Afterwards, a pair of spacer layers  122  are formed on opposite sidewalls of the gate structure  120 , in accordance with some embodiments. More specifically, the spacer layers  120  are formed on sidewall surfaces of the gate electrode layer  118 . The spacer layers  122  may be made of silicon oxide, silicon nitride, silicon oxynitride, and/or dielectric materials. In some embodiments, the spacer layers  122  are formed by a chemical vapor deposition (CVD) process, a spin-on-glass process, or another applicable process. 
     Afterwards, a top portion of the fin structures  110   a ,  110   b  is removed to form a recess (not shown), and the source/drain (S/D) structures (not shown) are formed in the recess. The source/drain (S/D) structures are formed on opposite of the gate structure  120 . In some embodiments, the S/D structures are strained source/drain structures. In some embodiments, the S/D structures are formed by growing a strained material in the recesses of the fin structures  110   a ,  110   b  by an epitaxial (epi) process. In addition, the lattice constant of the strained material may be different from the lattice constant of the substrate  102 . 
     In some embodiments, the source/drain structures include Ge, SiGe, InAs, InGaAs, InSb, GaAs, GaSb, InAlP, InP, or a combination thereof. The epitaxial process may include a selective epitaxy growth (SEG) process, CVD deposition techniques (e.g., vapor-phase epitaxy (VPE) and/or ultra-high vacuum CVD (UHV-CVD)), molecular beam epitaxy, or other suitable epi processes. 
     In some embodiments, after the S/D structures are formed, a contact etch stop layer (CESL) (not shown) is formed on the S/D structures and the gate structure  120 . In some embodiments, the contact etch stop layer is made of silicon nitride, silicon oxynitride, and/or other applicable materials. The contact etch stop layer may be formed by plasma enhanced CVD, low pressure CVD, ALD, or other applicable processes. 
     Afterwards, as shown in  FIGS. 11 and 21 , an inter-layer dielectric (ILD) material is formed over the fin structures  110   a ,  110   b  over the substrate  102 , in accordance with some embodiments. Next, the ILD material is planarized to form the ILD structure  126 . 
     The ILD structure  126  has a first portion in the first region  11  and a second portion in the second region  12 . A bottom surface of the first portion of the ILD structure  126  is higher than a bottom surface of the second portion of the ILD structure  126 . The ILD structure  126  may include multilayers made of multiple dielectric materials, such as silicon oxide, silicon nitride, silicon oxynitride, tetraethoxysilane (TEOS), phosphosilicate glass (PSG), borophosphosilicate glass (BPSG), low-k dielectric material, and/or other applicable dielectric materials. Examples of low-k dielectric materials include, but are not limited to, fluorinated silica glass (FSG), carbon doped silicon oxide, amorphous fluorinated carbon, parylene, bis-benzocyclobutenes (BCB), or polyimide. The ILD structure  126  may be formed by chemical vapor deposition (CVD), physical vapor deposition, (PVD), atomic layer deposition (ALD), spin-on coating, or other applicable processes. 
     Afterwards, the dummy gate dielectric layer  116  and the dummy gate electrode layer  118  are removed to form a trench, and the real gate dielectric layer and metal gate electrode layer (not shown) are filled into the trench. In some embodiments, a work function layer (not shown) may be formed between the gate dielectric layer and the gate electrode layer. The work function layer (not shown) is made of metal material, and the metal material may include N-work-function metal or P-work-function metal. The N-work-function metal includes tungsten (W), copper (Cu), titanium (Ti), silver (Ag), aluminum (Al), titanium aluminum alloy (TiAl), titanium aluminum nitride (TiAlN), tantalum carbide (TaC), tantalum carbon nitride (TaCN), tantalum silicon nitride (TaSiN), manganese (Mn), zirconium (Zr) or a combination thereof. The P-work-function metal includes titanium nitride (TiN), tungsten nitride (WN), tantalum nitride (TaN), ruthenium (Ru) or a combination thereof. Afterwards, the FinFET device structure  200  continues to form other devices or structures. 
     In the first embodiment, the FinFET device structure  100  with different fin heights is obtained by performing two etching processes. The first etching process is performed to simultaneously remove a portion of the first isolation structures  110   a  and a portion of the second isolation structures  110   b . The second etching process with an extra mask layer is used to further remove the second isolation structures  110   b  to a predetermined depth. Compared with forming the second isolation structure by using one etching process, the etching amount is controlled more precisely by the two etching process. Especially when the FinFET device structure has a fine spacing (or smaller pitch), which is in the range of nanometers (nm), between neighboring gate structures, removal of the bottom portion of the second isolation structures  114   b  becomes difficult due to the shadow effect. In order to control the fin height more precisely, the second isolation structures  114   b  are etched by a first rough etching process and a second accurate etching process. In addition, the second etching process is compatible with the FinFET processes. 
     Furthermore, the FinFET device structure  100  with different fin heights can improve the cutoff frequency (Ft), and therefore the FinFET device structure  100  can be used in the high-frequency device. 
       FIGS. 3A-3H  show perspective representations of various stages of forming a FinFET device structure  200 , in accordance with some embodiments of the disclosure.  FIGS. 4A-4H  show cross-sectional representations of various stages of forming the FinFET device structure  200  shown in  FIGS. 3A-3H .  FIGS. 4A-4H  show cross-sectional representations taken along line aa′ in  FIGS. 3A-3H . Some processes and materials used to form the FinFET device structure  200  are similar to, or the same as, those used to form the FinFET structure  100  and are not repeated herein. 
     The structures in  FIGS. 3A-3B  are similar to the structures in  FIGS. 1A-1B  and the structures in  FIGS. 4A-4B  are similar to the structures in  FIGS. 2A-2B , and are not repeated herein. 
     Afterwards, as shown in  FIGS. 3C and 4C , a mask layer  220  is formed over the second fin structures  110   b  in the second region  12 , and a portion of the first fin structures  110  is removed, in accordance with some embodiments. An etching process  222  is performed on the first fin structures  110  over the first region  11  to remove or shape a portion of the first fin structures  110   a . In some embodiments, the etching process  222  is a dry etching process with etching gas, such as Cl 2 , HB, CF 4  or another applicable gas. Each of the first fin structures  110   a  has a constant width from a bottom portion to a top portion. 
     Next, as shown in  FIGS. 3D and 4D , after the etching process  222 , the mask layer  220  is removed, in accordance with some embodiments. The mask layer  220  may be removed by a dry etching process or a wet etching process. 
     The first fin structures  110   a  in the first region  11  and the second fin structures  110   b  in the second region  12  have different fin profiles. The first fin structures  110   a  are etching twice and have straighter sidewall surface when compared with the second fin structures  110   b.    
     Afterwards, as shown in  FIGS. 3E and 4E , the isolation layer  112  is formed to cover the fin structures  110   a ,  110   b  over the substrate  102 , in accordance with some embodiments. 
     Afterwards, as shown in  FIGS. 3F and 4F , the isolation layer  112  is thinned or planarized, and then is etched to form the first isolation structures  114   a  and the second isolation structures  114   b , in accordance with some embodiments. As a result, the first fin structures  110   a  are surrounded by the first isolation structures  114   a , and the second fin structures  110   b  are surrounded by the second isolation structures  114   b.    
     Each of the first fin structures  110   a  has a first fin height H 1  and each of the second fin structures  110   b  has a second fin height H 2 . In some embodiments, the first fin height H 1  is equal to the second fin height H 2 . Each of the second structures  110   b  has a top width W 1  and a bottom width W 2 , and the top width W 1  is smaller than the bottom width W 2 . Each of the first fin structures  110   a  has a constant width from a bottom portion to a top portion. 
     A third angle θ 3  is between the sidewall surface of each of the first fin structures  110   a  and the top surface of the first isolation structures  114   a . The sidewall surface of each of the first fin structures  110  is substantially orthogonal to the top surface of each of the first isolation structures  114   a . A fourth angle θ 4  is between the sidewall surface of each of the second fin structures  110   b  and the top surface of the second isolation structures  114   b . In some embodiments, the third angle θ 3  is smaller than the fourth angle θ 4 . In some embodiments, the third angle θ 3  is in a range from about 80 degrees to about 90 degrees. In some embodiments, the fourth angle θ 4  is in a range from about 80 degrees to about 120 degrees. The angle difference between the third angle θ 3  and the fourth angle θ 4  is in a range from about 0 degree to about 30 degrees. 
     Next, as shown in  FIGS. 3G and 4G , the gate structure  120  is formed on the fin structures  110   a ,  110   b , the isolation structures  114   a ,  114   b , in accordance with some embodiments. The gate structure  120  includes the gate dielectric layer  116  and the gate electrode layer  118 . Afterwards, a pair of spacer layers  122  are formed on opposite sidewalls of the gate structure  120 . 
     Afterwards, as shown in  FIGS. 3H and 4H , the ILD structure  126  is formed on the fin structures  110   a ,  110   b  and the isolation structures  114   a ,  114   b , in accordance with some embodiments. Afterwards, the FinFET device structure  200  continues to form other devices or structures. 
     In the second embodiment, the FinFET device structure  200  is formed to have different fin profiles. The first fin structures  110   a  in the first region  11  have straighter sidewall surfaces, and the second fin structures  110   b  in the second region  12  have sloped sidewall surfaces. By forming different fin profiles, the parasite capacitance (C) of the FinFET device structures  200  is reduced, and therefore the cutoff frequency (Ft) of FinFET device structure  200  is improved. Therefore, the FinFET device structure  200  can be used in the high-frequency device. 
       FIGS. 5A-5E  show perspective representations of various stages of forming a FinFET device structure  300 , in accordance with some embodiments of the disclosure.  FIGS. 6A-6E  show cross-sectional representations of various stages of forming the FinFET device structure  300  shown in  FIGS. 5A-5E .  FIGS. 6A-6E  show cross-sectional representations taken along line aa′ in  FIGS. 5A-5E . Some processes and materials used to form the FinFET device structure  300  are similar to, or the same as, those used to form the FinFET structure  100  and are not repeated herein. 
     As shown in  FIGS. 5A and 6A , after the fin structures  110   a ,  110   b  are formed, the isolation layer  112  is formed to cover the fin structures  110   a ,  110   b  over the substrate  102 , in accordance with some embodiments. 
     Afterwards, as shown in  FIGS. 5B and 6B , the isolation layer  112  is thinned or planarized to expose the top surface of the patterned mask layer  106 . In some embodiments, the insulating layer  112  is thinned by a chemical mechanical polishing (CMP) process. 
     Afterwards, as shown in  FIGS. 5C and 6C , a portion of the insulating layer  112  is removed to form a number of isolation structures  114 , in accordance with some embodiments. In addition, the patterned dielectric layer  104  and the patterned mask layer  106  are removed. The isolation structures  114  include first isolation structures  114   a  over the first region  11  and second isolation structures  114   b  over the second region  12 . 
     It should be noted that the second pitch P 2  between two adjacent second fin structures  110   b  is greater than the first pitch P 1  between two adjacent first fin structures  110   a  due to the second fin structures  110   b  with a narrower top portion. More of the second isolation structures  114   b  are removed compared with the first isolation structures  114   a  since the etching gas may easily pass through the wider opening (or wider pitch P 2 ). The first isolation structures  114   a  are removed less, such that the top surface of each of the first isolation structures  114   a  is higher than the top surface of each of the second isolation structures  114   b.    
     The first fin height H 1  of each of the first fin structures  110   a  is smaller than the second fin height H 2  of each of the second fin structures  110   b . The first depth D 1  of each of the first isolation structures  114   a  is greater than the second depth D 2  of each of the second isolation structures  114   b . In addition, the third angle θ 3  is smaller than the fourth angle θ 4 . 
     Afterwards, as shown in  FIGS. 5D and 6D , the gate structure  120  is formed on the fin structures  110   a ,  110   b , the isolation structures  114   a ,  114   b , in accordance with some embodiments. The gate structure  120  includes the gate dielectric layer  116  and the gate electrode layer  118 . Afterwards, a pair of spacer layers  122  are formed on opposite sidewalls of the gate structure  120 . 
     Next, as shown in  FIGS. 5E and 6E , the ILD structure  126  is formed on the fin structures  110   a ,  110   b  and the isolation structures  114   a ,  114   b , in accordance with some embodiments. The ILD structure  126  has a first portion in the first region  11  and a second portion in the second region  12 . A bottom surface of the first portion of the ILD structure  126  is higher than a bottom surface of the second portion of the ILD structure  126 . Afterwards, the FinFET device structure  300  continues to form other devices or structures. 
     In the third embodiment, the FinFET device structure  300  is formed to have different fin profiles and different fin heights. The first fin structures  110   a  in the first region  11  have straighter sidewall surfaces and lower first fin height H 1  and the second fin structures  110   b  in the second region  12  have sloped sidewall surfaces and higher second fin height H 2 . By forming different fin profiles and fin heights, the cutoff frequency (Ft) of FinFET device structure  300  is improved. Therefore, the FinFET device structure  300  can be used in the high-frequency device. 
       FIGS. 7A-7D  show perspective representations of various stages of forming a FinFET device structure  400 , in accordance with some embodiments of the disclosure.  FIGS. 8A-8D  show cross-sectional representations of various stages of forming the FinFET device structure  400  shown in  FIGS. 7A-7D .  FIGS. 8A-8D  show cross-sectional representations taken along line aa′ in  FIGS. 7A-7D . Some processes and materials used to form the FinFET device structure  400  are similar to, or the same as, those used to form the FinFET structure  100  and are not repeated herein. 
     As shown in  FIGS. 7A and 8A , each of the first fin structures  110   a  in the first region  11  has constant fin width, and each of the second fin structures  110   b  in the second region  12  has a tapered width which is gradually tapered from the bottom portion to the top portion. The isolation layer  112  over the fin structures  110   a ,  110   b  and then is thinned or planarized to expose the top surface of the patterned mask layer  106 . In some embodiments, the insulating layer  112  is thinned by a chemical mechanical polishing (CMP) process. 
     Next, as shown in  FIGS. 7B and 8B , a portion of the insulating layer  112  is removed to form a number of isolation structures  114 , in accordance with some embodiments. In addition, the patterned dielectric layer  104  and the patterned mask layer  106  are removed. The first isolation structures  114   a  over the first region  11  and the second isolation structures  114   b  over the second region  12  both have the same height. 
     Afterwards, as shown in  FIGS. 7C and 8C , a mask layer  230  is formed over the first fin structures  110   a  and the first isolation structures  114   a , and a portion of the second isolation structures  114   b  is removed, in accordance with some embodiments. In some embodiments, an etching process  232  is performed on the second isolation structures  114   b  to remove the portion of the second isolation structures  114   b.    
     Next, as shown in  FIGS. 7D and 8D , after the etching process  232 , the mask layer  230  is removed, in accordance with some embodiments. The mask layer  230  may be removed by a dry etching process or a wet etching process. 
     The first fin structures  110   a  in the first region  11  and the second fin structures  110   b  in the second region  12  have different fin profiles and different fin heights. The second fin structures  110   b  in the second region  12  are etched twice and have sloped sidewall surfaces. Afterwards, the FinFET device structure  400  continues to form other devices and structures. 
     In the fourth embodiment, the FinFET device structure  400  is formed to have different fin profiles and different fin heights. The first fin structures  110   a  in the first region  11  have straighter sidewall surfaces and lower first fin height H 1  and the second fin structures  110   b  in the second region  12  have sloped sidewall surfaces and higher second fin height H 2 . By forming different fin profiles and fin heights, the cutoff frequency (Ft) of FinFET device structure  400  is improved. Therefore, the FinFET device structure  400  can be used in the high-frequency device. 
     In the first embodiment, the first fin structures  110   a  and the second fin structures  110   b  have different fin heights. In the second embodiment, the first fin structures  110   a  and the second fin structures  110   b  have different fin profiles. In the third and fourth embodiments, the first fin structures  110   a  and the second fin structures  110   b  have different fin heights and have different fin profiles. By forming the different fin height and/or different fin profiles, the parasite capacitance (C) of the FinFET device structures  100 ,  200 ,  300 ,  400  may be reduced and therefore the cutoff frequency (Ft) of the FinFET device structures  100 ,  200 ,  300 ,  400  is improved. 
     Embodiments for forming a FinFET device structure and method for formation of the same are provided. The FinFET device structure includes an isolation structure formed over a substrate, and a fin structure extends from the isolation structure. The FinFET device structure with different fin heights is achieved by using two etching processes. A portion of the isolation structures is etched twice to control the etching amount more precisely. The FinFET device structure with different fin heights and/or different fin profiles to reduce the parasite capacitance (C) of the FinFET device structures. Therefore, the performance of the FinFET device structures is improved. 
     In some embodiments, a FinFET device structure is provided. The FinFET device structure includes a first fin structure and a second fin structure over a substrate. The first fin structure and the second fin structure have a first sidewall surface and a second sidewall surface respectively, and the first sidewall surface and the second sidewall surface extend along a first direction. The FinFET device structure also includes a third fin structure and a fourth fin structure over the substrate. The third fin structure and the fourth fin structure have a third sidewall surface and a fourth sidewall surface respectively, and the third sidewall surface and the fourth sidewall surface extend along a second direction different from the first direction. The FinFET device structure further includes a first isolation structure over the substrate and surrounding the first fin structure and the second fin structure. In addition, the FinFET device structure includes a second isolation structure over the substrate and surrounding the third fin structure and the fourth fin structure. A first distance between top portions of the third sidewall surface of the third fin structure and the fourth sidewall surface of the fourth fin structure is greater than a second distance between top portions of the first sidewall surface of the first fin structure and the second sidewall surface of the second fin structure. 
     In some embodiments, a method for forming a FinFET device structure is provided. The method includes forming a first fin structure and a second fin structure in a first region of a substrate and a third fin structure and a fourth fin structure in a second region of the substrate. The first fin structure and the second fin structure have a first sidewall surface and a second sidewall surface respectively, and the first sidewall surface and the second sidewall surface extend along a first direction, wherein the third fin structure and the fourth fin structure have a third sidewall surface and a fourth sidewall surface respectively, and the third sidewall surface and the fourth sidewall surface extend along a second direction different from the first direction. The method also includes forming an isolation layer covering the first fin structure, the second fin structure, the third fin structure and the fourth fin structure. The method further includes etching a first portion of the isolation layer to expose top portions of the first fin structure, the second fin structure, the third fin structure and the fourth fin structure. In addition, the method includes forming a mask layer over the first fin structure and the second fin structure after removing the first portion of the isolation layer. The method also includes etching a second portion of the isolation layer surrounding the third fin structure and the fourth fin structure to form an isolation structure on the substrate. A first top surface of the isolation structure surrounding the first sidewall surface of the first fin structure in the first region is higher than a second top surface of the isolation structure surrounding the third sidewall surface of the third fin structure in the second region. A first distance between top portions of the third sidewall surface of the third fin structure and the fourth sidewall surface of the fourth fin structure is greater than a second distance between top portions of the first sidewall surface of the first fin structure and the second sidewall surface of the second fin structure. 
     In some embodiments, a method for forming a FinFET device structure is provided. The method includes forming a first fin structure and a second fin structure in a first region and a second region of a substrate, respectively, wherein the first fin structure and the second fin structure have a first sloped sidewall surface and a second slope sidewall surface respectively, and the first slope sidewall surface and the second slope sidewall surface have a first slope. The method also includes forming a mask layer over the second fin structure. The method further includes removing a portion of the first fin structure, so that the first fin structure has a first modified sidewall surface, wherein the first modified sidewall surface has a second slope different from the first slope. In addition, the method includes removing the mask layer. The method also includes forming a first isolation structure surrounding the first fin structure in the first region and a second isolation structure surrounding the second fin structure in the second region. 
     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.