Patent Publication Number: US-11024546-B2

Title: Vertical field effect transistors

Description:
FIELD OF THE INVENTION 
     The invention relates to semiconductor structures and, more particularly, to vertical field effect transistors (FETs) with minimum pitch and methods of manufacture. 
     BACKGROUND 
     A vertical field-effect transistor (FET) has a channel perpendicular to the substrate surface, as opposed to being situated along the plane of the surface of the substrate. By using this design, it is possible to increase packing density. That is, by having the channel perpendicular to the substrate surface, vertical FETs improve the scaling limit beyond planar finFETs. 
     However, vertical FETs are still severely challenged past the 7 nm node due to high aspect ratios, Vmax limits, and material thickness not scaling well. For example, insulator material and shared contacts formed between gate material of adjacent vertical FETs make it very difficult to scale the devices beyond the 7 nm node, basically due to material thicknesses, leakage concerns, breakdown voltage, decreased resistances and capacitance, etc. Accordingly, these constraints make it very difficult to decrease gate pitch in current vertical FET designs. 
     SUMMARY 
     In an aspect of the invention, a structure comprises at least one vertical fin structure and gate material contacting with the at least one vertical fin structure. The structure further comprising metal material in electrical contact with the ends of the at least one vertical fin. 
     In an aspect of the invention, a structure comprises: at least two adjacent fin structures of semiconductor material with a source region and a drain region at opposing ends; gate material about the two adjacent fin structures and between the opposing ends; a space between the gate material of the two adjacent fin structures; and drain contacts and source contacts at the opposing ends of the two adjacent fin structures on the source region and the drain region. 
     In an aspect of the invention, a method comprises: forming at least one vertical fin structure; forming gate material contacting with the at least one vertical fin structure; and forming source and drain contacts at ends of the at least one vertical fin structure by deposition of metal material in electrical contact with the silicide regions. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       The present invention is described in the detailed description which follows, in reference to the noted plurality of drawings by way of non-limiting examples of exemplary embodiments of the present invention. 
         FIG. 1  shows a respective structure and fabrication processes of forming fins according to aspects of the invention; 
         FIG. 2  shows a structure and respective fabrication processes of forming dielectric material between fins according to aspects of the invention; 
         FIG. 3  shows a structure and respective fabrication processes of forming gate material around the fins according to aspects of the invention; 
         FIG. 4  shows a structure and respective fabrication processes of patterning the gate material according to aspects of the invention; 
         FIG. 5  shows a structure and respective fabrication processes of forming dielectric material about vertical gate structures according to aspects of the invention; 
         FIG. 6  shows a structure and respective fabrication processes of forming epitaxial material on exposed portions of fins according to aspects of the invention; 
         FIG. 7  shows a structure and respective fabrication processes of forming of silicide regions according to aspects of the invention; 
         FIG. 8  shows a structure and respective fabrication processes of forming contact regions according to aspects of the invention; 
         FIGS. 9 a  and 9 b    show cross-sectional and top-down views of an alternative structure and respective fabrication processes according to aspects of the invention; 
         FIGS. 10 a  and 10 b    show cross-sectional and top-down views of another alternative structure and respective fabrication processes according to aspects of the invention; 
         FIGS. 11 a  and 11 b    show cross-sectional and top-down views of another alternative structure and respective fabrication processes according to aspects of the invention; and 
         FIG. 12  shows another alternative structure and respective fabrication processes according to aspects of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     The invention relates to semiconductor structures and, more particularly, to vertical field effect transistors (FETs) with minimum pitch and methods of manufacture. More specifically, the vertical FETs of the present invention have the source/drain contacts formed at ends of the gate structures (compared to between the gate structures) in order to reduce the pitch between adjacent gate structures, e.g., vertical FETs. Advantageously, by moving the source/drain contact to the outside ends of the FETs, the pitch of the FETs can be scaled significantly. Also, gate to contact capacitance is significantly reduced by moving the source/drain contacts to the outside ends of the FETs. 
     Vertical FETs significantly improve the scaling limit beyond planar finFETs; however, conventional vertical FETs are still severely challenged past the approximately 7 nm node due to high aspect ratios, Vmax limits, and material thickness not scaling well. For example, in current layouts the source and drain contact(s) are placed between adjacent FETs due to resistance issues. The challenge is that as the pitch is scaled the width (thickness) of the contact decreases. This results in a high overall contact resistance. This also results in very high contact to gate capacitance and lack of pitch scaling. That is, the placement of the contact between the gate structures effectively limits the scaling properties between adjacent vertical FETs, limiting the pitch to about 27 nm. 
     In comparison, the vertical FETs of the present invention have contacts at the ends of the gate structures. By placing a shared contact (or source/drain contacts) at ends of the FET and making the FET conductor bottoms tall, the resistance issues are reduced while providing lower contact to gate capacitance and, importantly, the ability to scale the gate pitch, e.g., space between adjacent FETs. Effectively, eliminating the contact between the metal gate structures of the vertical FETs also eliminates a layer of insulator material, thereby making it possible to significantly decrease the pitch (spacing) between adjacent vertical FETs. In fact, the scaling can be improved by approximately 20% or greater (e.g., approximately 29% in some instances) compared to conventional structures which place source and drain contact(s) between adjacent FETs. Moreover, by removing the shared contact between the metal gate structures, it is also possible to provide an air gap between the adjacent FETs effectively reducing capacitance. 
     In embodiments, the vertical FETs can be single or double sided gates. In addition, the vertical gate structures can be long without increasing pitch, and gate width, fin thickness and insulator materials can all be scaled accordingly. In this way, it is possible to minimize or scale the pitch between adjacent vertical FETs, by forming the contacts at their ends. In further embodiments, the vertical FETs comprise a first vertical double gate CMOS FET pair having a shared contact strap at one or both ends (source and drain regions) of the FINFET between the adjacent pair of FETs and a shared or individual S/D silicide region (silicide at shared S/D region). An air-gap can be formed between the adjacent vertical FETs, with a high aspect ratio bottom contact region for low horizontal resistance. 
     The vertical FETs of the present invention can be manufactured in a number of ways using a number of different tools. In general, though, the methodologies and tools are used to form structures with dimensions in the micrometer and nanometer scale. The methodologies, i.e., technologies, employed to manufacture the vertical FETs have been adopted from integrated circuit (IC) technology. For example, the vertical FETs are built on wafers and are realized in films of material patterned by photolithographic processes on the top of a wafer. In particular, the fabrication of the vertical FETs uses three basic building blocks: (i) deposition of thin films of material on a substrate, (ii) applying a patterned mask on top of the films by photolithographic imaging, and (iii) etching the films selectively to the mask. 
       FIG. 1  shows a beginning structure and respective processing steps in accordance with aspects of the present invention. The structure  10  includes a substrate  12  and an insulator layer  14  formed thereon. In embodiments, fins  16  are formed on the insulator layer  14  using conventional lithography and etching steps. In embodiments, the insulator layer  14  can be a buried oxide layer (BOX) and the fins  16  can be formed from silicon on insulator material (e.g., SOI) or any semiconductor material including, but not limited to, Si, SiGe, SiGeC, SiC, GE alloys, GaAs, InAs, InP, and other III/V or II/VI compound semiconductors. 
     In embodiments, the fins  16  are formed by two etching processes. For example, the first etching process forms the lower portion  16   a  of the fins  16 , which is wider than the narrower portion (body)  16   b  of the fins  16 ; whereas, the second etching process forms the narrower portion (body)  16   b  of the fin  16 . In any of the embodiments described herein, the bottom region, e.g., wide portion  16   a  of the fin  16  can be made significantly taller, e.g., 30 nm, than conventional structures thereby further reducing resistance. 
     By way of example of forming the fins  16 , the first etching process can be a sidewall image transfer (SIT) technique. In the SIT technique, a mandrel material, e.g., oxide or nitride material, is formed on the semiconductor material using conventional deposition, lithography and etching processes. In an example of a SIT technique, the mandrel material can be deposited using conventional CVD processes. A resist is formed on the mandrel material, and exposed to light to form a pattern (openings). A reactive ion etching (RIE) is performed through the openings to form the mandrels. Spacers are formed on the sidewalls of the mandrels which are preferably material that is different than the mandrels, and which are formed using conventional deposition processes known to those of skill in the art. The spacers can have a width which matches the dimensions of the lower portion  16   a  of the fins  16 , for example, e.g., about 7 nm. The mandrels are removed or stripped using a conventional etching process, selective to the mandrel material. An etching is then performed within the spacing of the spacers to form the sub-lithographic features. The sidewall spacers can then be stripped. In embodiments, the narrower fin portions (e.g., body)  16   b  of the fins  16  can be formed after the patterning process of the wider portion  16   a , using conventional patterning processes as contemplated by the present invention. In embodiments, the narrower fin portions  16   b  can be approximately 5 nm or less. 
     As shown in  FIG. 2 , a dielectric material  18  is deposited on the fins  16 , and etched back to expose the vertical portions of the fins  16 , e.g., the narrower fin portions (e.g., body)  16   b  of the fins  16 . In embodiments, the dielectric material  18  will protect the lower portion  16   a  of the fins  16  during subsequent gate formation and will ensure that a later formed source or drain portion formed at this wider portion of the fin will not short to a gate structure. The dielectric material  18  can be an oxide material, which can be blanket deposited over the fins  16  and on any exposed surfaces of the structure by using a conventional deposition process. For example, the deposition process can be a chemical vapor deposition (CVD) process. 
     In  FIG. 3 , a gate dielectric material  20  and a gate material  22  are formed over the fins  16 , e.g., the narrower fin portions (e.g., body)  16   b  of the fins  16 , and on the etched back dielectric material  18 . In embodiments, the gate dielectric material  20  can be a high-k material such as a hafnium based material, e.g., hafnium oxide. The gate material  22  can be any appropriate metal material or combinations of metal materials, depending on the desired workfunction properties. In embodiments, the gate dielectric material  20  and the gate material  22  have a thickness of about 5 nm or less, and wrap around the entire exposed vertical surfaces of the fins  16  (e.g., narrow portion  16   b ), on top of the dielectric material  18 . The gate dielectric material  20  and the gate material  22  can be formed by a conventional deposition process, e.g., CVD, followed by a recessing process such that the vertical extents of the narrower fin portions (e.g., body)  16   b  of the fins  16  are exposed. In embodiments, the recess process can be a chemical mechanical process (CMP), which exposes a top surface of the fin  16  following by an etch back process. 
     In  FIG. 4 , the gate dielectric material  20  and the gate material  22  are etched back, forming a space  24  between adjacent gate structures  10 ′. In embodiments, the space  24  between the gate dielectric material  20  and the gate material  22  can be formed by conventional lithography and etching processes, e.g., reactive ion etching (RIE) processes. The space  24  can be about 6 nm or less and is capable of being further scaled. In embodiments, the recessing of the gate dielectric material  20  and the gate material  22  can also be performed prior to or after space formation. 
     As shown in  FIG. 5 , a dielectric material  26  is formed within the space between the adjacent structures  10 ′, and on any exposed vertical extent of the narrow portion  16   b  of the fin  16 , resulted from the etch back process of the gate dielectric material  20  and the gate material  22 . The dielectric material  26  can be an oxide material, acting as isolation structures between the adjacent structures  10 ′, as well as separating a later formed source or drain region from the gate structure, e.g., the gate dielectric material  20  and the gate material  22 . The dielectric material  26  can be formed by a conventional deposition process (e.g., CVD), followed by a planarization process. In embodiments, the planarization process will expose top surfaces of the fins  16 , e.g., semiconductor material. 
     In  FIG. 6 , a wide (thick) portion of semiconductor material  16   c  is formed on the fin  16  by an epitaxial growth process. As should be understood by those of skill in the art, the wide portion  16   c  can be used as a source or drain region of the structure  10 ′; whereas, the wide portion  16   a  can be used as a drain or source region of the structure  10 ′, respectively. The dielectric material  20  and the gate material  22  will wrap around the vertical sidewalls of the fins  16 , between the wider portions  16   a ,  16   c.    
     Referring now to  FIGS. 7 and 8 , contact openings  28  are formed in the dielectric material  26 , in order to form silicide regions  16   a ′ and  16   c ′ on the source and drain contact regions of the gate structure  10 ′. In embodiments, the openings  28  are formed by conventional lithography and etching processes, e.g., reactive ion etching (RIE) processes. The silicide regions  16   a ′ and  16   c ′ can be formed by a platinum silicide process; although cobalt and nickel silicide processes are also contemplated by the present invention. 
     As should be understood by those of skill in the art, the silicide process begins with deposition of a thin transition metal layer, e.g., platinum, cobalt or nickel, over fully formed and patterned semiconductor devices (e.g., doped or ion implanted source and drain regions formed from the wide portions  16   a ,  16   c  as should be understood by those of skill in the art). After deposition of the material, the structure is heated allowing the transition metal to react with exposed silicon (or other semiconductor material as described herein) in the active regions of the semiconductor device (e.g., source, drain, gate contact region) forming a low-resistance transition metal silicide. Following the reaction, any remaining transition metal is removed by chemical etching, leaving silicide contacts  16   a ′ and  16   c ′ in the active regions of the device. 
       FIG. 8  shows a cross-sectional view of a single device  10 ′. In  FIG. 8 , the drain contact  30 , the gate contact  32  and the source contact  34  are formed through the openings. The contacts  30 ,  32 ,  34  are formed by a metal deposition process, in direct electrical contact with the respective silicide regions  16   a ′ and  16   c ′ and on metal gate material deposited on the gate structure. The contacts  30 ,  32 ,  34  can be formed by a metal deposition process, with liner material. For example, the contacts can be tungsten or copper or alloys thereof, with the liner material being TiN or TaN; although other materials are also contemplated by the present invention. 
     In embodiments, the metal material can be formed by a conventional deposition process, e.g., CVD, followed by a planarization process to remove any excessive material from the surface of the dielectric material  26 . Following the metal fill process to form the contacts  30 ,  32 ,  34 , additional dielectric material  26 ′ is formed on the structure, e.g., over the devices  10 ′, followed by a planarization process to expose portions of the contacts  30 ,  32 ,  34  for middle of the line (MOL) and back end of the line (BEOL) processes. In embodiments, the dielectric material  26 ,  26 ′ can be an oxide material or ultra low-k dielectric material, as examples. As should now be understood by those of skill in the art, by implementing the processes of the present invention, e.g., moving the contact to the ends of the structures, the resultant pitch of adjacent devices  10 ′ can now be scaled significantly, e.g., 6 nm or less. 
       FIGS. 9 a  and 9 b    show a cross-sectional view and a top down view of an alternative structure  10 ″, after formation of the additional dielectric material  26 ′. In embodiments, the deposition of the additional dielectric material  26 ′ will form an air gap  36  between adjacent devices  10 ″, effectively reducing the contact resistance. (This air gap  36  can also be formed with the process flow of  FIG. 8 .) The formation of the air gap  36  is due to a pinching effect of the deposition process, between the minimum pitch, e.g., spacing, of the adjacent devices  10 ″. 
     In addition, the devices  10 ″ shown in  FIGS. 9 a  and 9 b    include a shared contact strap  32 ′ formed at the drain regions of the devices (FETs)  10 ″. It should be understood by those of skill in the art that the shared contact strap  32 ′ can equally be formed at the source regions, e.g., other end of the device  10 ″, or both the source and drain regions as schematically represented by  FIGS. 9 a  and 9 b   . Thus, the present invention contemplates shared or individual S/D silicide and contact regions. Also, in each of the embodiments described herein, the channel current is in a vertical direction, as these devices are vertical FETs. 
       FIGS. 10 a  and 10 b    show a cross-sectional view and a top down view of alternative devices  10 ′″ and respective processing steps in accordance with aspects of the present invention. These alternative devices  10 ′″ include merged regions  40 , e.g., bottom merged drain region, or alternatively a merged source region or both a source region and a drain region. As should be understood, these merged devices  10 ′″ provide even lower resistance between adjacent FETs; compared to a non-merged FET. The merged devices  10 ′″ can be formed using the processes described herein, with the merged regions  40  being formed together during the formation of the fins structures (and subsequent ion implantation or doping of the source/drain regions), instead of separately for separate devices. A contact  40 ′ also spans across the merged region  40 . The contact  40 ′ is formed by conventional lithography, etching and deposition processes as already described herein including the silicide and contact formation. In embodiments, an air gap  36  can optionally be formed between the devices  10 ′″, as already described herein. 
       FIGS. 11 a  and 11 b    show a cross-sectional view and a top down view of alternative devices  10 ″″ and respective processing steps in accordance with aspects of the present invention. These alternative devices  10 ″″ are single sided gates, as shown representatively by the metal material  22 ′ and dielectric material  20 ′ provided on only a single side of the fin structures  16 . These devices  10 ″″ also include an optional merged drain region  40  and contact  40 ′, resulting in lower resistance and higher capacitance. In embodiments, the drain region  40  and contact  40 ′ of the devices  10 ″″ can be representative of a merged source region, or both a source and a drain region. In embodiments, an air gap  36  can optionally be formed between the devices  10 ″″, as already described herein. 
       FIG. 12  shows another alterative structure and respective processing steps. In this alternative structure, the devices  10 ′″″ include gate material  22  and dielectric material  20  wrapping around two fin structures  16 ′ (in parallel). In embodiments, the gate material  22  and dielectric material  20  can be deposited and patterned in the same processing steps to form the gate structures spanning over the two fin structures  16 ′. Also, an air gap  36  can optionally be formed between adjacent devices  10 ′″″, extending along the two fin structures  16 ′, as already described herein. 
     Moreover, the devices  10 ′″″ include a merged bottom drain region  60 , or alternatively a source region or both a source region and a drain region, thereby resulting in lower resistance and higher capacitance. The alternative devices  10 ′″″ also can include a shared contact strap as shown by reference numeral  60 , formed using the processes described herein and as should understood by those of ordinary skill in the art such that no further explanation is needed for an understanding of the present invention. 
     The method(s) as described above is used in the fabrication of integrated circuit chips. The resulting integrated circuit chips can be distributed by the fabricator in raw wafer form (that is, as a single wafer that has multiple unpackaged chips), as a bare die, or in a packaged form. In the latter case the chip is mounted in a single chip package (such as a plastic carrier, with leads that are affixed to a motherboard or other higher level carrier) or in a multichip package (such as a ceramic carrier that has either or both surface interconnections or buried interconnections). In any case the chip is then integrated with other chips, discrete circuit elements, and/or other signal processing devices as part of either (a) an intermediate product, such as a motherboard, or (b) an end product. The end product can be any product that includes integrated circuit chips, ranging from toys and other low-end applications to advanced computer products having a display, a keyboard or other input device, and a central processor. 
     The descriptions of the various embodiments of the present invention have been presented for purposes of illustration, but are not intended to be exhaustive or limited to the embodiments disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the described embodiments. The terminology used herein was chosen to best explain the principles of the embodiments, the practical application or technical improvement over technologies found in the marketplace, or to enable others of ordinary skill in the art to understand the embodiments disclosed herein.