Patent Publication Number: US-7723787-B2

Title: SOI MOSFET device with reduced polysilicon loading on active area

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   The present application is a division of U.S. patent application Ser. No. 11/053,461, filed on Feb. 8, 2005. 

   TECHNICAL FIELD 
   The present invention relates to semiconductor devices and fabrication methods thereof, and particularly to silicon-on insulator (SOI) devices and methods for fabricating the same. 
   BACKGROUND 
   Silicon-on-insulator (SOI) technology has become an increasingly important technique utilized in the fabrication and production of semiconductor devices. SOI technology deals with the formation of transistors in a relatively thin monocrystalline semiconductor layer, which overlays an insulating layer. In other words, the active devices are formed in a thin semiconductor on an insulator layer rather than in the bulk semiconductor of the device. SOI technology makes possible certain performance advantages, such as the reduction of parasitic elements present in integrated circuits formed in bulk semiconductors, useful in high performance and high-density integrated circuits. SOI technology further allows for the mapping of standard advanced technologies into a SOI technology without significant modifications, and exhibits its advantages for higher speed, lower power consumption and better radiation immunity due to the enhanced isolation of buried oxide layers. 
   On a typical SOI transistor, however, the body is generally isolated from the silicon substrate and usually kept floating, and this may result in serious problems for current-sensitive circuit applications. The body retains charges and some of the electrical properties from the last time the transistor was used, interfering with subsequent use of the device. A variety of solutions have been proposed to address the problems associated with the SOI semiconductor device. For example, the use of a body contact in the SOI device addresses this problem, and also allows the threshold voltage to be changed so that standby power can be reduced for low-power applications. The body contact in the SOI device has conventionally been made by the use of a T-shaped or a H-shaped polysilicon structure on an active area, thereby creating three distinct regions including a source region, a drain region, and a body contact region. 
     FIG. 1A  is a top view illustrating a conventional T-shaped polysilicon structure with a body contact.  FIG. 1B  is a cross-sectional view along line  1 B- 1 B in  FIG. 1A .  FIG. 1C  is a cross-sectional view along line  1 C- 1 C in  FIG. 1A . An SOI substrate  10 , typically including a silicon base layer, a buried oxide layer and a semiconductor silicon layer (e.g., a P −  type layer), is provided with an active area  12  defined by a shallow trench isolation (STI) structure  14  formed in the semiconductor silicon layer over the buried oxide layer. A T-shaped polysilicon layer  16  is formed on the active area  12 , thus creating three distinct regions including a source region  17 , a drain region  18 , and a body contact region  20  in the semiconductor silicon layer. For an example of NMOS device, the source region  17  and the drain region  18  are N + -type regions, and the body contact region  20  is a P + -type region.  FIG. 2A  is a top view illustrating a conventional H-shaped polysilicon structure with body contacts.  FIG. 2B  is a cross-sectional view along line  2 B- 2 B in  FIG. 2A .  FIG. 2C  is a cross-sectional view along line  2 C- 2 C in  FIG. 2A . Explanation of the same or similar portions to the description in  FIGS. 1A-1C  is omitted herein. By comparison, the T-shaped polysilicon layer  16  is replaced with a H-shaped polysilicon layer  16 ″, thus creating four distinct regions. The conventional approaches increase parasitic diode due to increased P + /P − /N +  junction areas, thus improving the current sink capacity in the body contact region. The T-shaped or H-shaped polysilicon layer, however, including a polysilicon gate portion and a polysilicon dummy portion, induces high gate capacitance, usually causing poor performance. 
   Therefore, there exists a need for a SOI MOSFET device that allows a reduction of polysilicon loading on the active area to overcome problems arising from high gate capacitance. 
   SUMMARY OF THE INVENTION 
   Embodiments of the present invention include silicon-on-insulator (SOI) devices with reduced polysilicon loading on an active area, in which at least one dielectric layer resistant to silicidation is employed to separate at least one body contact region from source/drain regions, thus reducing gate capacitance and improving device performance. The SOI devices may be used in full depletion type transistors or partial depletion type transistors. 
   In one aspect, the present invention provides a silicon-on-insulator (SOI) device on a SOI substrate having a semiconductor layer overlying an insulator layer. At least one gate electrode layer is over the semiconductor layer of an active area. A source region and a drain region are in the semiconductor layer of the active area and separated from each other by the gate electrode layer. A dielectric layer resistant to silicidation is over the semiconductor layer of the active area. A body contact region is in the semiconductor layer of the active area and separated from the source region and the drain region by the dielectric layer resistant to silicidation. 
   In another aspect, the present invention provides a silicon-on-insulator (SOI) chip. A SOI substrate having a semiconductor layer overlying an insulator layer. At least two dielectric layers resistant to silicidation are over the semiconductor layer of an active area. At least one transistor located between the dielectric layers resistant to silicidation includes a gate electrode layer over the semiconductor layer, and source/drain regions in the semiconductor layer. At least two body contact regions are in the semiconductor layer of the active area and separated from the source/drain regions by the dielectric layers resistant to silicidation. 
   In another aspect, the present invention provides a method of forming a silicon-on-insulator (SOI) device. At least one gate electrode layer and at least one dielectric layer resistant to silicidation are formed over a semiconductor layer of an active area. A source region and a drain region are formed in the semiconductor layer of the active area, and the gate electrode layer separates the source region and the drain region. At least one body contact region is formed in the semiconductor layer of the active area, and the dielectric layer resistant to silicidation separates the body contact region from the source region and the drain region. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The aforementioned objects, features and advantages of this invention will become apparent by referring to the following detailed description of the preferred embodiments with reference to the accompanying drawings, wherein: 
       FIG. 1A  is a top view illustrating a conventional T-shaped polysilicon structure with a body contact; 
       FIG. 1  B is a cross-sectional view along line  1 B- 1 B in  FIG. 1A ; 
       FIG. 1C  is a cross-sectional view along line  1 C- 1 C in  FIG. 1A ; 
       FIG. 2A  is a top view illustrating a conventional H-shaped polysilicon structure with body contacts; 
       FIG. 2B  is a cross-sectional view along line  2 B- 2 B in  FIG. 2A ; 
       FIG. 2C  is a cross-sectional view along line  2 C- 2 C in  FIG. 2A ; 
       FIG. 3A  is a top view illustrating a RPO layer used to distinguish a body contact region from source/drain regions; 
       FIG. 3B  is a cross-sectional diagram along line  3 B- 3 B of  FIG. 3A ; 
       FIG. 3C  is a cross-sectional diagram along line  3 C- 3 C of  FIG. 3A ; 
       FIG. 4A  is a top view illustrating a suicide formation on the completed structure shown in  FIG. 3A ; 
       FIG. 4B  is a cross-sectional diagram along line  4 B- 4 B of  FIG. 4A ; 
       FIG. 4C  is a cross-sectional diagram along line  4 C- 4 C of  FIG. 4A ; 
       FIG. 5A  is a top view illustrating a dielectric spacer formation and a silicide formation on the completed structure shown in  FIG. 3A ; 
       FIG. 5B  is a cross-sectional diagram along line  5 B- 5 B of  FIG. 5A ; 
       FIG. 5C  is a cross-sectional diagram along line  5 C- 5 C of  FIG. 5A ; 
       FIG. 6A  is a top view illustrating a pair of RPO layers provided on the active region; 
       FIG. 6B  is a cross-sectional diagram along line  6 B- 6 B of  FIG. 6A ; 
       FIG. 6C  is a cross-sectional diagram along line  6 C- 6 C of  FIG. 6A ; 
       FIG. 7A  is a top view illustrating four RPO layers provided on the active region; 
       FIG. 7B  is a cross-sectional diagram along line  7 B- 7 B of  FIG. 7A ; 
       FIG. 7C  is a cross-sectional diagram along line  7 C- 7 C of  FIG. 7A ; 
       FIG. 8A  is a top view illustrating a modification of four RPO layers for forming a circle-like body contact region; 
       FIG. 8B  is a cross-sectional diagram along line  8 B- 8 B of  FIG. 8A ; 
       FIG. 8C  is a cross-sectional diagram along line  8 C- 8 C of  FIG. 8A ; 
       FIG. 9A  is a top view illustrating three RPO layers provided on the active region; 
       FIG. 9B  is a cross-sectional diagram along line  9 B- 9 B of  FIG. 9A ; 
       FIG. 9C  is a cross-sectional diagram along line  9 C- 9 C of  FIG. 9A ; 
       FIG. 10A  is a top view illustrating a modification of three RPO layers for forming a reversed U-shaped body contact region; 
       FIG. 10B  is a cross-sectional diagram along line  10 B- 10 B of  FIG. 10A ; 
       FIG. 10C  is a cross-sectional diagram along line  10 C- 10 C of  FIG. 10A ; 
       FIG. 11A  is a top view illustrating a partial depletion type SOI device; 
       FIG. 11B  is a cross-sectional diagram along line  11 B- 11 B of  FIG. 11A ; and 
       FIG. 11C  is a cross-sectional diagram along line  11 C- 11 C of  FIG. 11A . 
   

   DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS 
   Embodiments of the present invention provide SOI devices with a reduction of polysilicon loading on an active area where a dielectric layer resistant to silicidation, such as a RPO (resist protective oxide) layer, is used to replace the polysilicon dummy portion of the conventional T-shaped or H-shaped polysilicon structure, thus reducing gate capacitance and improving device performance. Fabricating methods for such SOI devices are more facile than the conventional body contact process, and there is no need for generating extra layout area. Embodiments of the present invention provide several modifications of the RPO pattern, which may be used in full depletion type SOI transistors or partial depletion type SOI transistors. 
   As used throughout this disclosure, the term “a dielectric layer resistant to silicidation” refers to a protective layer that is used where a salicide process is not desired to take place. It may be desired to perform the salicide process on one part of a wafer while protecting another portion of the wafer from silicidation. Except for oxide (such as RPO), any other material that protects the semiconductor device from silicidation may be used as the dielectric layer resistant to silicidation, for example nitride-containing dielectric materials, silicon nitride, silicon oxynitride or the like. 
   Reference will now be made in detail to the present embodiments, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers are used in the drawings and the description to refer to the same or like parts. In the drawings, the shape and thickness of one embodiment may be exaggerated for clarity and convenience. This description will be directed in particular to elements forming part of, or cooperating more directly with, apparatus in accordance with the present invention. It is to be understood that elements not specifically shown or described may take various forms well known to those skilled in the art. Further, when a layer is referred to as being on another layer or “on” a substrate, it may be directly on the other layer or on the substrate, or intervening layers may also be present. 
   Herein, a SOI MOSFET device with a body contact region according to an exemplary embodiment of the present invention is illustrated.  FIG. 3A  is a top view illustrating a RPO layer used to distinguish a body contact region from source/drain regions,  FIG. 3B  is a cross-sectional diagram along line  3 B- 3 B of  FIG. 3A , and  FIG. 3C  is a cross-sectional diagram along line  3 C- 3 C of  FIG. 3A . A SOI substrate  30  is provided with an active area  32  defined by an isolation structure that is omitted for clarity. The SOI substrate  30  is comprised of a base substrate  31 , a buried insulator layer  33  and a semiconductor layer  34 . The base substrate  31  may comprise silicon, gallium arsenide, gallium nitride, strained silicon, silicon germanium, silicon carbide, carbide, diamond, an epitaxy layer, and/or other materials. The buried insulator layer  33  may comprise silicon oxide, silicon nitride, silicon oxynitride, and/or other dielectric materials. The semiconductor layer  34  may comprise silicon, gallium arsenide, gallium nitride, strained silicon, silicon germanium, silicon carbide, carbide, diamond, and/or other materials. The buried insulator layer  33  and the semiconductor layer  34  may be formed using various SOI technologies. For example, the buried insulator layer  33  may be formed on a semiconductor wafer by a process referred to as separation by implanted oxygen (SIMOX). The SIMOX technology is based on ion-implanting a high-dose of oxygen ions into a silicon wafer, such that the peak concentration lies beneath the silicon surface. After implantation the wafer is subjected to a high-temperature anneal to form a continuous stoichiometric subsurface-layer of silicon dioxide. Thus formed dielectric layer  33 , also referred to as buried oxide (BOX), electrically separates the semiconductor layer  34  and the base substrate  31 . After the SOI process, isolation structures, such as shallow trench isolation (STI) structures, may be formed in the semiconductor layer  34  for defining element-to-element active areas  32 . 
   At least one gate structure is then formed on the active area  32  through advances in deposition, lithography and masking techniques and dry etching processes. The gate structure includes a gate dielectric layer  36  and a gate electrode layer  38 . The active area  32  may include one or more gate structures separated from each other with an appropriate distance allowed by a predetermined design rule. In one embodiment, the gate dielectric layer  36  is a silicon oxide layer with a thickness chosen specifically for the scaling requirements of the SOI MOSFET device technology, for example, formed through a thermal oxidation process or a chemical vapor deposition (CVD) process. It is to be appreciated other well-known gate dielectric material such as oxides, nitrides, and combinations thereof. In one embodiment, the gate electrode layer  38  is a polysilicon layer with a gate length chosen specifically for the scaling requirements of the SOI MOSFET device technology, for example deposited through Low Pressure CVD (LPCVD) methods, CVD methods and Physical Vapor Deposition (PVD) sputtering methods employing suitable silicon source materials. If desired the polysilicon layer may be ion implanted to the desired conductive type. It is to be appreciated other well-known gate electrode material such as metal, metal alloys, single crystalline silicon, or any combinations thereof. It is noted that each gate electrode layer  38  is a stripe-shaped polysilicon pattern without adding a dummy polysilicon portion as an end-to-end connection as compared with the conventional T-shaped or H-shaped polysilicon pattern, therefore the polysilicon-occupied area on the active area  32  is significantly decreased to achieve a reduction of gate capacitance and an improvement of device performance. 
   Following the formation of gate structures, an RPO layer  40 , source/drain regions  42  and a body contact region  44  are formed by the use of any well-known processes. In one embodiment, the RPO layer  40  formed on the active area  32  is a stripe-shaped dielectric layer across ends of the gate electrode layers  38  respectively, thus a combination of the RPO layer  40  and the gate electrode layers  38  can create three separated regions including the source region  42 , the drain region  42  and the body contact region  44  formed in the semiconductor layer  34 . The RPO layer  40  may be deposited through CVD methods and then patterned by using well-known lithography, masking and etching processes. In one embodiment, the RPO layer  40  may comprise TEOS oxide or silicon oxide (SiO x ). In one embodiment, the RPO layer  40  may be replaced by using nitride-containing dielectric materials, such as silicon nitride, silicon oxynitride or the like. Alternatively, the RPO layer  40  may be replaced by using any other material suitable for dielectric layer resistant to silicidation. The source/drain regions  42  and body contact regions  44  may be formed by the use of ion implantation processes performed with various dopant species into the semiconductor layer  34 . Typically, the dopant for the body contact region  44  has the same polarity as the body (e.g., the region  35  of the semiconductor layer  34  underlying the gate electrode layer  38  and the RPO layer  40 ), and has a doping concentration greater than that of the body. The polarity of dopant for the body contact region  44  is different from that of the source/drain regions  44 . In one exemplary embodiment of N-channel MOSFET device as depicted in  FIGS. 3A to 3C , the source/drain regions  42  are N +  regions, the body contact region  44  is a P +  region, and the region  35  is a P −  region. If P-channel MOSFET devices are desired, the source/drain regions are P +  regions, the body contact region is a N +  region, and the body region is a N −  region. 
     FIG. 4A  is a top view illustrating a silicide formation on the completed structure shown in  FIG. 3A ,  FIG. 4B  is a cross-sectional diagram along line  4 B- 4 B of  FIG. 4A , and  FIG. 4C  is a cross-sectional diagram along line  4 C- 4 C of  FIG. 4A . First, a metal layer such as a refractory metal layer including cobalt, tungsten, titanium, nickel or the like is deposited over the entire surface through the use of RF sputtering, CVD, PVD or any advanced deposition technology. An annealing process, such as a rapid thermal anneal (RTA) method, is then performed, causing the deposited metal to be converted into its silicide wherever it is in direct contact with silicon. A selective etchant such as hydrogen peroxide is then used to remove all unreacted metal, i.e. all metal that is in contact with oxide rather than silicon, obtaining silicide regions  46  on the gate electrode layers  38 , the source/drain regions  42  and the body contact region  44 . 
   In one embodiment, dielectric spacers are provided on sidewalls of the gate structures prior to the formation of source/drain regions  42 .  FIG. 5A  is a top view illustrating a dielectric spacer formation and a silicide formation on the completed structure shown in  FIG. 3A ,  FIG. 5B  is a cross-sectional diagram along line  5 B- 5 B of  FIG. 5A , and  FIG. 5C  is a cross-sectional diagram along line  5 C- 5 C of  FIG. 5A , while explanation of the same or similar portions to the description in  FIGS. 3A-3C  and  FIGS. 4A-4C  is omitted. Following the formation of gate structures, dielectric spacers  48  are formed along the sidewalls of the gate structures respectively. For example, a lightly doped ion implantation process may be performed with various dopant species into the semiconductor layer  34  to form LDD regions. The margins of the LDD regions are substantially aligned to the sidewall of the gate structures. Advances in deposition, lithography, masking techniques and dry etch processes are followed to form the dielectric spacers  48  along the sidewalls of the gate structures. The dielectric spacers  48  may be formed of a silicon nitride layer, a silicon oxide layer, a silicon oxynitride layer, alternating layers of silicon oxide and silicon nitride, or combinations thereof. A heavily doped ion implantation process is then performed with the dielectric spacers  48  and the gate structures as the mask to implant various dopant species into the semiconductor layer  34 , resulting in the source/drain regions  42 . Thus, in the subsequent salicide process, the silicide regions  46  are selectively formed on the surfaces of the gate electrode layers  38 , source/drain regions  42  and the body contact region  44  to lower their resistance values, without forming on surfaces of the RPO layer  40  and the dielectric spacers  48 . 
   Embodiments of the present invention provide various modifications of the RPO pattern. In one embodiment, a pair of RPO layers  40   a  and  40   b  are provided on the active region  32  as depicted in  FIGS. 6A ,  6 B and  6 C, while explanation of the same or similar portions to the description in  FIGS. 3A-3C  is omitted. The first RPO layer  40   a  is a stripe-shaped layer across the first end of each gate electrode layer  38 , thus distinguishing the source/drain regions  42  and the first body contact region  44   a . Similarly, the second RPO layer  40   b  is a stripe-shaped layer across the second end of each gate electrode layer  38 , thus distinguishing the source/drain regions  42  and the second body contact region  44   b . That is, the gate electrode layers  38  are sandwiched between two parallel RPO layers  40   a  and  40   b , thus creating four separated regions including one source region, one drain region and two body contact regions. 
   In one embodiment, another pair of RPO layers  40   c  and  40   d  are further provided between the two parallel RPO layers  40   a  and  40   b  as depicted in  FIGS. 7A ,  7 B and  7 C, while explanation of the same or similar portions to the description in  FIGS. 6A-6C  is omitted. The third RPO layer  40   c  is a stripe-shaped layer laterally adjacent to the left-most gate electrode layer  38  and connected to the two parallel RPO layers  40   a  and  40   b . Similarly, the fourth RPO layer  40   c  is a stripe-shaped layer laterally adjacent to the right-most gate electrode layer  38  and connected to the two parallel RPO layers  40   a  and  40   b . The arrangement of the four RPO layers  40   a ,  40   b    40   c  and  40   d  forms a closed loop, thus source/drain regions  42  are distinguished by the gate electrode layers  38  inside this closed loop, and four separated body contact regions  44  are created outside this closed loop. 
   In one embodiment, the arrangement of the four RPO layers  40   a ,  40   b    40   c  and  40   d  are modified to form the four body contact regions  44  as a continuous region (e.g., a circle-like body contact region) as depicted in  FIGS. 8A ,  8 B and  8 C. For example, the two ends of the first RPO layer  40   a  are substantially aligned to the outer sidewalls of the third RPO layer  40   c  and the fourth RPO layer  40   d , respectively. Similarly, the two ends of the second RPO layer  40   b  are substantially aligned to the outer sidewalls of the third RPO layer  40   c  and the fourth RPO layer  40   d , respectively. Therefore, the rectangular-loop RPO pattern can define a circle-like body contact region  44 . 
   In one embodiment, the second RPO layer  40   b  is removed from the active area  32 , as depicted in  FIGS. 9A ,  9 B and  9 C, thus the three RPO layers  40   a ,  40   c  and  40   d  create three separated body contact regions  44 . In one embodiment, the arrangement of the three RPO layers  40   a ,  40   c  and  40   d  can be further modified to change the three separated body contact regions  44  into a continuous region, as depicted in  FIGS. 10A ,  10 B and  10 C. For example, the two ends of the first RPO layer  40   a  are substantially aligned to the outer sidewalls of the third RPO layer  40   c  and the fourth RPO layer  40   d  respectively. Therefore, the reversed U-shaped RPO pattern can define a reversed U-shaped (e.g., a door-like loop) body contact region  44 . 
   A transistor wherein the maximum depletion layer width is greater than the thickness of the semiconductor layer at the channel region is called a full depletion type SOI transistor, while a transistor wherein the maximum depletion layer width is smaller than the thickness of the semiconductor layer at the channel region is called a partial depletion type SOI transistor. The above-described embodiments use the full depletion type SOI devices where channel parts are fully depleted, but the partial depletion type SOI devices may be used in all embodiments of the present invention if dispersion of the threshold voltage can be suppressed.  FIG. 11A  is a top view illustrating a partial depletion type SOI device,  FIG. 11B  is a cross-sectional diagram along line  11 B- 11 B of  FIG. 11A , and  FIG. 11C  is a cross-sectional diagram along line  11 C- 11 C of  FIG. 11A . For a full depletion type SOI transistor, a thickness of the semiconductor layer may be made thin such as the same as or smaller than approximately 50 nm, thus a short channel effect can be reduced. By comparison, the partial depletion type SOI transistor may need a thicker semiconductor layer of more than 50 nm and generally requires a body contact. 
   Although the present invention has been described in its preferred embodiments, it is not intended to limit the invention to the precise embodiments disclosed herein. Those skilled in this technology can still make various alterations and modifications without departing from the scope and spirit of this invention. Therefore, the scope of the present invention shall be defined and protected by the following claims and their equivalents.