Abstract:
A method of making a semiconductor device includes thickening source and drain regions. After a field effect device having a source region, a drain region, and a gate, is formed, a layer of semiconductor material is deposited on the device by a directional deposition method, such as collimated sputtering. Then the semiconductor material is selectively removed from side walls on either side of the gate, such as by isotropic back etching, leaving thickened semiconductor material in the source and drain regions, and on the gate.

Description:
This application claims the benefit of U.S. Provisional Application No. 60/286,941, filed Apr. 27, 2001, which is hereby incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Technical Field 
     The present invention generally relates to a semiconductor device and methods for manufacturing the semiconductor device and, more particularly, to a field effect transistor having elevated source and drain regions and methods for manufacturing the same. 
     2. Description of the Related Art 
     The semiconductor industry is increasingly characterized by a growing trend toward fabricating larger and more complex circuits on a given semiconductor chip. This is being achieved by reducing the size of individual devices within the circuits and spacing the devices closer together. The reduction of the size of individual devices and the closer spacing brings about improved electrical performance. 
     As the physical dimensions of field effect devices are scaled down, the operating voltages are being scaled down accordingly in order not to have excessive internal electric fields. At low operating voltages, it is increasingly important to have small parasitic resistances at the source, drain and gate regions. This is usually achieved by forming a metal silicide in these regions and making contacts to the low resistivity metal silicide. 
     However, in field effect devices with very shallow junctions or in fully depleted semiconductor-on-insulator SOI devices, the silicon layer at the source and drains regions are insufficient for metal silicide formation. A previous method of addressing this problem has been to thicken the source and drain by epitaxial growth of semiconductor material. However, thickening by epitaxial growth involves high-temperature processes that may cause undesirable redistribution of dopants. 
     SUMMARY OF THE INVENTION 
     A method of making a semiconductor device includes thickening source and drain regions. After a field effect device having a source region, a drain region, and a gate, is formed, a layer of semiconductor material is deposited on the device by a directional deposition method, such as collimated sputtering. Then the semiconductor material is selectively removed from side walls on either side of the gate, such as by isotropic back etching, leaving thickened semiconductor material in the source and drain regions, and on the gate. 
     According to an aspect of the invention, a method of thickening a source and drain of a transistor device includes directionally depositing semiconductor material, and isotropically etching the semiconductor material. 
     According to another aspect of the invention, a method of making a semiconductor-on-insulator device includes the steps of forming a structure including a source region and a drain region in a surface semiconductor layer of the device, and a gate and a pair of spacers on the surface semiconductor layer, wherein the spacers on respective opposite sides of the gate, and wherein the gate is operatively coupled to the source region and the drain region; directionally depositing semiconductor material on the gate, the spacers, and on exposed portions of the source region and the drain region; and selectively removing the semiconductor material to uncover at least part of each of the spacers, the selectively removing leaving a source-side slab of the semiconductor material overlying the source, and a drain-side slab of the semiconductor material overlying the drain. 
     According to still another aspect of the invention, a method of making a semiconductor-on-insulator device includes the steps of a) forming a structure including a source region and a drain region in a surface semiconductor layer of the device, and a gate and a pair of spacers on the surface semiconductor layer, wherein the spacers on respective opposite sides of the gate, and wherein the gate is operatively coupled to the source region and the drain region, the forming including: i) forming a gate on the surface semiconductor layer; ii) forming a source extension and a drain extension on respective opposite sides of the gate; iii) forming the spacers on opposite sides of the gate; and iv) forming the source region and the drain region; b) directionally depositing semiconductor material on the gate, the spacers, and on exposed portions of the source region and the drain region; and c) selectively removing the semiconductor material to uncover at least part of each of the spacers, the selectively removing leaving a source-side slab of the semiconductor material overlying the source, a drain-side slab of the semiconductor material overlying the drain, and a gate slab of the semiconductor material at least partially overlying the gate. 
     According to a further aspect of the invention, a method of making a semiconductor-on-insulator device includes the steps of forming a structure including a source region and a drain region in a surface semiconductor layer of the device, and a gate and a pair of spacers on the surface semiconductor layer, wherein the spacers on respective opposite sides of the gate, and wherein the gate is operatively coupled to the source region and the drain region; collimated sputtering semiconductor material on the gate, the spacers, and on exposed portions of the source region and the drain region; isotropically etching the semiconductor material to uncover at least part of each of the spacers, the selectively removing leaving a source-side slab of the semiconductor material overlying the source, a drain-side slab of the semiconductor material overlying the drain, and a gate slab of the semiconductor material at least partially overlying the gate; depositing a metal layer; and annealing the device to induce formation of semiconductor-metal compound regions at intersections of the slabs and the metal layer. 
     To the accomplishment of the foregoing and related ends, the invention comprises the features hereinafter fully described and particularly pointed out in the claims. The following description and the annexed drawings set forth in detail certain illustrative embodiments of the invention. These embodiments are indicative, however, of but a few of the various ways in which the principles of the invention may be employed. Other objects, advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the annexed drawings: 
     FIG. 1 is a cross-sectional view of a semiconductor device in accordance with the present invention; and 
     FIGS. 2-11 are cross-sectional views of various steps in a method of fabricating the semiconductor device of FIG.  1 . 
    
    
     DETAILED DESCRIPTION 
     A field effect device includes thickened source and drain regions formed by a highly directional deposition of semiconductor material, such as by collimated sputtering, followed by removal of the semiconductor material from the sidewalls of spacers on opposite sides of a gate, such as by isotropic etching. In addition to the thickened source and drain regions, the method may also cause widening of the gate at the top, resulting in reduced gate resistance. 
     This invention is described below in the context of a fully depleted SOI device, since such devices are expected to benefit from the invention the most. However, all aspects of the invention apply equally well to bulk field effect devices, which will also benefit when the source and drain junctions are too shallow to support silicidation. 
     Referring initially to FIG. 1, a semiconductor device  10  includes an SOI wafer  12  with a transistor  14  formed thereupon. The SOI wafer  12  includes a semiconductor substrate  16  and a surface semiconductor layer  18 , with a buried insulator layer  20  therebetween. The semiconductor substrate  16  and the surface semiconductor layer  18  may be made of silicon, and the buried insulator layer  20  may be made of a silicon oxide such as SiO 2 , although it will be appreciated that other suitable materials may be used instead or in addition. 
     The transistor  14  includes a gate  22  formed on an active semiconductor region  24  of the surface semiconductor layer  18 . The gate  22  includes a gate dielectric  26  and a gate electrode  28 . In addition, spacers  30  and  32  are on respective opposite sides of the gate  22 . Exemplary materials for the gate dielectric  26  are SiO 2  and Si 3 N 4 . The gate electrode  28  may be made of polysilicon or another semiconductor, or may be made in whole or in part of metal. An exemplary material for the spacers  30  and  32  is SiN. 
     The active region  24  includes a body  38 , with a source  40  (also referred to as “source region”) and a drain  42  (also referred to as “drain region”) on respective opposite sides of the body. The source  40  and the drain  42  have respective source and drain extensions  46  and  48 . As is conventional, the body  38  is primarily of different conductivity semiconductor material than the source  40  and the drain  42 . For instance, the body  38  may be P−conductivity silicon while the source  40  and the drain  42  may be N−conductivity silicon. Alternatively, the body  38  may be N−conductivity silicon while the source  40  and the drain  42  may be P−conductivity silicon. 
     The body  38 , the source  40 , and the drain  42 , are operatively coupled with the gate  22  to function as a transistor. The source  40  and the drain  42  are covered by respective source and drain electrically-conducting metal-semiconductor compound regions  54  and  56  (also referred to as “silicide regions”), to facilitate electrical connection to the source and drain. The gate electrode  28  likewise may be include an upper gate region  60  with a metal-semiconductor compound to facilitate electrical connection. 
     The source and drain silicide regions  54  and  56  include portions that are elevated above a top surface  62  of the active region  24 . The gate silicide region  60  includes overhangs  70  and  72  which overlie the spacers  30  and  32 , respectively. 
     The metal semiconductor compounds may include compounds of titanium, cobalt, and/or tungsten, such as titanium suicide (TiSi 2 ) or cobalt silicide (CoSi 2 ). It will be appreciated that these are only examples, and that other suitable metal semiconductor compounds may alternatively be used. 
     The active region  24  is laterally isolated from other structures of the device  10  by insulator-filled trenches  82  and  84  on opposite sides of the active region. The insulator-filled trenches  82  and  84  may be trenches filled with silicon dioxide (SiO 2 ) using known isolation techniques. 
     Various steps in the fabrication of the above-described semiconductor device  10  are illustrated in FIGS. 2-11. Referring initially to FIG. 2, starting initially with the SOI wafer  12 , insulator-filled trenches  82  and  84  are created using well known techniques such as shallow trench isolation (STI) or local oxidation (LOCOS). Thereafter the gate  22  is formed as shown in FIG.  3 . It will be appreciated that there are many well-known sources and methods for forming the gate  22 . For example, a layer of dielectric material may be deposited on a wafer surface  96  of the SOI wafer  12 , with a layer of semiconductor material then deposited thereupon. The layers may selectively be etched to form the gate dielectric  26  and the gate electrode  28 . The gate electrode  28  may be made out of polysilicon which may be deposited using well-known processes such as low pressure chemical vapor deposition (LPCVD). 
     Thereafter, as illustrated in FIGS. 4-6, well-known suitable means are employed for formation of the source  40  and the drain  42 . Portions of the silicon on opposing sides of the channel regions that are not masked by the gate  22  then may be doped by ion implantation to produce the source  40  and the drain  42 . Such doping may be formed in a two-step implantation process, with a low-energy implant  100  (FIG. 4) to create the extensions  46  and  48 , followed by formation of the spacers  30  and  32  (FIG.  5 ), and then a high-energy implant  102  (FIG. 6) to create the remainder of the source  40  and the drain  42 . The region of the semiconductor layer  18  underneath the gate is protected from both implants, by the gate material. 
     There are many well-known efforts to form the spacers  30  and  32 . An exemplary method is to deposit a conformal dielectric layer (e.g., SiN) on the SOI wafer  12  and on the gate  22 . Parts of the dielectric layer are then selectively removed to leave respective gate source-side and drain-side spacers  30  and  32 . The deposit of the dielectric material and its selective removal may be accomplished by conventional means, for example chemical vapor deposition (CVD) such as LPCVD or plasma enhanced chemical vapor deposition (PECVD), of silicon nitride, followed by anisotropic etching using suitable, well-known etchants, exemplary etchants being freons and their derivatives. 
     Alternatively, suitable tilted implants may be used to form the source extension  46  and the drain extension  48 . 
     Turning now to FIG. 7, a semiconductor material layer  110  is deposited on the exposed surfaces of the SOI wafer  12 , the gate  22 , and the spacers  30  and  32 . The semiconductor material layer  110  is deposited by a directional deposition method, such as by a collimated sputtering  112 . Collimated sputtering involves interposing a collimator between a magnetron cathode and the item to be sputtered. The collimator may be a metal structure with holes or openings therethrough which collects atoms traveling laterally to the item to be sputtered. Thus only atoms traveling in a desired direction or range of directions (e.g., substantially perpendicular to the item to be sputtered) are allowed to pass through the collimator. Alternatively, directionality may be achieved by other methods such as long throw sputtering. 
     The semiconductor material layer  110  may be composed of the same material as the material of the active layer  24  (e.g., silicon). It will be appreciated that the depth of the semiconductor material layer  110  will be nonuniform due to the directional nature of the deposition. Thus the semiconductor material layer  110  will be thicker in parts overlying the source  40 , the drain  42 , and the gate  22 , than in regions overlying the spacers  30  and  32 . The depth of the parts of the semiconductor material layer  110  overlying the source  40 , the drain  42 , and/or the gate  22 , may be between 100 and 1000 Å (Angstroms). 
     Following the deposit of the semiconductor material layer  110 , portions of the layer are selectively removed, as illustrated in FIG.  8 . The selective removal uncovers portions of the spacers  30  and  32 , dividing the layer  110  into a source-side slab  120 , a drain-side slab  122 , and a gate slab  124 . The gate slab  124  may include tip portions  130  and  132  that overlie the spacers  30  and  32 , respectively. 
     The selective removal may be accomplished, for example, by isotropic etching, such as an isotropic wet etching process using a suitable etchant. An example of a suitable etchant for silicon is a mixture of hydroflouric, nitric, and acetic acids. It will be appreciated that other suitable isotropic etching processes, such as dry etching, may alternatively be utilized. 
     Referring to FIG. 9, mask elements  140  and  142  are formed, to shield the underlying portions of the source-side and drain-side slabs  120  and  122  during the subsequent formation of the metal-semiconductor compounds. The mask elements  140  and  142  may be formed by well-known methods, such as involving photolithography of a suitable resist material. 
     Turning now to FIGS. 10 and 11, the metal-semiconductor compound regions are formed. As shown in FIG. 10, a metal layer  150  is deposited on the exposed surfaces of the device. The metal layer may be of a metal such as titanium, cobalt, or nickel, which is suitable for forming a conducting compound, such as a silicide, with the semiconductor material. The metal layer may be deposited, for example, by sputtering. 
     Then, as illustrated in FIG. 11, a compound such as a silicide is formed between the metal of the metal layer  150  and the exposed portions of the source  40 , the drain  42 , and the gate electrode  28 , thus producing the metal-semiconductor compound regions  54 ,  56 , and  60 . Suitable methods for formation of such electrically-conducting compounds (e.g., silicidation) are well known, an exemplary method being raising temperature of the semiconductor device  10  to a suitable level for a suitable length of time (annealing). An exemplary temperature is between about 400 and 700° C., and an exemplary suitable length of time is between 10 seconds and 10 minutes. Rapid thermal annealing (RTA) may also be employed, for example subjecting the semiconductor device  10  to a temperature between 400 and 900° C. for about 5 to 120 seconds. It will be appreciated that other temperatures and heating times may be employed. 
     Finally, excess metal of the metal layer is removed by conventional, well-known means. Suitable well-known means may also be employed to remove the mask elements  140  and  142 , and the remaining portions  160  and  162  of the deposited semiconductor layer. Thus the device shown in FIG. 1 is produced. 
     It will be appreciated that the above-described structure and method are only exemplary, and that many suitable variations may be employed. For example, the semiconductor material may be silicon or another suitable semiconductor material, for example involving a material such as germanium. It may be possible to substitute oxides for nitrides, and/or vice versa, in the above structure and/or in the above fabrication method. 
     Some of the above-described method steps may be omitted or altered, if suitable. For example, the formation of the mask elements  140  and  142  may be omitted if desired. As another example, the shallow trench isolation step may be performed at a different stage during the manufacturing process. As a further example, the steps for forming the source and drain may be simplified, for example performed in a single implant. It will be appreciated that other suitable modifications of the above-described method are possible. 
     Although the invention has been shown and described with respect to a certain embodiment or embodiments, it is obvious that equivalent alterations and modifications will occur to others skilled in the art upon the reading and understanding of this specification and the annexed drawings. In particular regard to the various functions performed by the above described elements (components, assemblies, devices, compositions, etc.), the terms (including a reference to a “means”) used to describe such elements are intended to correspond, unless otherwise indicated, to any element which performs the specified function of the described element (i.e., that is functionally equivalent), even though not structurally equivalent to the disclosed structure which performs the function in the herein illustrated exemplary embodiment or embodiments of the invention. In addition, while a particular feature of the invention may have been described above with respect to only one or more of several illustrated embodiments, such feature may be combined with one or more other features of the other embodiments, as may be desired and advantageous for any given or particular application.