SOI device with self-aligned selective damage implant, and method

[0002] This application is related to commonly-assigned, copending U.S. application Ser. No. 09/484,634, filed Jan. 18, 2000. This application is also related to commonly-assigned U.S. application Ser. No. 09/776,197, titled “SOI Device With Body Recombination Region”, filed Feb. 2, 2001.

[0003] 1. Technical Field

[0004] The present invention relates generally to semiconductor-on-insulator (SOI) devices and methods of making, and more specifically to SOI transistor devices having reduced floating body effects.

[0005] 2. Description of the Related Art

[0006] Conventional or bulk semiconductor devices are formed in semiconductor material by implanting a well of either P-type or N-type conductivity silicon in a silicon substrate wafer of the opposite conductivity. Gates and source/drain diffusions are then manufactured using commonly known processes. These form devices known as metal-oxide-semiconductor (MOS) field effect transistors (FETs). When a given chip uses both P-type and N-type, it is known as a complimentary metal oxide semiconductor (CMOS). Each of these transistors must be electrically isolated from the others in order to avoid shorting the circuits. A relatively large amount of surface area is needed for the electrical isolation of the various transistors. This is undesirable for the current industry goals for size reduction. Additionally, junction capacitance between the source/drain and the bulk substrate and “off” state leakage from the drain to the source both increase power consumption. Junction capacitance also slows the speed at which a device using such transistors can operate. These problems result in difficulties in reducing the size, power consumption, and voltage of CMOS technology devices.

[0007] In order to deal with the junction capacitance and “off state” leakage problem as well as obtain reduced size, semiconductor-on-insulator technology (SOI) has been gaining popularity. A SOI wafer may be formed from a bulk silicon wafer by using conventional oxygen implantation techniques to create a buried oxide layer at a predetermined depth below the surface. The implanted oxygen oxidizes the silicon into insulating silicon dioxide in a gaussian distribution pattern centered at the predetermined depth to form the buried oxide layer. Field effect transistors formed on SOI substrates also may be able to achieve higher speed operation with higher drive currents, when compared with FETs formed on conventional bulk silicon substrates.

[0008] However, one problem with forming field effect transistors on an SOI wafer is the floating body effect. The floating body effect occurs because the buried oxide layer isolates the channel, or body, of the transistor from the fixed potential silicon substrate and therefore the body takes on charge based on recent operation of the transistor. The floating body effect causes the current-to-voltage curve for the transistor to distort or kink, which in turn causes the threshold voltage for operating the transistor to fluctuate. This problem is particularly apparent for passgate devices such as those used in dynamic random access memory (DRAM) wherein it is critical that the threshold voltage remain fixed such that the transistor remains in the “off” position to prevent charge leakage from the storage capacitor.

[0009] Accordingly, there is a strong need in the art for a semiconductor circuit structure, and a method for forming such structure, that includes the low junction capacitance and low “off” state leakage characteristics of the SOI FET based circuits but does not suffer the disadvantages of a floating body potential.

[0010] A transistor on an SOI wafer has a subsurface recombination area at least partially within its body. The recombination area includes one or more damaged recombination regions. The damaged recombination region(s) may be formed by a damaging implant into a surface semiconductor layer, for example through an open portion of a doping mask, the opening portion created for example by removal of a dummy gate. Alignment of the damaged recombination region(s) is improved by forming the source and drain of the transistor prior to removal of the dummy gate, using the dummy gate as a doping mask.

[0011] According to an aspect of the invention, a semiconductor-on-insulator (SOI) transistor device includes an insulating layer made of an insulating material; an active layer of semiconductor material atop the insulating layer, the active layer including a body between a source and a drain, wherein the source and the drain are of a same conductivity type, and wherein the body is of an opposite conductivity type; and a gate atop the body. The body includes a damaged area therein, and the damaged area is not in contact with the source or the drain.

[0012] According to another aspect of the invention, a semiconductor-on-insulator (SOI) transistor device includes an insulating layer made of an insulating material; an active layer of semiconductor material atop the insulating layer, the active layer including a body between a source and a drain, wherein the source and the drain are of a same conductivity type, and wherein the body is of an opposite conductivity type; a gate atop the body; and a recombination region along the interface between the body and the insulating layer. The recombination region is fully underneath the gate.

[0013] According to still another aspect of the invention, a method of producing a semiconductor-on-insulator (SOI) transistor device includes the steps of forming a subsurface damaged area at least partially in a surface semiconductor layer of an SOI wafer; and forming a gate atop the surface semiconductor layer such that the damaged area is at least partially underneath the gate.

[0014] 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.

[0015] In the annexed drawings:

[0016] FIG. 1 is a cross-sectional view of a semiconductor device in accordance with the present invention; and

[0017] FIGS. 2-10 are cross-sectional views of various steps in a method of fabricating the semiconductor device of FIG. 1.

[0018] A semiconductor device includes a transistor on an SOI substrate, the transistor having a subsurface damaged region wholly or partially within a body of the transistor. The damaged region may be along an interface between the body and an underlying insulating layer, and may extend into the insulating layer. The subsurface damaged region acts as a recombination center, reducing minority carrier times in the body and therefore reducing floating body effects in the transistor.

[0019] 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 SiO₂, although it will be appreciated that other suitable materials may be used instead or in addition.

[0020] 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 SiO₂and Si₃N₄. 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.

[0021] The active region 24 includes a body 38, with a source 40 and a drain 42 on respective opposite sides of the body. The source 40 and the drain 42 have respective source and drain extensions 46 and 48. The body includes a surface channel region 50 operatively coupled to the source 46 and the drain 48. As is conventional, the body 38 is primarily of different conductivity semiconductor material than the source 40, the drain 42, and the channel region 50. For instance, the body 38 may be P-conductivity silicon while the source 40, the drain 42, and the channel region 50 may be N-conductivity silicon. Alternatively, the body 38 may be N-conductivity silicon while the source 40, the drain 42, and the channel region 50 may be P-conductivity silicon.

[0022] 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 have 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 includes an upper conductive portion 60 to facilitate electrical connection.

[0023] 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 (SiO₂) using known shallow trench isolation (STI) techniques.

[0024] The body 38 includes a subsurface damaged region 90. The damaged region 90 may be partially within the body 38. For example it may be along an interface 92 between the body 38 and the insulator layer 20. The damaged region 90 may extend into the insulator layer 20. It will be appreciated that alternatively the damaged region 90 may be fully within the body 38, not in contact with the source 40, the drain 42, or the insulator layer 20.

[0025] The presence of the damaged region 90 within the body 38 greatly increases the recombination rate within the body. This may occur due to the damaged region 90 acting as a recombination center for the excess holes created in the channel region 50. The damaged region 90 thus reduces minority carrier times within the body 38, thereby reducing the tendency of the body to build up a floating body potential.

[0026] The damaged region 90 may have a size (a dimension such as length, width or depth) of between approximately 50 nanometers and approximately 200 nanometers. The damaged region 90 may be fully underneath the gate 22. In addition the damaged region 90 may be fully underneath the channel region 50. Alternatively, it will be appreciated that the damaged region 90 may be wholly or partially other than underneath the gate 22 and/or the channel region 50. The damaged region 90 may be centered within the body 38 and/or relative to the gate 22. It will appreciated that alternatively the damaged region 90 may be other than centered, for example being closer to the source 40 than to the drain 42.

[0027] It will be appreciated that multiple damaged regions within the body 38 may be used, if desired.

[0028] Various steps in the fabrication of the above-described semiconductor device 10 are illustrated in FIGS. 2-10. Referring initially to FIG. 2, starting initially with the SOI wafer 12, a dummy gate 94 is formed thereupon. It will be appreciated that there are many well-known sources and methods for producing SOI wafers such as the SOI wafer 12. The dummy gate 94 may be made out of a semiconductor material, such as polysilicon, and is formed on the wafer surface 96 of the surface semiconductor layer 18 of the SOI wafer 12. The dummy gate 94 may be formed by any of a variety of well-known processes such as low pressure chemical vapor deposition (LPCVD). It will be appreciated that the dummy gate 94 may be made out of a wide variety of suitable conductive, semiconductive, or non-conductive materials.

[0029] As illustrated in FIG. 3, insulator-filled trenches 82 and 84 are then created in the SOI wafer 12. The insulator-filled trenches 82 and 84 define and laterally isolated the active region 24 of the surface semiconductor layer 18. The insulator-filled trenches may be formed using conventional well-known shallow trench isolation (STI) techniques. An exemplary process for forming an insulating trench 82 and 84 includes forming a thin layer of oxide, approximately 150-200 Angstroms thick, on the wafer surface 96 and a top surface of the dummy gate 94, and forming a silicon nitride mask thereon. The mask covers and protects the substrate in the area where the active region 24 are to be formed while leaving exposed the area where the insulator-filled trenches 82 and 84 are to be formed.

[0030] Thereafter, the unmasked portions of the semiconductor surface layer 18 (e.g. the portions where the silicon nitride mask has been etched away) are etched away to form an open trench extending at least past the upper surface of the buried insulator layer 20. The etching process for a silicon substrate is typically an anisotropic dry etch using hydrogen bromide (HBr) which has selectivity characteristics such that it etches the silicon substrate but not the silicon nitride mask.

[0031] The open trench is filled by depositing silicon dioxide (SiO₂), formed by a chemical reaction involving SiH₄or TEOS, to form insulating trenches 82 and 84. After filling the open trench the surface of the wafer is polished using a chemical mechanical polish to remove any excess silicon dioxide layer and the remaining silicon nitride mask.

[0032] It will be appreciated that the trenching may be performed at another point in the process, either earlier or later, if desired.

[0033] 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 dummy gate 94 then may be doped to produce the source 40 and the drain 42. Such doping may be formed in a two-step doping process, with a low-energy doping 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 doping 102 (FIG. 6) to create the remainder of the source 40 and the drain 42. Because the ions cannot penetrate the dummy gate 94, the dummy gate effectively operates as a doping mask, protecting the region of the semiconductor layer 18 underneath the gate from doping.

[0034] To form the spacers 30 and 32, a conformal dielectric layer (e.g., SiN) may be deposited on the SOI wafer 12 and on the dummy gate 94. 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 dioxide, followed by anisotropic etching using suitable, well-known etchants, an exemplary etchant being CHF₃.

[0035] Alternatively tilted implants may be used to form the source extension 46 and the drain extension 48.

[0036] A doping implant may be used to form the channel region 50. It will be appreciated that the doping implant to form the channel region 50 may be performed at any of a number of suitable points in the above process, either before the formation of or after the removal of the dummy gate 94.

[0037] Turning now to FIG. 7, the silicide regions 54 and 56 are then formed. Silicidation may then be accomplished as follows. A layer of metal is deposited upon the dummy gate 94, the spacers 30 and 32, and the exposed portions of the surface semiconductor layer 18. 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.

[0038] Then a compound such as a silicide is formed between the metal of the metal layer and the exposed portions of the surface semiconductor layer 18. 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 500 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 600 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.

[0039] As illustrated in FIG. 8, mask elements 110 and 112 are then formed on opposite sides of the dummy gate 94. This may occur, for example, first by deposition of a layer of insulator material, for example silicon nitride, by a process such as CVD. Then well-known chemical-mechanical polishing (CMP) processes may be employed to expose the top of the dummy gate 94.

[0040] As shown in FIG. 9, the dummy gate 94 is then removed, for example by etching, to create an opening 114 for access to a portion of the underlying surface semiconductor layer 18. The etching used in removing the dummy gate 94 may include one or more dry etch processes such as plasma etching, ion milling, reactive ion beam etching, and/or may include other processes suitable for carrying out the invention.

[0041] Then a damaging implant 120 is used to form the subsurface damaged region 90. The mask elements 110 and 112 protect the underlying parts of the active region 24 from being damaged, thus confining event damage from the damaging implant to the area beneath the opening 114.

[0042] The damaging implant 120 may be performed using conventional techniques, and may involve implanting suitable ions in the silicon or other semiconductor material to damage the crystalline structure of the silicon and/or provide impurities within the silicon, to thereby create the damaged region 90. A wide variety of suitable implant materials may be used. The implant materials may be electrically conductive but fairly immobile in silicon. Alternatively, the implant material may be electrically non-conductive in silicon. Further, it is desirable for the implant material to be relatively large so that it causes a relatively large amount of damage to or defects in the silicon in the damaged region 90. The damage or defects in the crystalline structure provide pathways for electron/hole recombination, thus reducing minority carrier times in the body 38. Exemplary implant materials include silicon, germanium, xenon, and arsenic. The damaging implant ions may have an energy of between approximately 10 keV and approximately 50 keV. The concentration of the damaging implant may be between approximately 1×10¹³ions/cm²and approximately 1×10¹⁵ions/cm².

[0043] The damaging implant 120 may be a perpendicular implant, or alternatively may include tilted implanting.

[0044] As illustrated in FIG. 10, after the damaging implant 120 the gate 22 is formed. The gate dielectric layer 26 may be formed by conventional means, such as deposition and etching. Then the gate electrode 28 is formed. After formation of the gate 22, the mask elements 110 and 112 may be removed, for example by etching, forming the structure that is shown in FIG. 1.

[0045] It will be appreciated that the dummy gate 94 is advantageously used both for forming the source 40 and the drain 42, and for positioning the opening 114. The opening 114, and thus the damaged region 90, are thereby automatically accurately positioned within the active region 24 relative to the source 40 and the drain 42.

[0046] 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. It may be possible to substitute oxides for nitrides, and/or vice versa, in the above structure and/or in the above fabrication method.

[0047] The above-described method steps may be performed in a different order, with suitable modification. For example, some or all of the formation of the source 40, the drain 42, and the silicide regions 54 and 56 may be accomplished after the damaging implant, using the gate 22 as a mask. It will be appreciated that other suitable modifications of the above-described method are possible.

[0048] 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.