Silicon-on-insulator (SOI) transistor having partial hetero source/drain junctions fabricated with high energy germanium implantation

[0002] The present invention relates generally to silicon-on-insulator (SOI) devices and methods of forming the same and, more particularly, to SOI devices and methods for forming which avoid or reduce floating body effects.

[0003] Silicon-on-insulator (SOI) materials offer potential advantages over bulk materials for the fabrication of high performance integrated circuits. Dielectric isolation and reduction of parasitic capacitance improve circuit performance, and virtually eliminate latch-up in CMOS circuits. In addition, circuit layout in SOI can be greatly simplified and packing density greatly increased if the devices are made without body contacts (i.e., if the body regions of these devices are “floating”). However, partially-depleted metal oxide semiconductor field effect transistors (MOSFETs) on SOI materials typically exhibit parasitic effects due to the presence of the floating body (“floating body effects”). These floating body effects may result in undesirable performance in SOI devices.

[0004] It will be appreciated from the foregoing that a need exists for SOI MOSFETs having reduced floating body effects.

[0005] According to one aspect of the invention, the invention is a silicon-on-insulator (SOI) transistor. The SOI transistor having a source and a drain having a body disposed therebetween, the source being implanted with germanium to form an area of silicon-germanium adjacent a source/body junction in a lower portion of the source, the area of silicon-germanium in the source forming a hetero junction along a lower portion of the source/body junction.

[0006] According to another aspect of the invention, the invention is a method of fabricating a silicon-on-insulator (SOI) transistor. The method including the steps of providing an active layer disposed on a buried oxide (BOX) layer, the BOX layer being disposed on a substrate, the active layer having an active region defined by isolation regions; forming a transistor in the active region, the transistor having a source and a drain having a body disposed therebetween, and a gate disposed on the body; and implanting the source with germanium to form an area of silicongermanium adjacent a source/body junction in a lower portion of the source, the area of silicon-germanium in the source forming a hetero junction along a lower portion of the source/body junction.

[0007] These and further features of the present invention will be apparent with reference to the following description and drawings, wherein:

[0008] FIG. 1 is a cross-section of a silicon-on-insulator (SOI) device according to a first embodiment of the present invention;

[0009] FIG. 1ais an enlarged partial cross-section view of the SOI device of FIG. 1;

[0010] FIG. 2 is a cross-section of an SOI device according to a second embodiment of the present invention;

[0011] FIG. 3 is a cross-section of an SOI device according to a third embodiment of the present invention;

[0012] FIG. 4 is a flow chart illustrating a method of fabricating the SOI device of FIG. 1;

[0013] FIGS. 5a-5eare cross-sections of the SOI device of FIG. 1 in intermediate stages of fabrication;

[0014] FIG. 6 is a flow chart illustrating a method of fabricating the SOI devices of FIGS. 2 and 3; and

[0015] FIGS. 7a-7fare cross-sections of the SOI devices of FIGS. 2 and 3 in intermediate stages of fabrication.

[0016] In the detailed description which follows, identical components have been given the same reference numerals, regardless of whether they are shown in different embodiments of the present invention. To illustrate the present invention in a clear and concise manner, the drawings may not necessarily be to scale and certain features may be shown in somewhat schematic form.

[0017] Referring to FIG. 1, a silicon-on-insulator (SOI) device 10, also referred herein as a transistor or a MOSFET, is fabricated on an SOI wafer 12 which has a silicon active layer 14 disposed on a buried oxide (BOX) layer 16. The BOX layer 16 is disposed on a silicon substrate 18. Within the active layer 14 shallow trench isolation (STI) regions 20 define the placement of active regions 22 used for the fabrication of active devices, such as the device 10 described more fully below.

[0018] The device 10 has a source 24 and a drain 26 with a body 28, or channel, disposed therebetween. Disposed on top of the body 28 is a gate 30. The gate 30 includes a layer of gate oxide 32 and a polysilicon gate portion 34, or other gate stack formation, as is known in the art. After the gate 30 has been formed, the source 24 and drain 26 are doped to form extensions 38 as are known in the art. Following extension 38 formation, side wall spacers 40 are deposited adjacent to gate 30 and the source 24 and the drain 26 are doped with deep implantations. By way of example, the source 24 and the drain 26 receive N+ doping and the body 28 is initially P doped. Alternatively, the source 24 and the drain 26 may receive P+ doping and the body 28 is N doped. In another alternative, the body 28 is undoped. In yet another alternative, the source 24 and the drain 26 are each N doped or P doped.

[0019] With additional reference to FIG. 1a,the device 10 is implanted with germanium (Ge). The germanium is planted at a zero degree angle with respect to vertical (i.e., in a straight downward direction) and at a relatively high energy such that most of the germanium traverses an upper portion 42 of the source 24 and an upper portion 44 of the drain 26. In one example, the implantation energy of the germanium is about 20 kev to about 80 kev. The germanium is implanted with a dosage of about 1×10¹⁴atoms/cm²to about 1×10¹⁶atoms/cm². The germanium collects in a lower portion 46 of the source 24 and the lower portion 48 of the drain 26. In each of the lower portions 46 and 48, the active layer 14 becomes silicone germanium (SiGe). In one embodiment, the atomic concentration of silicon in the lower portions 46 and 48 is about 30% to about 70% and the atomic concentration of germanium is about 70% to about 30%.

[0020] Due to the germanium implantation, a junction 50 between the source 24 and the body 28 forms a SiGe/Si junction along a lower portion 56 of the source/body junction 50 where silicon-germanium is present in the source 24 and silicon is present in the body 28. More specifically, the silicon-germanium of the lower portion 46 of the source 24 has a narrow band gap and forms a hetero junction with the body 28 along the lower portion 56 of the source/drain body junction 50. To elaborate, germanium is implanted such that the lower portion 46 of the source 24 is generally co-extensive with the deep implantation of the source 24 and very little germanium is present in the upper portion 42 of the source 24 such that the extension 38 is relatively germanium free and an upper portion 54 of the source/body junction 50 forms a Si/Si junction.

[0021] As one skilled in the art will appreciate, a drain/body junction 52 will also be divided into an upper portion 58 where the extension 38 of the drain 26 contacts the body 28 and a lower portion 60 where the deep implant, or lower portion 48, of the drain 26 contacts the body 28. Since the lower portion 48 of the drain 26 is also implanted with germanium, the lower portion 60 of the drain/body junction 52 will also form a hetero junction, similar to the lower portion 56 of the source/body junction 50.

[0022] The partial hetero junctions present along the source/body junction 50 and the drain/body junction 52 serve to suppress floating body effects (FBE). More specifically, holes will be transported from the body 28 respectively to the source 24 or the drain 26 thereby reducing floating body effects by removing charge from the body 28. The hetero junctions of the lower portions 56, 60 of the source/body junction 50 and the drain/body junction 52, however, form a large barrier for electron flow. To maintain performance of the device 10, the upper portions 42, 44 of the source 24 and the drain 26 remain relatively germanium free so that barriers to electron flow are minimized.

[0023] Referring now to FIG. 2, a second embodiment of the device 10′ is illustrated. As with the device 10 of the first embodiment, the device 10′ of the second embodiment has silicon-germanium formed in a region 80 of the source 24 and a region 82 of the drain 26. The silicon-germanium regions 80 and 82 are respectively formed adjacent a lower portion 84 of the source/body junction 50 and a lower portion 86 of the drain/body junction 52. Therefore, the lower portions 84 and 86 form hetero junctions to reduce floating body effects in the same manner as for the device 10 of the first embodiment. That is, the lower energy barriers to holes in the lower portions 84 and 86 of the source/body junction 50 and the drain/body junction 52 respectively help remove body charge from the body 28 so that floating body effects are reduced.

[0024] The silicon-germanium regions 80 and 82 are formed using tilted angled germanium implantation. More specifically, to form the region 80, germanium is implanted at an angle of 30 to 40 degrees from vertical tilted towards the source 24. To form the region 82, germanium is implanted at an angle of 30 to 40 degrees from vertical tilted toward the drain 26. The germanium is implanted with an energy of about 20 kev to about 80 kev and a dosage of about 1×10¹⁴atoms/cm²to about 1×10¹⁶atoms/cm². In each of the regions 80 and 82, the active layer 14 becomes silicon-germanium (SiGe). In one embodiment, the atomic concentration of silicon in the regions 80 and 82 is about 30% to about 70% and the atomic concentrations of germanium is about 70% to about 30%. The device 10′ is a symmetrical transistor where both the source 24 and the drain 26 are provided with silicon-germanium regions 80 and 82. Depending on the device 10′ being fabricated, however, additional angled implantations of germanium may be desired to form, for example, a halo of silicon-germanium around a body 28.

[0025] Referring now to FIG. 3, a device 10″ according to a third embodiment of the present invention is illustrated. The device 10″ is an asymmetric transistor and, similar to the device 10′, has a region 80 of silicon-germanium disposed adjacent a lower portion 84 of the source/body junction 50. However, the device 10″ does not have a silicon-germanium region on the drain 26 side of the device. The silicongermanium region 80 serves to reduce floating body effects as described above and is deposited using angled germanium implantation where the germanium is implanted at an angle 30 to 40 degrees from vertical tilted towards the source 24. The germanium is implanted with an energy of about 20 kev to about 80 kev and a dosage of about 1×10¹⁴atoms/cm²to about 1×10¹⁶atoms/cm². In the region 80, the active layer 14 becomes silicon-germanium (SiGe). In one embodiment, the atomic concentration of silicon in the region 80 is about 30% to about 70% and the atomic concentrations of germanium is about 70% to about 30%.

[0026] Referring now to FIG. 4, a method 100 of fabricating the device 10 illustrated in FIG. 1 is shown in flowchart format. With additional reference to FIG. 5a,the method 100 begins in step 102 where a wafer 12 of SOI material is formed. As mentioned, the wafer 12 has a silicon substrate 18 with a buried oxide (BOX) layer 16 disposed thereon. Disposed on the BOX layer 16 is a silicon active layer 14. The active layer 14 may be initially doped for the fabrication of an N-channel device or, as illustrated, a P-channel device.

[0027] Next, in step 104 and as illustrated in FIG. 5b,the active region 22 is defined. More specifically, STI regions 20 are formed to define the size and placement of the active region 22. Next, in step 106, the gate oxide layer 32 is formed using conventional techniques. Next, in step 108, the polysilicon gate portion 34, along with any other desired gate layers to form the gate stack, are formed on the gate oxide 32 using conventional techniques.

[0028] Next, in step 110, and as illustrated in FIG. 5c,source 24 and drain 26 extensions 38 are implanted. For an N-channel device, the extensions 38 are formed by implanting arsenic (As+) at, for example, an energy of about 1.0 kev to about 3.0 kev. For a P-channel device, the extensions 38 are formed by implanting boron (B+) at, for example, an energy of about 0.3 kev to about 1.5 kev. Regardless of the channel type, the implantation dose for the extensions 38 is, for example, about 1×10¹⁴atoms/cm²to about 1×10¹⁵atoms/cm².

[0029] Next, in step 112 and as illustrated in FIG. 5d,side wall spacers 40 are formed adjacent the gate 30. The spacers 40 are formed using conventional techniques and are made from a material such as silicon oxide (SiO₂) or a nitride (e.g., Si₃Na).

[0030] With continued reference to FIG. 4 and FIG. 5d,the source 24 and drain 26 are further defined by source/drain deep implantation in step 114. For an N-channel device, the deep implantation is made by implanting arsenic at, for example, an energy of about 5 kev to about 30 kev and a dose of about 1×10¹⁵atoms/cm²to about 5×10¹⁵atoms/cm². For a P-channel device, the deep implantation is made by implanting boron at, for example, an energy of about 3 kev to about 15 kev and a dose of about 1×10¹⁵atoms/cm²to about 5×10¹⁵atoms/cm². As one skilled in the art will appreciate, the source/drain extensions 38 and source/drain deep implantation can be carried out using alternative dopants and/or at other appropriate energy levels and dose levels, as is desirable for the device being fabricated. Following deep implantation in step 114, the wafer 12, in step 116, is subjected to a thermal anneal cycle of at about 1,000° C. to about 1,150° C. for a period of about five seconds to about fifteen seconds or, alternatively, a rapid temperature anneal (RTA) cycle for about 0.1 seconds to about five seconds.

[0031] Next, in step 118 and as illustrated in FIGS. 1, 1aand 5e,the device 10 is subjected to germanium implantation. The germanium is implanted with an energy of about 20 kev to about 80 kev and a dose of about 1×10¹⁴atoms/cm²to about 1×10¹⁶atoms/cm². The germanium is implanted at substantially a zero angle (i.e., implanted generally perpendicular to the wafer 14). Most of the germanium traverses an upper portion 42 of the source 24 and an upper portion 44 of the drain 26 such that the germanium amorphizes the silicon and forms silicon-germanium (SiGe) in a lower portion 46 of the source 24 and a lower portion 48 of the drain 26.

[0032] Following germanium implantation, the wafer 12 may be annealed to recrystallize the source 24 and drain 26. Example annealing cycles can be either a furnace anneal of about 500° C. to about 600° C. for about one minute to about ten minutes or an RTA for about 0.1 seconds to about five seconds. It is also noted that the deep implantation step (step 114) and the germanium implantation step (step 118), along with their associated thermal cycles, could be reversed.

[0033] Referring now to FIG. 6, a method 150 of fabricating the device 10′ and the device 10″ illustrated in FIGS. 2 and 3 respectively is shown in flowchart format. With additional reference to FIG. 7a,the method 150 begins in step 152 where a wafer 12 of SOI material is formed. As mentioned, the wafer 12 has a silicon substrate 18 with a buried oxide (BOX) layer 16 disposed thereon. Disposed on the BOX layer 16 is a silicon active layer 14. The active layer 14 may be initially doped for the fabrication of an N-channel device or, as illustrated, a P-channel device.

[0034] Next, in step 154 and as illustrated in FIG. 7b,the active region 22 is defined. More specifically, STI regions 20 are formed to define the size and placement of the active region 22. Next, in step 156, the gate oxide layer 32 is formed using conventional techniques. Next, in step 158, the polysilicon gate portion 34, along with any other desired gate layers to form the gate stack, are formed on the gate oxide 32 using conventional techniques.

[0035] Next, in step 160, and as illustrated in FIG. 7c,source 24 and drain 26 extensions 38 are implanted. For an N-channel device, the extensions 38 are formed by implanting arsenic (As+) at, for example, an energy of about 1.0 kev to about 3.0 kev. For a P-channel device, the extensions 62 are formed by implanting boron (B+) at, for example, an energy of about 0.3 kev to about 1.5 kev. Regardless of the channel type, the implantation dose for the extensions 62 is, for example, about 1×10¹⁴atoms/cm²to about 1×10¹⁵atoms/cm².

[0036] Next, in step 162 and as illustrated in FIG. 7d,side wall spacers 40 are formed adjacent the gate 30. The spacers are formed using conventional techniques and are made from a material such as silicon oxide (SiO₂) or a nitride (e.g., Si₃Na).

[0037] With continued reference to FIG. 6 and FIG. 7d,the source 24 and drain 26 are further defined by source/drain deep implantation in step 164. For an N-channel device, the deep implantation is made by implanting arsenic at, for example, an energy of about 5 kev to about 30 kev and a dose of about 1×10¹⁵atoms/cm²to about 5×10¹⁵atoms/cm². For a P-channel device, the deep implantation is made by implanting boron at, for example, an energy of about 3 kev to about 15 kev and a dose of about 1×10¹⁵atoms/cm²to about 5×10¹⁵atoms/cm². As one skilled in the art will appreciate, the source/drain extensions 38 and source/drain deep implantation can be carried out using alternative dopants and/or at other appropriate energy levels and dose levels, as is desirable for the device being fabricated. Following deep implantation in step 164, in step 166, the wafer 12 is subjected to a thermal anneal cycle of at about 1,000° C. to about 1,150° C. for a period of about five seconds to about fifteen seconds or, alternatively, a rapid temperature anneal (RTA) cycle for about 0.1 seconds to about five seconds.

[0038] Next, in step 168 and as illustrated in FIGS. 2, 3 and 7e,the device 10′ or 10″ is subjected to tilted angle germanium implantation. The germanium is implanted with an energy of about 20 kev to about 80 kev and a dose of about 1×10¹⁴atoms/cm²about 1×10¹⁶atoms/cm². The germanium is implanted at an angle α of about 30 degrees to about 40 degrees from vertical and tilted towards the source 24. The tilted angle germanium implantation introduces germanium into the source 24 such that the silicon-germanium region 80 is formed adjacent the source/body junction 50 to establish a hetero junction along the lower portion 84 of the source/body junction 50 as described in more detail above.

[0039] If an asymmetrical device, or device 10″, is desired having a partial hetero junction on the only the source 24 side and not the drain 26 side, then in step 170, the method 150 will end resulting in the device 10″ illustrated in FIG. 3. Following germanium implantation, however, the wafer 12 may be annealed to re-crystallize the source 24 and drain 26. Example annealing cycles include a furnace anneal of about 500° C. to about 600° C. for about one minute to about ten minutes and an RTA for about 0.1 seconds to about five seconds. It is also noted that the deep implantation step (step 164) and the germanium implantation step (step 168), along with their associated thermal cycles, could be reversed.

[0040] If in step 170, a symmetrical device, or device 10′, is desired having a second partial hetero junction on the drain 26 side, the device 10′ is subjected to a second tilted angle germanium implantation step in step 172 and as illustrated in FIGS. 2 and 7f.More specifically, germanium is implanted with an energy of about 20 kev to about 80 kev and a dose of about 1×10¹⁴atoms/cm²about 1×10⁶atoms/cm². The germanium is implanted at an angle β of about 30 degrees to about 40 degrees from vertical and tilted towards the drain 26. The tilted angle germanium implantation introduces germanium into the drain 24 such that the silicon-germanium region 82 is formed adjacent the drain/body junction 52 to establish a hetero junction along the lower portion 86 of the drain/body junction 52 as described in more detail above.

[0041] In addition to the germanium implantation to form the silicon-germanium region 82, optional tilted angle implants from other directions may be used to form a partial halo hetero junction around the body 28 or to form similar hetero junctions in other devices on the wafer which have a different orientation than the device 10′. Following germanium implantation, the wafer 12 may be annealed to re-crystallize the source 24 and drain 26. Example annealing cycles include a furnace anneal of about 500° C. to about 600° C. for about one minute to about ten minutes and an RTA for about 0.1 seconds to about five seconds. It is also noted that the deep implantation step (step 164) and the germanium implantation steps (steps 168 and 172), along with their associated thermal cycles, could be reversed.

[0042] Although particular embodiments of the invention have been described in detail, it is understood that the invention is not limited correspondingly in scope, but includes all changes, modifications and equivalents coming within the spirit and terms of the claims appended hereto.

[0043] For example, in making the device 10 illustrated in FIG. 1, a mask may be used to prevent or minimize germanium implantation into the drain 16 to produce an asymmetric transistor.