Laser annealing for forming shallow source/drain extension for MOS transistor

[0002] This application is a continuation of application No. 09/162,919, filed on Sep. 29, 1998, with the same title.

[0003] The present invention relates generally to semiconductor fabrication, and more particularly to methods for fabricating improved ultra-large scale integration (ULSI) semiconductor devices such as ULSI metal oxide silicon field effect transistors (MOSFETs).

[0004] Semiconductor chips or wafers are used in many applications, including as processor chips for computers, and as integrated circuits and as flash memory for hand held computing devices, wireless telephones, and digital cameras. Regardless of the application, it is desirable that a semiconductor chip hold as many circuits or memory cells as possible per unit area. In this way, the size, weight, and energy consumption of devices that use semiconductor chips advantageously is minimized, while nevertheless improving the memory capacity and computing power of the devices.

[0005] A common circuit component of semiconductor chips is the transistor. In ULSI semiconductor chips, a transistor is established by forming a polysilicon gate on a silicon substrate, and then forming a source region and a drain region side by side in the substrate beneath the gate by implanting appropriate dopant materials into the areas of the substrate that are to become the source and drain regions. The gate is insulated from the source and drain regions by a thin gate oxide layer, with small portions of the source and drain regions, referred to as “extensions”, extending toward and virtually under the gate. This generally-described structure cooperates to function as a transistor.

[0006] As the dimensions of MOSFETs desirably are reduced as discussed above, it will be readily appreciated that the depth to which the source and drain extensions of a transistor penetrates the silicon substrate decreases. While shallow source/drain extensions promote immunity from the deleterious ramifications of so-called short-channel effect and facilitate ever-smaller integrated circuits, unfortunately the limits of current ion implantation technology render it difficult at best to form ultra-shallow source/drain regions, i.e., regions having depths of 30 nanometers or less.

[0007] The present invention recognizes that laser annealing might instead be used to form ultra-shallow source/drain regions. However, the present invention understands that the use of a metal film to absorb the laser energy as has been proposed in other applications is undesirable. This is because the film must be stripped after use by a wet chemical etch, which adds cost and complexity, and the film and chemical process furthermore might possibly contaminate the silicon chip and render it less suitable for its intended purpose than might otherwise be hoped. Fortunately, the present invention recognizes that laser annealing can be used to precisely form ultra-shallow source/drain extensions without using a metal film.

[0008] A method is disclosed for establishing at least one transistor on a semiconductor device. The method includes forming one or more MOS gates on a semiconductor substrate, and depositing dopant on first regions of the substrate. Then, a silicidizing substance is implanted in the first regions to silicidize the first regions. The method then envisions directing a laser beam against the first regions to move the dopant into the substrate for establishing source and drain regions.

[0009] In a preferred embodiment, a protective layer is deposited on the gate, and a first pre-amorphization (PAI) substance is implanted into the substrate prior to the step of depositing dopant. The protective layer substantially prevents implanting the first PAI substance through the layer into the gate. Also, a sidewall spacer is established on the gate after the step of implanting the first PAI substance, and then a second PAI substance is implanted into the substrate, with the protective layer and sidewall spacer substantially preventing implanting the second PAI substance through the layer and spacer. As described in greater detail below, the first PAI substance defines the contours of source and drain extensions, and the second PAI substance defines the contours of deep source/drain junctions.

[0010] In accordance with the present invention, the laser beam preferably is established such that regions with PAI substances (i.e., pre-amorphized regions) are at least partially melted thanks to the selective absorption of laser light induced by the silicidizing of these regions. The melting of the pre-amorphized regions causes the dopant to move into the pre-amorphized regions, and single crystal silicon regions are not melted, thereby substantially preventing the dopant from moving into the single crystal regions. If desired, the protective layer can be removed from the gate prior to implanting the dopant such that at least one dopant substance is implanted into the gate. A semiconductor device made according to the method, and a digital processing apparatus incorporating the semiconductor device, are also disclosed.

[0011] In another aspect, a method for making an ultra-large scale integration (ULSI) semiconductor device includes forming at least one gate on at least one semiconductor substrate, and pre-amorphizing first regions of the substrate while not pre-amorphizing second regions of the substrate. Dopant is provided on the substrate next to the gate. Then, the first regions are caused to at least partially melt, in contrast to the second regions, which do not melt, thereby causing the dopant to enter only the first regions.

[0012] In yet another aspect, a semiconductor device includes at least one transistor that in turn includes a gate disposed on a silicidized silicon substrate. The transistor further includes first regions that have dopant driven into them by virtue of the first regions having been melted by a laser. These first regions define source and drain regions for ULSI MOSFETs. Preferably, the first regions are pre-amorphized prior to laser annealing.

[0013] Other features of the present invention are disclosed or apparent in the section entitled “DETAILED DESCRIPTION OF THE INVENTION”.

[0014] FIG. 1 is a schematic diagram of a semiconductor device made according to the present invention, shown in combination with a digital processing apparatus;

[0015] FIG. 2 is a flow chart showing the steps of the present invention;

[0016] FIG. 3 is a side view of the device after the gate polysilicon and gate oxide has been formed, during implantation of the first PAI substance;

[0017] FIG. 4 is a side view of the device after the nitride spacers have been formed, during implantation of the second PAI substance;

[0018] FIG. 5 is a side view of the device during deposition of the gate and source/drain dopant;

[0019] FIG. 6 is a side view of the device after silicidation; and

[0020] FIG. 7 is a side view of the device during laser annealing.

[0021] Referring initially to FIG. 1, a semiconductor device embodied as a chip is shown incorporated into a digital processing apparatus such as a computer, 12. The chip 10 is made in accordance with the below disclosure.

[0022] Now referring to FIGS. 2 and 3, as indicated at block 14 in FIG. 2 and as shown in FIG. 3, using conventional semiconductor fabrication techniques including low pressure chemical vapor deposition (LPCVD) and appropriate etching and lithography, a transistor gate, generally designated 16, is formed on a silicon or other semiconductor substrate 18. As shown, the gate 16 includes a thin insulating gate oxide layer 20 that faces the substrate 18 and a gate polysilicon stack 22 on the gate oxide layer 20.

[0023] Additionally, a protective silicon-oxy-nitride (SiON) cap 24 is deposited on the top of the gate 16. Then, a first pre-amorphization (PAI) substance such as high-dose silicon (Si) or germanium (Ge), represented by the dashed line 26, is implanted into the substrate 18 from the surface 18aof the substrate 18 down to the dashed line 26, as indicated by the arrows 28 and as shown at block 29 in FIG. 2. As intended by the present invention, the implantation of the pre-amorphization substance promotes the formation of and defines the contours of ultra-shallow source and drain extensions in extension regions 30, 32 during subsequent steps disclosed below. Those skilled in the art will recognize that the protective cap 24 substantially prevents implanting the pre-amorphization substance through the cap 24 into the gate 16.

[0024] Proceeding to block 34 an now referring to FIG. 4, nitride sidewall spacers 36 are deposited on the substrate 18 and etched in accordance with well-known principles to establish the illustrated shoulder configuration around the sides 38 of the gate 16. Next, a second PAI substance is implanted into the substrate 18 as represented by the dashed line 40 from the surface 18aof the substrate 18 down to the dashed line 40, as indicated by the arrows 42. The first and second PAI substances can be also a high-dose silicon (Si) or a germanium (Ge). Also, the cap 24 and spacers 36 prevent implanting the second PAI substance through the cap 24 and spacers 36. As understood herein, the second PAI substance promotes the formation and defines the contours of relatively deep source/drain junctions in deep regions 44, 46 of the substrate 18 during subsequent steps disclosed below.

[0025] Following the above-described steps, the process moves to block 48 of FIG. 2, wherein the SiON protective cap 24 is removed from the gate 16 as shown in FIG. 5 by, e.g., wet etching. After removal of the protective cap 24, an appropriate dopant such as boron fluoride (BF₂), arsenic (As), boron (B), and phosphorous (P) or dopants, represented by heavy horizontal lines 50 in FIG. 5, is deposited onto the gate 16 and substrate 18, as indicated by the arrows 52. However, the dopant 50 is not driven into the substrate 18 at block 48.

[0026] Proceeding to block 54 of FIG. 2, most of the regions that are to become the source and drain regions (i.e., the regions that have been pre-amorphized as discussed above with the exception of the extension regions 30, 32 which are otherwise shielded by the sidewall spacers 36) are silicidized by depositing at least one refractory metal onto the gate 16 at the top region 58, the source region, and the drain region, and reacting the at least one refractory metal with the underlying silicon ions as indicated by cross-hatched areas 56 in FIG. 6. In one preferred embodiment, the silicidation is undertaken by sputtering a refractory material such as titanium or cobalt onto the areas/regions 56, 58.

[0027] As understood by the present invention, the silicide, which is already in wide use for other purposes in the semiconductor fabrication art and which accordingly does not introduce additional metal film deposition and etch-away steps, and which furthermore does not contaminate the substrate 16 in contrast to metal film, is a laser energy absorber. Moreover, as the skilled artisan will appreciate, the above-disclosed method steps result in the silicide laser absorber being self-aligned with the regions that will become the gate and source/drain regions. With this in mind, it may now be appreciated that laser energy will preferentially be absorbed precisely in the region that will become the gate and precisely in the pre-amorphized regions that will become the source/drain, owing to the silicidizing of these regions.

[0028] Moving to block 60 in FIG. 2 and now referring to FIG. 7, the gate 16 and substrate 18 are annealed with a laser beam, represented by the arrows 62. The silicidized regions 56, 58 selectively absorb the laser beam 62. In accordance with the preferred embodiment, the characteristics of the laser beam 62 are established such that pre-amorphized regions (referred to as α-Si), which have a lower melting point than single crystal silicon, are at least partially melted, whereas the laser beam 62 does not melt the single crystal silicon. This causes the dopant 50 (FIG. 5) to move into the pre-amorphized regions as well as to diffuse laterally into the extension regions 30, 32 (FIG. 5).

[0029] Consequently, under the influence of laser annealing at block 60, the extension regions 30, 32 become ultra-shallow (with thickness δ≦30 nm) source/drain extensions 64, 66, as shown in FIG. 7, promoting superior short-channel performance. Also, the deep regions 44, 46 (FIG. 5) become source/drain junction regions 68, 70 as shown in FIG. 7. The dopant, however, substantially does not move into single crystal silicon regions. With the above-disclosed invention, no subsequent annealing steps such as high temperature annealing are required, because the dopant is activated during the laser annealing.

[0030] While the particular METHOD FOR FORMING SHALLOW SOURCE/DRAIN EXTENSION FOR MOS TRANSISTOR as herein shown and described in detail is fully capable of attaining the above-described objects of the invention, it is to be understood that it is the presently preferred embodiment of the present invention and is thus representative of the subject matter which is broadly contemplated by the present invention, that the scope of the present invention fully encompasses other embodiments which may become obvious to those skilled in the art, and that the scope of the present invention is accordingly to be limited by nothing other than the appended claims, in which reference to an element in the singular is not intended to mean “one and only one” unless explicitly so stated, but rather “one or more”. Indeed, although a single transistor structure is shown in the drawings for clarity, the skilled artisan will appreciate that the chip 10 can include plural transistors, each substantially identical to that shown, as well as other circuit components. All structural and functional equivalents to the elements of the above-described preferred embodiment that are known to those of ordinary skill in the art are expressly incorporated herein by reference and are intended to be encompassed by the present claims. Moreover, it is not necessary for a device or method to address each and every problem sought to be solved by the present invention, for it to be encompassed by the present claims.