Abstract:
A simplified and cost reduced process for fabricating a field-effect transistor semiconductor device ( 104 ) using laser radiation is disclosed. The process includes the step of forming removable first dielectric spacers ( 116 R) on the sides ( 120   a   , 120   b ) of the gate ( 120 ). Dopants are implanted into the substrate ( 100 ) and the substrate is annealed to form an active deep source ( 108 ) and an active deep drain ( 110 ). The sidewall spacers are removed, and then a blanket pre-amorphization implant is performed to form source and drain amorphized regions ( 200   a   , 200   b ) that include respective extension regions ( 118   a   , 118   b ) that extend up to the gate. A layer of material ( 210  is deposited over the source and drain extensions, the layer being opaque to a select wavelength of laser radiation ( 220 ). The layer is then irradiated with laser radiation of the select wavelength so as to selectively melt the amorphized source and drain extensions, but not the underlying substrate. This causes dopants in the deep source to diffuse into the molten source extension, and dopants in the deep drain to diffuse into the molten drain extension. Upon recrystallization of the extensions, the layer of material is removed, and the FET device is completed using known processing techniques. The above process eliminates the lithography and ion implantation steps normally required for source and drain extension formation, and thereby reduces the manufacturing costs of field-effect transistors.

Description:
FIELD OF THE INVENTION 
     The present invention relates to processes for fabricating semiconductor field-effect transistors, and in particular to such processes involving laser thermal processing. 
     BACKGROUND OF THE INVENTION 
     Laser thermal processing (LTP) involves using short pulses of laser radiation (e.g., on the order of nanoseconds to tens of nanoseconds) to thermally anneal and activate dopants in a semiconductor. The pulses of laser radiation provide sufficient heat to briefly melt the doped semiconductor, which allows the dopants to diffuse within the molten region. When the semiconductor cools, it recrystallizes with the electrically active dopants occupying lattice sites within the crystal. 
     LTP techniques can be used to form junctions and source and drain (S/D) extension regions of a field-effect transistor (FET). Junctions formed using LTP techniques are shallow, abrupt and have low resistance, which are all very desirable device characteristics. In addition, because of the extremely high heating and cooling rate involved in LTP (10 6 −10 12 ° K/s), a meta-stable state can be established where dopant activation above the solid solubility limit occurs. These properties allow a transistor to be scaled to a smaller dimension with improved performance. 
     Incorporated by reference herein is the article by Talwar et al., “Ultra-Shallow, Abrupt, and Highly Activated Junctions by Low-energy Ion Implantation and Laser Annealing,” Proceedings of the 13 th  International Conference on Ion Implantation Technology, pp. 1171-1174 (1999), the article by Talwar et al., “Laser Thermal Processing for Shallow-Junction and Silicide Formation,” Proceedings of the SPIE, Microelectronic Device Technology II, volume 3506, p. 74-81 (1998), and the article by Goto et al., “Ultra-Low Contact Resistance for Deca-nm MOSFETs by Laser Annealing,” IEDM Digest, paper 20.7.1, pp. 931-933 (1999). These articles describe applications of LTP to semiconductor fabrication. Also incorporated by reference is U.S. Pat. No. 5,956,603, entitled “Gas Immersion Laser Annealing Process Suitable for Use in the Fabrication of Reduced-Dimension Integrated Circuits,” and U.S. Pat. No. 5,908,307, entitled “Fabrication Processes for Reduced-Dimension FET Devices.” 
     Unfortunately, LTP techniques cannot be directly inserted into the conventional complimentary metal-oxide-semiconductor (CMOS) fabrication process flow. This is because in the conventional process flow, deep source/drain regions need to be formed to make the contacts so that the transistors can be connected to each other to form a functional circuit. The formation of deep source/drain regions requires a high-temperature rapid thermal annealing (RTA) step to activate the implanted dopant. The RTA process is typically performed at a temperature of about 1000° C. for a duration of several tens of seconds. The RTA process is also used to activate and diffuse the dopants in the poly gate to decrease the poly resistance and eliminate the poly depletion problem. Such high RTA temperatures, however, cause the dopants in the laser-annealed shallow source/drain regions to out-diffuse into the silicon substrate. This results in deeper and less abrupt junctions, which degrades device performance. In addition, electrically active dopants in a meta-stable state (i.e., above the solid-solubility limit) can also deactivate (i.e., precipitate and become electrically inactive), resulting in higher electrical resistance and thus diminished device performance. 
     The prior art contains a technique that can be used to avoid the degradation of LTP junctions caused by post-LTP thermal process. The idea is to limit the post-LTP thermal budget by forming deep source/drain regions before LTP of the shallow source/drain extensions. This can be done using a so-called disposable spacer process (DSP), as described in the article by Yu et al, “70 nm MOSFET with Ultra-Shallow, Abrupt, and Super-Doped S/D Extension Implemented by Laser Thermal Process,” IEDM Digest, paper 20.4.1, pp. 509-511 (1999). In the process described in the Yu article, an additional dielectric layer is deposited and etched back to form a disposable spacer. This spacer is used to self-align the deep source/drain dopant implant and is subsequently removed. While working devices have been demonstrated with this approach, it requires additional steps, which add to the process complexity and thus increases the manufacturing costs. 
     Accordingly, it would be desirable to have a non-complex LTP-based CMOS process flow that does not compromise the above-mentioned desired LTP junction characteristics. 
     SUMMARY OF THE INVENTION 
     The present invention relates to processes of fabricating semiconductor field-effect transistors, and in particular to such processes involving laser thermal processing. 
     Accordingly, a first aspect of the present invention is a process for fabricating a field-effect transistor semiconductor device from a semiconductor substrate having an upper surface, spaced apart shallow trench isolations, and a gate formed on the upper surface between the shallow trench isolations. The process includes the step of forming removable first dielectric spacers on respective sides of the gate. The next step involves implanting dopants into the substrate in respective first and second regions between the spacers and the shallow isolation trenches. The third step involves annealing the first and second regions to form an active deep source and an active deep drain. The next step involves removing the removable spacers. A blanket pre-amorphization implant is then performed to form amorphized source and drain regions that include extension regions that extend up to the gate. The next step then includes depositing at least one layer of material (referred to as a “stack” herein) over at least the source and drain extensions, wherein the stack is opaque to a select wavelength of laser radiation. The next step is then irradiating the stack with laser radiation having the select wavelength so as to selectively melt the amorphized source and drain extension regions but not the underlying crystalline substrate. This LTP step causes diffusion of dopants from the deep source and drain into the source and drain extensions. The next step is the removal of the stack. 
     A second aspect of the present invention involves completing the formation of the MOSFET structure by forming first and second permanent dielectric spacers on the first and second sides of the gate, respectively, and then forming electrical contacts atop the gate, the source and the drain. 
     A third aspect of the present invention is a MOSFET device product-by-process, made using the process summarized immediately above and described in more detail below. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram of a metal oxide semiconductor field effect transistor (MOSFET) that results from performing the process steps of the present invention as illustrated in FIGS. 2A-2H; 
     FIG. 2A is a cross-sectional schematic diagram of a silicon substrate having formed therein shallow isolation elements and a gate on the upper surface of the substrate; 
     FIG. 2B is a cross-sectional diagram of the silicon substrate of FIG. 2A, further including removable sidewall spacers; 
     FIG. 2C is a cross-sectional diagram of the silicon substrate of FIG. 2B, showing the step of performing a deep dopant implant and an optional shallow dopant implant; 
     FIG. 2D is a cross-sectional diagram of the silicon substrate of FIG. 2C, after the step of performing RTA; 
     FIG. 2E is a cross-sectional diagram of the silicon substrate of FIG. 2D, but with the removable spacers removed; 
     FIG. 2F is cross-sectional diagram of the silicon substrate of FIG. 2E, showing the process step of performing a pre-amorphization implant (PAI); 
     FIG. 2G is cross-sectional diagram of the silicon substrate of FIG. 2F, further including a stack comprising one or more absorbing layers deposited on the upper surface of the substrate, and the process step of performing LTP; and 
     FIG. 2H is cross-sectional diagram of the silicon substrate of FIG. 2G, with the stack removed, and showing the formation of the doped extension regions. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention relates to processes of fabricating semiconductor field-effect transistors, and in particular to such processes involving laser thermal processing. 
     Referring to FIG. 1, a semiconductor structure  10  includes a silicon substrate  100  having an upper surface  101   a  and a bottom surface  101   b . On upper surface  101   a  of substrate  100 , a metal oxide semiconductor field effect transistor (MOSFET)  104  is formed using the process of the present invention, as described in greater detail below. MOSFET  104  is isolated from other devices that may be also formed on the silicon substrate  100  by isolation elements  102 . MOSFET  104  is shown to include an N well  106 , a P+ source  108 , a P+ drain  110  and a gate layer  112  (MOSFET  104  could also alternatively include a P well and a N+ source  108  and a N+ drain  110 ). Gate layer  112  is formed on upper surface  101   a  of substrate  100  and is insulated from well  106  by a gate insulation layer  114 . Gate layer  112  and insulation layer  114  are collectively referred to herein as “gate  120 .” Electrical contacts  117   a ,  117   b  and  117   c  are formed on top of gate  120 , the P+ source  108  and the P+ drain  110 , respectively. Dielectric sidewall spacers  116  help achieve self-alignment of the position of the electrical contacts, and also prevent electrical shorts between gate  120  and P+ source  108  and P+ drain  110  upon silicidation. Source  108  and drain  110  couple to the channel region (CR) below gate  120  via shallow source and drain extensions  118   a  and  118   b , respectively. In one example of MOSFET  104 , gate layer  112  has a height of approximately 2000 Angstroms, shallow extensions  118   a  and  118   b  have a depth of approximately 200 to 500 Angstroms, and source  108  and drain  110  have a depth of about 500-1000 Angstroms. 
     Referring to FIG. 2A, a process of fabricating MOSFET  104  of FIG. 1 according to the method of the present invention includes first forming spaced apart shallow isolation elements  102  to electrically isolate an area of silicon substrate  100  in which the MOSFET device is to be formed. Isolation elements  102  are formed by first etching spaced apart trenches into upper surface  101   a  of silicon substrate  100  and then filling the trenches with an insulating material. In one example, the insulating material is an oxide such as silicon dioxide. Upper surface  101   a  of silicon substrate  100  is then chemical-mechanical polished, resulting in a planarized upper surface. 
     After the formation of the isolation elements  102 , a gate insulation layer  114  is formed on the upper surface  101   a  of silicon substrate  100  in an area between isolation elements  102 . On top of gate insulation layer  114 , a gate layer  112  is deposited. In one example, the gate insulator layer is SiO 2  and gate layer  112  is polycrystalline-silicon deposited via low pressure chemical vapor deposition to a thickness of about 2000 Angstroms. Another exemplary material for gate layer  112  is amorphous silicon. In alternative embodiments, gate layer  112  could be a metal or a metal compound such as tungsten, tungsten silicide, tungsten nitride, tantalum, tantalum nitride, titanium, titanium nitride and platinum. Gate insulation layer  114  and gate layer  112 , as mentioned above, form what is called simply gate  120 . Gate  120  has first and second sides  120   a  and  120   b.    
     With reference now to FIG. 2B, the next step in the process of the present invention involves forming removable (“disposable”) sidewall spacers  116 R on either side  120   a  and  120   b  of gate  120 . A preferred method for forming sidewall spacers  116 R involves depositing a dielectric layer and anisotropically etching back that layer. The etch stops at upper surface  101   a  of silicon substrate  100 . Examples of dielectric materials for forming sidewall spacers  116 R include silicon oxide or silicon nitride. Sidewall spacers  116 R have a width W at the base, preferably on the order of 100-1000 Angstroms. The width W of the spacers allows for the formation of deep source and deep drain regions that are spaced apart from the gate by a predetermined distance, as discussed below. 
     With reference now to FIG. 2C, the next step in the process involves doping first and second regions  108 ′ and  110 ′ of silicon substrate  100 , located between gate  120  and isolation elements  102 , with appropriate N− or P− type dopants, as indicated by dopant implant beam  172 . Regions  108 ′ and  110 ′ will ultimately become source  108  and drain  110 , respectively, of MOSFET  104  (see FIG.  1 ). The implants are self-aligned to sidewall spacers  116 R 
     Doping of regions  108 ′ and  110 ′ is performed using P− type dopant ions (e.g., boron, aluminum, gallium) or N-type dopant ions (e.g., phosphorous, arsenic, antimony) from an ion implanter, such as the 9500 XR ION IMPLANTER™, commercially available from Applied Materials, Inc., Santa Clara, Calif. The ions are accelerated to a given energy level (e.g., 200 eV to 40 KeV) and implanted in regions  108 ′ and  110 ′ through upper surface  101   a  to a given dose (e.g., about 1×10 14  atoms/cm 2  to 1×10 16  atoms/cm 2 ). Regions  108 ′ and  110 ′ have, in practice, a concentration of dopant that is graded with depth into substrate  100  from upper surface  101   a , and is typically between 10 19 −10 21  ions/cm 3 . If well  106  is − type, regions  108 ′ and  110 ′ are doped with P− type dopants. On the other hand, if well  106  is P− type, then regions  108 ′ and  110 ′ are doped with N− type dopants. 
     With continuing reference to FIG. 2C, an optional shallow dopant implant, as indicated by arrows  172 ′, may also be performed. The purpose of shallow dopant implant  172 ′ is to increase the surface concentration of dopant so that more dopant can diffuse to extension regions  118   a  and  118   b  (see FIG. 1) during the LTP step, described below. This leads to lower extension resistance. For a typical CMOS process, two lithography steps, (namely, coating the surface with photoresist, and then exposing and developing the resist to form a mask) are necessary before implantation to separately define P− type and N− type source and drain regions  108 ′ and  110 ′. 
     With reference now to FIG. 2D, the next step in the process involves performing RTA to activate the dopants implanted in regions  108 ′ and  110 ′ (see FIG.  2 C), thereby forming (deep) source  108  and (deep) drain  110 . Typical RTA temperatures range from 900° C. to 1050° C. Wavy lines  176  in FIG. 2D indicate the application of annealing heat. The high temperature thermal cycle drives the dopant out laterally by a small distance δ. The temperature and anneal time should be controlled so that this lateral dopant out-diffusion does not exceed the width W of sidewall spacers  116 R. 
     With reference now to FIG. 2E, removable sidewall spacers  116 R are removed, preferably using either a wet or dry etch process. This step could be performed prior to the RTA step associated with FIG.  2 D. 
     With reference now to FIG. 2F, the next step in the process involves performing a blanket pre-amorphization implant (PAI) as indicated by arrows  180 , resulting in amorphous regions  200   a  and  200   b  (i.e., amorphous source and drain regions, respectively), being formed in the upper portion of silicon substrate  100  between gate  120  and isolation elements  102 , as indicated by the dashed line. Amorphous regions  200   a  and  200   b  include respective source and drain extensions  118   a  and  118   b  that extend up to gate  120 . 
     The PAI depth d of regions  200   a  and  200   b  determines the depth of the laser melt in silicon substrate  100  during the LTP step (described below), which in turn determines the junction depth of source and drain extensions  118   a  and  118   b  (see FIG.  1 ). The PAI implant depth is determined by the implant species, dose, and energy, and may be performed to achieve a predetermined depth for extensions  118   a  and  118   b . Preferred PAI species include Si and Ge. By way of example, a Si implant having a dose of 10 15  atoms/cm 2  at an energy of 10 keV will amorphize silicon substrate  100  to a depth of about 240 Angstroms. Likewise, a Ge implant having a does of 3×10 14  atoms/cm 2  at an energy of 20 keV will amorphize silicon substrate  100  to a depth of about 300 Angstroms. Depending on the desired amorphization depth, d, the typical implant dose is in the range from about 10 13  atoms/cm 2  to about 10 16  atoms/cm 2 , while the range of implant energy is from about 5 keV to about 400 keV. 
     The PAI implant is self-aligned to sides  120   a  and  120   b  of gate  120 . However, though not shown explicitly in FIG. 2F, amorphous regions  200   a  and  200   b  typically extend slightly underneath gate  120  due to the lateral straggle of the PAI process. This overlap of gate  120  with amorphous regions  200   a  and  200   b  can impact device performance and can be controlled by varying the tilt angle θ of the PAI implant with respect to the normal vector of upper surface  101   a  of silicon substrate  100  (the tilt angle in FIG. 2F, for example, is 0°). The tilt angle θ should be selected based on the overlap specification that yields optimal device performance. 
     With reference now to FIG. 2G, the next step in the process involves forming at least one layer of material (hereinafter, referred to as a “stack”)  210  on upper surface  101   a  and covering at least source and drain regions  108  and  110  (see FIG.  2 F). Of the one or more layers of material constituting stack  210 , at least one of the layers is a material that is opaque to a select wavelength of radiation used to perform LTP, as described below. Such opaque materials may include metals, metal oxides, metal nitrides, tungsten, tantalum or the like, so long at the material has a melting temperature higher than that of silicon. An exemplary stack  210  comprises a thin layer (e.g., 150 Angstroms) of silicon dioxide deposited directly on upper surface  101   a , and a relatively thick layer (e.g., 300 Angstroms) of tantalum nitride formed atop the silicon dioxide layer. The role of the silicon dioxide layer in this case is to prevent contamination of silicon substrate  100  (including source  108  and drain  110 ) that could result if tantalum nitride were in direct contact with upper surface  101   a  of the silicon substrate. The purpose of stack  210  is to facilitate the absorption of LTP radiation and the uniform distribution of heat from the radiation. The layers also serve to maintain the physical integrity of the semiconductor structure during processing. 
     With continuing reference to FIG. 2G, the next step in the process involves performing LTP by irradiating stack  210  with laser radiation, indicated by arrows  220 . The laser radiation is preferably provided in pulses. The laser radiation preferably has a fluence from about 0.05 to 1 Joule/cm 2 , and a select wavelength in the range from about 157 nanometers to about 1,064 nanometers. The laser radiation is controlled such that the entirety of amorphous regions  200   a  and  200   b  are melted, but the underlying portion of the crystalline silicon substrate  100  (i.e., well  106 ) is not. This is possible because the melt temperature of crystalline silicon is about 250° C. higher than that of amorphous silicon. When amorphous regions  200   a  and  200   b  are molten, dopants in source  108  and drain  110  diffuse laterally into the molten extension regions  118   a  and  118   b  as indicated by arrows  230 . The diffusion stops sharply at the liquid-solid interface that exists underneath sides  120   a  and  120   b  of gate  120 . This abrupt interface is possible because the diffusivity of the dopants in solid silicon is eight orders of magnitude lower than that in liquid (molten) silicon. The dopant diffusion length, L, in molten silicon is give by L=2(Dô) ½ , where D is the dopant diffusion coefficient in liquid silicon and ô is the melt duration, which is of the order of the temporal pulse length of the laser radiation. For boron, D is about 2.5×10 −6  cm 2 /sec. To ensure that the dopants will diffuse all the way to the edge of the molten amorphous regions  200   a  and  200   b  under the gate, the dopant diffusion length, L, in molten silicon must be greater than the difference between the width of the removable spacer  116 R (see FIG. 2D) and the lateral out-diffusion of source and drain  108  and  110 . As mentioned above, width, W, of spacer  116 R at its base is several hundred Angstroms. The temporal pulse length of the laser radiation pulse is on the order of tens of nanoseconds. It is important to note that in conventional CMOS fabrication process flow, as well as in the prior art pertaining to LTP processing, shallow extensions (e.g., extensions  118   a  and  118   b ) are formed using low-energy ion implantation. For CMOS devices, two lithography steps are necessary before implanting to separately define the P− type and N− type extension regions, as mentioned above. By contrast, in the present invention, (shallow) doped extensions  118   a  and  118   b  are formed by melting amorphized silicon regions  200   a  and  200   b  so that dopants in (deep) source  108  and (deep) drain  110  laterally diffuse from the deep source into the source extension and from the deep drain into the drain extension. Thus, the process of the present invention eliminates two ion implantation steps and two lithography steps associated with forming the extensions. The lithography step, including mask generation, resist coating, photo exposure, resist develop, resist strip, is typically an expensive process. The cost reduction in elimination of two lithography steps is enough to offset the added process complexity due to disposable spacer. The cost saving and process simplification are two of the key advantages the current invention has over the prior art of LTP process. 
     With reference now to FIG. 2H, stack  210  is removed by using a wet or dry etch. 
     With reference again to FIG. 1, the next step in the process involves forming new and permanent dielectric spacers  116  on the sides  120   a  and  120   b  of gate  120 . The process further includes forming electrical contact  117   a  atop gate  120  and electrical contacts  117   b ,  117   c  on upper surface  101   a  of the silicon substrate  100  and contacting source  108  and drain  110 . Electrical contacts  117   a - 117   c  may be formed from silicide such as titanium disilicide, cobalt disilicide or nickel monosilicide, or may be formed by direct metal deposition using sputtering, evaporation or chemical vapor deposition. 
     The many features and advantages of the present invention are apparent from the detailed specification, and, thus, it is intended by the appended claims to cover all such features and advantages of the described apparatus that follow the true spirit and scope of the invention. Furthermore, since numerous modifications and changes will readily occur to those of skill in the art, it is not desired to limit the invention to the exact construction and operation described herein. Accordingly, other embodiments are within the scope of the appended claims.