Abstract:
A method for selectively heating a substrate without damaging surrounding regions of the substrate. In particular, the invention provides for a method of selectively activating doped regions of a semiconductor device without damaging surrounding doped and activated regions. Specifically, the invention provides a laser anneal which activates locally doped regions, while surrounding doped and activated regions are protected using a reflective mask.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Technical Field  
           [0002]    The present invention relates generally to semiconductor manufacturing, and more particularly, to a method of selectively heating particular regions of a substrate without damaging surrounding regions.  
           [0003]    2. Related Art  
           [0004]    In the manufacture of semiconductor devices, a semiconductor substrate is often subjected to an ion implanting process to form heavily doped source-drain regions of CMOSFET devices. Subsequent to the ion implanting process, the substrate is exposed to high temperatures. This type of annealing process is necessary to restore the crystallinity of the substrate and electrically activate the doped ions implanted in the surface of the substrate. This is typically accomplished via a furnace, rapid thermal, or laser annealing process.  
           [0005]    However, since the annealing process currently used heats the entire substrate, it is difficult to achieve optimal results for multiple dopant anneals since the heat cannot be focused exclusively on a desired area. In fact, when the substrate is exposed to high temperatures the profiles of surrounding doped shallow source-drain regions may become deformed, move or increase in depth in the substrate thereby degrading the device performance characteristics. Consequently, it is very difficult to form the shallow junctions necessary for high performance semiconductor devices using a conventional wafer annealing method.  
           [0006]    Based on the above description, there is a need for a new method of selectively heating particular regions of a substrate without damaging surrounding regions. More particularly, there is a need for a method of forming and activating locally doped regions of a semiconductor substrate that will not damage the surrounding doped and activated regions.  
         SUMMARY OF THE INVENTION  
         [0007]    The present invention provides a method of overcoming the above-identified problems of the related art by selectively activating certain doped regions of a semiconductor substrate using a laser, while protecting the surrounding doped and activated regions using a photolithographically defined reflective mask.  
           [0008]    The first general aspect of the present invention provides a method for local activation comprising the steps of: providing a substrate having a first section and a second section, wherein the first section contains at least one doped and activated region; depositing a reflective mask over the first section to protect the at least one doped and activated region; implanting a dopant species in at least one region of the second section; and activating the dopant species. This aspect provides for a method of activating doped regions of a semiconductor device. Further, this aspect provides for the use of a reflective mask to prevent the destruction of underlaying doped and activated regions when the substrate is exposed to the activation process.  
           [0009]    The second general aspect of the present invention provides a method for local activation of a semiconductor device including the steps of: depositing a reflective mask over a previously doped and activated first section of a substrate; implanting a dopant on a second section of the substrate; and activating the dopant of the second section. This aspect provides similar advantages as those described in the first aspect.  
           [0010]    The third general aspect of the present invention provides for a reflective mask used to protect underlaying regions of a semiconductor substrate upon dopant activation of adjacent portions of the substrate. This aspect provides similar advantages as the first aspect.  
           [0011]    The fourth general aspect of the present invention provides for a semiconductor device comprising: a substrate having a previously doped and activated first section, and a second section; a reflective mask over the first section to protect the first section from damage caused by further processing; and at least one inactivated doped region in the second section. This aspect provide for a device formed in accordance with the present invention.  
           [0012]    The fifth general aspect of the present invention provides for a semi-transparent layer of reflective material, having a topography with varying thickness, to allow for the selective activation of a doped region. This aspect allows for the selective activation of particular regions to varying degrees.  
           [0013]    The foregoing and other features and advantages of the invention will be apparent from the following more particular description of the preferred embodiments of the invention. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]    The preferred embodiments of this invention will be described in detail, with reference to the following figures, wherein like designations denote like elements, and wherein:  
         [0015]    [0015]FIG. 1 depicts a semiconductor substrate in accordance with the present invention;  
         [0016]    [0016]FIG. 2 depicts the formation of a PMOSFET in accordance with the present invention;  
         [0017]    [0017]FIG. 3 depicts the deposition of a reflective mask and photoresist mask on the semiconductor substrate in accordance with the present invention;  
         [0018]    [0018]FIG. 4 depicts n+ ion implantation in accordance with the present invention;  
         [0019]    [0019]FIG. 5 depicts a laser anneal in accordance with the present invention;  
         [0020]    [0020]FIG. 6 depicts a semiconductor substrate having doped and activated regions produced in accordance with the present invention; and  
         [0021]    [0021]FIG. 7 depicts a plot of the amount of laser transmission, reflection and absorption vs. the thickness of the reflective mask in accordance with the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0022]    Although certain preferred embodiments of the present invention will be shown and described in detail, it should be understood that various changes and modifications may be made without departing from the scope of the appended claims. The scope of the present invention will in no way be limited to the number of constituting components, the materials thereof, the shapes thereof, the relative arrangement thereof, etc., and are disclosed simply as an example of the preferred embodiments.  
         [0023]    Referring to FIG. 1, a semiconductor substrate  10 , typically made of silicon, including N-well  12 , P-well  14 , and shallow trench isolations (hereafter “STI”)  16 , is depicted in accordance with the present invention. STI  16  are formed within substrate  10  at a depth of about 0.25 microns, by an etching process or other comparable process. STI  16  are filled with silicon dioxide via a chemical vapor deposition (CVD) technique and the surface is planarized by chemical-mechanical polishing or other known methods. N-well  12  and P-well  14 , separated by STI  16 , are formed within substrate  10  and activated using conventional annealing processes.  
         [0024]    As depicted in FIG. 2, a gate oxide layer  20 , having a thickness of about 3 to 10 nm (30 to 100 angstrom), is formed on the surface of substrate  10  using thermal growth techniques commonly used in the industry. A layer of polysilicon is deposited onto gate oxide layer  20  using chemical-vapor deposition (CVD). The layer of polysilicon is patterned to form polysilicon gates  22  and  23  via reactive-ion etching, or other suitable processes. Gate oxide layer  20  is also etched during this process, thereby limiting gate oxide layer  20  to the area directly under polysilicon gates  22  and  23 . A layer of silicon dioxide  24 , or other insulating material, is deposited over the entire surface of substrate  10  at a thickness of about 5 to 10 nm (50 to 100 angstrom), using CVD. N-well  12  and polysilicon gate  23  are doped with p+ ions then annealed using conventional methods. Heavily doped P-type source and drain regions  18  and doped polysilicon gate  23  form PMOSFET  40 , which the present invention is designed to protect from damage that typically results from the subsequent doping and activation processes performed on P-well  14 .  
         [0025]    As depicted in FIG. 3, a reflective mask  26  comprising a semi-transparent reflective material, such as aluminum, tungsten, copper, gold, etc., having a thickness of about 50 to 500 nm (500 to 5000 angstrom), is deposited over the entire silicon dioxide layer  24 . A photoresist mask  28  is deposited on reflective mask  26 , using conventional methods, to photopattern and remove the portion of reflective mask  26  over P-well  14 , thereby exposing that section of substrate  10  for subsequent doping. Photoresist mask  28  is removed using a conventional process, leaving reflective mask  26  over PMOSFET  40  of N-well  12 . The reflective mask  26  is used to protect doped source and drain regions  18  and doped polysilicon gate  23  of PMOSFET  40  from damage when P-well  14  is doped and annealed during subsequent processing steps.  
         [0026]    As depicted in FIG. 4, n+ ions, such as singly charged arsenic (As+) or phosphorous (P+), are implanted over P-well  14  and polysilicon gate  22 , via ion implantation. Heavily doped N-type source and drain regions  30  and doped polysilicon gate  22  form NMOSFET  41 . Substrate  10  is exposed to a pulse laser beam, preferably a zenon chloride excimer laser (having a wavelength of approximately 308 nm), with energy between 50 mJ/cm 2  and 5 J/cm 2 , for a time between 10 milliseconds to 60 seconds to activate the n+ ions implanted in doped source and drain regions  30  and doped polysilicon gate  22 , as shown in FIG. 5. Reflective mask  26 , located over PMOSFET  40 , acts to reflect a portion (approximately 50%) of the incoming laser beam to protect p+ doped source and drain regions  18  and doped polysilicon gate  23  of PMOSFET  40  from damage, while the implanted n+ ions of source and drain regions  30  and doped polysilicon gate  22  of NMOSFET  41  over P-well  14  are being activated. After the activation of the n+ ions of NMOSFET  41 , reflective mask  26  is removed using a wet or dry etch or other customary methods, leaving the semiconductor substrate depicted in FIG. 6. This will typically be followed by additional contact and wiring steps to complete the wafer manufacture process.  
         [0027]    [0027]FIG. 7 illustrates the percentage of laser light (at 308 nm) that is transmitted, reflected and absorbed, represented by lines T, R and A, respectively, for various thicknesses of reflective mask  26 . It is important to note that when the thickness of reflective mask  26  is less than 50 nm, a significant amount of laser light is transmitted, at least 5%, to the underlaying doped layers. However, as the thickness of reflective mask  26  is reduced, for instance to 10 nm, the amount of transmission increases (to approximately 35%). Therefore, since transmitted light is converted to heat energy, and the amount of transmission is dependent upon the thickness of reflective mask  26 , local activation may be achieved in varying amounts by adjusting the thickness of reflective mask  26  in select regions. In other words, varying the local topography of reflective mask  26  will allow the amount of dopant activated in the underlying regions to be tailored.  
         [0028]    In an alternative embodiment of the present invention, photoresist  28  is not removed prior to exposing the substrate to the laser. This enables the use of a thinner reflective mask  26  since photoresist  28  acts as an implant blocking mask. This may be compared with the process depicted in FIGS. 4 and 5, in which reflective mask  26  acts as both an implant blocking mask and a laser reflective mask.  
         [0029]    It should be noted that the above described process is designed to operate using a single ion implantation-laser annealing tool. In other words, the ion implantation and the laser annealing process occur in the same tool, almost simultaneously to save processing time and to improve process control.  
         [0030]    The present embodiment is described to optimize both PMOSFET and NMOSFET to manufacture high performance CMOS devices, as an example only. It should also be appreciated that the present invention can be used to improve the manufacture of various other types of semiconductor devices, such as optimizing an array and support device of DRAM (dynamic random access memory), optimizing DRAM and LOGIC devices for merging LOGIC-DRAM products, optimizing bipolar and CMOS devices for BiCMOS products, and so on.  
         [0031]    While this invention has been described in conjunction with the specific embodiments outlined above, it is evident that many alternatives, modifications, and variations will be apparent to those skilled in the art. Accordingly, the preferred embodiments of the invention set forth above are intended to be illustrative, not limiting. Various changes may be made without departing form the spirit and scope of the invention as defined in the following claims.