Abstract:
A method of particle mitigation which includes obtaining a semiconductor wafer having a nonfunctional backside and a functional frontside on which semiconductor devices are formed by one or more lithography processes; coating the backside with a layer comprising silicon or amorphous carbon; planarizing the coated backside by a planarizing process; placing the semiconductor wafer onto a wafer chuck such that the wafer chuck makes direct contact with the coated backside; and while maintaining the coated backside in direct contact with the wafer chuck, performing a first lithographic process on the frontside.

Description:
RELATED APPLICATION 
       [0001]    This application is related to U.S. patent application Ser. No. ______ (Attorney Docket No. FIS920140071US1), entitled “WAFER BACKSIDE PARTICLE MITIGATION”, filed even date herewith. 
     
    
     BACKGROUND 
       [0002]    The present exemplary embodiments relate to mitigating particle contamination on the backside of a semiconductor wafer, and more particularly, relate to mitigating particle contamination by providing a coating on the backside of the semiconductor wafer to encapsulate the contaminating particles and fill any scratches. 
         [0003]    Particulate matter may be generated from wafer handling devices (such as pics, pins and pads) as the semiconductor wafers travel through a multitude of tools in the line. Some particulate matter, and especially scratches and dents, are inevitable, regardless of any sort of preemptive cleaning or wiping methods. 
         [0004]    The semiconductor wafers are typically handled with a so-called wafer chuck, one example of a wafer chuck being an electrostatic wafer chuck, which secures the semiconductor wafer during processing. However, conventional electrostatic chucks maintain a high percent point of contact with the backside of the semiconductor wafer. This large area of contact is highly susceptible to semiconductor wafer backside particulate manner and scratches, which can create wafer topography during lithography exposure and lead to “hot spots”. A hot spot in the present context is a lithography term for a localized pattern distortion (i.e., defocus) of which one cause is wafer topography. 
       BRIEF SUMMARY 
       [0005]    The various advantages and purposes of the exemplary embodiments as described above and hereafter are achieved by providing, according to a first aspect of the exemplary embodiments, a method of particle mitigation which includes obtaining a semiconductor wafer having a nonfunctional backside and a functional frontside on which semiconductor devices are formed by one or more lithography processes; coating the backside with a mitigating layer comprising silicon or amorphous carbon; planarizing the coated backside by a planarizing process; placing the semiconductor wafer onto a wafer chuck such that the wafer chuck makes direct contact with the coated backside; and while maintaining the coated backside in direct contact with the wafer chuck, performing a first lithographic process on the frontside. 
         [0006]    According to a second aspect of the exemplary embodiments, there is provided a method of particle mitigation which includes obtaining a semiconductor wafer having a nonfunctional backside and a functional frontside on which semiconductor devices are formed by one or more lithography processes; coating the backside with a mitigating layer comprising silicon or amorphous carbon; planarizing the coated backside by a planarizing process; placing the semiconductor wafer onto an electrostatic wafer chuck such that the electrostatic wafer chuck makes direct contact with the coated backside; and while maintaining the coated backside in direct contact with the electrostatic wafer chuck, performing an extreme ultraviolet (EUV) lithographic process on the frontside. 
         [0007]    According to a third aspect of the exemplary embodiments, there is provided a method of particle mitigation which includes obtaining a semiconductor wafer having a nonfunctional backside and a functional frontside on which semiconductor devices are formed by one or more lithography processes; coating the backside with a mitigating layer comprising silicon or a carbon-like material; planarizing the coated backside by a planarizing process; coating the mitigating layer with a stop layer that is compositionally different than the mitigating layer; coating the stop layer with another mitigating layer comprising silicon or carbon-like material; planarizing the coated backside by a planarizing process; repeating, a predetermined number of times, coating the mitigating layer with the stop layer that is compositionally different than the mitigating layer, coating the stop layer with another mitigating layer comprising silicon or amorphous carbon, and planarizing the coated backside by the planarizing process; placing the semiconductor wafer onto a wafer chuck such that the wafer chuck makes direct contact with the coated backside; and while maintaining the coated backside in direct contact with the wafer chuck, performing a lithographic process on the frontside. 
     
    
     
       BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS 
         [0008]    The features of the exemplary embodiments believed to be novel and the elements characteristic of the exemplary embodiments are set forth with particularity in the appended claims. The Figures are for illustration purposes only and are not drawn to scale. The exemplary embodiments, both as to organization and method of operation, may best be understood by reference to the detailed description which follows taken in conjunction with the accompanying drawings in which: 
           [0009]      FIG. 1  is a perspective view of a semiconductor wafer according to the exemplary embodiments having a coating on the backside of the semiconductor wafer. 
           [0010]      FIGS. 2 to 4  illustrate a first exemplary process of particle mitigation in which: 
           [0011]      FIG. 2  is a cross sectional view of  FIG. 1  showing a backside coating on the semiconductor wafer; 
           [0012]      FIG. 3  is a cross sectional view illustrating the semiconductor wafer of  FIG. 2  in which the backside coating is planarized; and 
           [0013]      FIG. 4  is a cross sectional view illustrating the semiconductor wafer of  FIG. 3  being placed on an electrostatic chuck and a lithography process being performed on the frontside of the semiconductor wafer. 
           [0014]      FIGS. 5 to 7  illustrate a second exemplary process of particle mitigation in which: 
           [0015]      FIG. 5  is a cross sectional view illustrating a protective coating formed on the frontside of the semiconductor wafer; 
           [0016]      FIG. 6  is a cross sectional view illustrating a backside coating on the semiconductor wafer of  FIG. 5 ; and 
           [0017]      FIG. 7  is a cross sectional view illustrating the semiconductor wafer of  FIG. 6  in which the backside coating is planarized. 
           [0018]      FIG. 8  illustrates a third exemplary embodiment of particle mitigation in which the backside of the semiconductor wafer of  FIG. 7  is coated with a plurality of layers. 
           [0019]      FIG. 9  is a flow chart illustrating a process for forming the exemplary embodiments. 
       
    
    
     DETAILED DESCRIPTION 
       [0020]    Semiconductor technology is well known. Through semiconductor fabrication processes, semiconductor devices are formed on a semiconductor wafer. A typical semiconductor wafer has a back, nonfunctional side (hereafter “backside”) and a front, functional side (hereafter “frontside”). The semiconductor fabrication processes such as front end of the line processes to form transistors and back end of the line processes to form interconnects occur on the frontside of the semiconductor wafer. Lithography may be used in many of these semiconductor fabrication processes to pattern the frontside. Optical lithography, immersion lithography, ultraviolet (UV) lithography and extreme ultraviolet (EUV) lithography being examples of types of lithographic processes that may be utilized. 
         [0021]    During these semiconductor fabrication processes, the semiconductor wafer may be supported by a wafer chuck such as an electrostatic wafer chuck. Electrostatic wafer chucks employ a platen with integral electrodes which are biased with high voltage to establish an electrostatic holding force between the platen and wafer, thereby “chucking” the wafer. The semiconductor wafer is typically placed backside down on the wafer chuck since the backside has no semiconductor devices and is thus nonfunctional. 
         [0022]    As noted previously, particulate matter and scratches on the backside can cause problems such as lithographic “hotspots” and so it is desirable to mitigate the harmful effect of such particulate matter and scratches. 
         [0023]    Referring to the Figures in more detail, and particularly referring to  FIG. 1 , there is illustrated a semiconductor wafer  10  having a coating  12  on the backside of the semiconductor wafer. The coating  12  may also be referred to as a mitigating layer. 
         [0024]    It may be desirable to have the semiconductor  10  undergo a cleaning process prior to applying the coating  12  in order to remove as many particulate matter as possible. This cleaning process may be a conventional cleaning process such as a wet cleaning process or a dry cleaning process where the wafer may be wiped to remove the particulate matter. It is believed that in many cases, encapsulation by applying the coating  12  may be desirable since there may be incomplete removal of the particulate matter during any cleaning process and in any case, scratches may not be removed by cleaning. In one exemplary embodiment, the particulate matter and scratches may be characterized before applying the coating  12  to determine if the coating  12  is desirable. 
         [0025]      FIG. 2  is a cross sectional view of  FIG. 1  in the direction of arrows  2 - 2  shown in  FIG. 1 . Semiconductor wafer  10  may have particulate matter  14  and/or scratches  16  on the backside  18  of the semiconductor wafer  10 . The frontside of the semiconductor wafer is indicated by reference number  20 . 
         [0026]    A coating  12  has been applied to the backside  18  so as to encapsulate the particulate matter  14  and fill scratches  16  that may be present on the backside  18 . Coating  12  may have a thickness that may be selected to be on the order of at least about two times the size of the maximum targeted particle size. For example, typical particle sizes range from 0-10 um so the layer thickness may be on the order of 20 um. The coating  12  may be deposited or applied by a spin on film process. In one exemplary embodiment the coating  12  may be silicon, such as amorphous silicon. In an alternative embodiment, the coating  12  may be an amorphous carbon film. Both of the silicon and amorphous carbon films may be planarized and may also be removed selectively using a wet process or a dry process such as a reactive ion etching process or some combination thereof. In one exemplary embodiment, TEOS (tetraethyl orthosilicate) for the silicon film or acetylene, ethylene or propylene for the amorphous carbon film may be applied by a spin on process at a temperature that is compatible with any frontside films, for example, 250 to 600° C. to turn the spin on film into silicon or amorphous carbon. For backside deposition, the processing should be performed in a single wafer chamber. It is noted that the surface  22  of the coating  12  may be uneven due to the presence of particulate matter  14  and scratch  16 . Accordingly, the coating  12  may undergo a planarizing process to planarize the surface  22  of the coating  12 . For purposes of illustration and not limitation, the planarizing process may be a chemical-mechanical planarizing process (CMP). The coating  12  after planarization is shown in  FIG. 3 . 
         [0027]    Referring now to  FIG. 4 , the semiconductor wafer  10  is flipped over so as to be supported on a wafer chuck  24 , such as an electrostatic chuck. Frontside surface  20  (now facing up) of the semiconductor wafer  10  may then undergo a lithographic process such as by lithographic tool  28 . Most preferably, the lithographic process is an EUV process in which extreme ultraviolet light (around 13.5 nanometers in wavelength) is used for exposing a photoresist. Surface  22  of coating  12  is now in direct contact with surface  26  of wafer chuck  24 . It is noted that since the particulate matter  14  and scratch  16  are fully encapsulated by coating  12 , and there is even topography due to the planarizing described with respect to  FIG. 3 , the semiconductor wafer  10  may be processed without fear of hotspots. 
         [0028]    Later in the process flow, the coating  12  may be removed by conventional means such as by reactive ion etching, wet etching or chemical-mechanical polishing, depending on the specific film chosen. 
         [0029]    Referring now to  FIGS. 5 to 7 , another exemplary embodiment will be described. In this exemplary embodiment, a protective coating  30  may be applied to the frontside  20  of the semiconductor wafer  10  to avoid any possible harm to the frontside  20  when the backside coating  12  is applied. In a preferred exemplary embodiment, a protective coating may not be required, but if it is required (for instance when the exposed frontside cannot be mechanically contacted to the wafer chuck without damaging the frontside film stack), then the protective coating  30  may need to be applied, and the material for such protective coating may need to be chosen with special consideration with respect to the choice of backside pattern material as chosen above, so as to allow selective removal in subsequent processes. The protective coating  30  may also be silicon or amorphous carbon. The protective coating  30  may be the same or different than the backside coating  12 . 
         [0030]    As shown in  FIG. 5 , protective coating  30  has been applied to the frontside  20  of the semiconductor wafer  10 . The backside  18  may contain particulate matter  14  and/or scratch  16  which may need to be encapsulated. 
         [0031]    Coating  12  may be applied as described previously to the backside  18  to encapsulate the particulate matter  14  and fill scratch  16 , as shown in  FIG. 6 , followed by planarizing to result in the structure shown in  FIG. 7 . 
         [0032]    Another exemplary embodiment is illustrated in  FIG. 8 . In the exemplary embodiment illustrated in  FIG. 8 , multiple layers of coating  12  may be applied to the backside  18 . The coating  12  may be applied and planarized, as described previously, to encapsulate the particulate matter  14  and fill scratch  16 . Thereafter, a stop layer  32  such as an oxide may be applied to the coating  12  and planarized. Stop layer  32  may have a thickness of about 20 to 100 nm. The stop layer  32  provides a method of removing the exposed coated layer selective to another coated layer deeper in the stack. The stop layer  32  stops the removal of one coated layer selective to the stop layer. Any material that is compatible with being deposited alternately with the coating  12  as well as providing a selective stop against the coating  12  may be used as the stop layer  32 . For example, if the coating  12  is silicon, then the stop layer  32  may be an oxide or nitride if the removal process of the silicon is reactive ion etching. If using a wet etching process, hot ammonia will etch silicon selective to oxide, for example. Then the stop layer  32  may be removed by a process selective to the underlying coating  12 . For example, if the coating  12  is silicon and the stop layer  32  is oxide, then the oxide stop layer  32  may be removed, for example, by a fluorine-based reactive ion etching process or dilute HF. 
         [0033]    Then another coating  12 ′ may be applied to the stop layer  32  and planarized. This process sequence may be repeated until subsequent stop layer  32 ′ and coating  12 ″ have been formed as shown in  FIG. 8 . Additional stop layers and coatings may be applied until the desired number of stop layers and coatings have been added. All of the stop layers  32 ,  32 ′ may comprise the same material or different materials. Similarly, all of the coatings  12 ,  12 ′,  12 ″ may comprise the same material or different materials. 
         [0034]    The method of alternating coated layers separated by stop layers provides a stack of “pre-built” films that may be iteratively removed as needed, without the need to go through as many iterative deposition steps. That is, the last coating  12 ,  12 ′ or  12 ″ may be removed followed by removal of the last stop layer  32 ′ or  32  so that the next coating  12  or  12 ′ is a clean and flat backside layer for the next lithographic process, such as an EUV process. There may be cases where one is prohibited from depositing such a coated layer prior to EUV exposure due to exposed films, etc. where having several “pre-built” layers can serve the needed purpose of having a clean and flat backside layer before each lithographic process. 
         [0035]    The frontside protective coating  30 , while shown in  FIG. 8 , is optional and may be dispensed with if desired. 
         [0036]    Referring now to  FIG. 9 , there is illustrated a process flow for the various exemplary embodiments. The process begins by obtaining a semiconductor wafer, box  40 . 
         [0037]    In an optional process, the backside of the semiconductor wafer may be cleaned to remove as many particulate matter as possible, box  42 . Optionally also, the backside of the semiconductor wafer may be characterized to determine if an encapsulating coating is desirable. 
         [0038]    In another optional process, the frontside of the semiconductor wafer may receive a protective coating to protect the frontside during subsequent application of the backside coating, box  44 . The frontside protective coating would have to be removed prior to performing any lithographic process on the frontside. 
         [0039]    The backside of the semiconductor wafer may be coated, box  46 , by any of the processes and materials described previously. 
         [0040]    The backside coating may then be planarized, box  48 . 
         [0041]    It is then determined if more backside layers are to be applied, box  50 . The additional backside layers may be those described with respect to  FIG. 8 . If more layers are to be applied, a stop layer may be applied such as stop layer  32  in  FIG. 8 , box  60 , then the process returns to box  46  to apply additional coating layers. If no more layers are to be applied, the process proceeds to place the coated semiconductor wafer on the wafer chuck, box  52 . It is preferred that the wafer chuck be an electrostatic wafer chuck. The backside coating is placed in direct contact on the wafer chuck. 
         [0042]    If the semiconductor wafer has a frontside protective coating, the frontside coating may be removed, box  54 , before or after the semiconductor wafer is placed on the wafer chuck. In any event, the frontside protective coating must be removed before a lithographic process is performed. 
         [0043]    A lithographic process is next performed on the frontside, box  56 . It is most preferred that the backside coating be present when the semiconductor wafer undergoes an EUV process as this process most likely leads to hotspots. 
         [0044]    When lithographic processing is completed, or at least when EUV lithographic processing is completed, the backside coating may be removed, box  58 . 
         [0045]    It will be apparent to those skilled in the art having regard to this disclosure that other modifications of the exemplary embodiments beyond those embodiments specifically described here may be made without departing from the spirit of the invention. Accordingly, such modifications are considered within the scope of the invention as limited solely by the appended claims.