Abstract:
A method and apparatus for forming a thermally-evaporated bina          (or greater) thin film are disclosed in which the surface area of an evaporatio          container is effectively increased by using an inert medium added to source materials that are to form the binary (or greater) film. Using this method a

Description:
FIELD OF THE INVENTION  
         [0001]    This invention relates to the field of deposition of thin fil          composed of multiple materials by thermal evaporation.  
         BACKGROUND  
         [0002]    Evaporative deposition techniques are extremely importan          the semiconductor industry where there is a necessity for highly uniform an          very thin films of various materials. In the semiconductor industry, evapora          deposition is useful in forming a material layer of a desired stoichiometry frc          plurality of different materials.  
           [0003]    In thermal evaporation techniques, vapor particles can be generated in high vacuum by sublimation or vaporization of a material via a variety of heating sources and then condensed on a substrate. Heating sou          include resistive heating sources, lasers, and electron beam sources. Typical material source is placed in an evaporation crucible or boat and a heat sourc          such as resistive heating coils, applies thermal energy to the crucible or boat (indirect resistive heating) causing the material source to melt and vaporize. Upon contacting a cooler surface the vaporized material condenses and for          film.  
           [0004]    Formation of a homogenous thin film having high unifor          and desired stoichiometry by thermal evaporation of a single material is a sir          procedure because a homogenous material source will have only a single bo          point, a single freezing point, and there is no opportunity for dissociation. Therefore, under appropriate conditions, a very thin film that is useful for v          purposes can be easily formed. However, when a binary (or tertiary or grea          film is desired, problems are presented because of the differing physical characteristics (e.g., melting and boiling points) of the multiple source mate          and the ever-present problem of dissociation. Often, when forming binary           by thermal evaporation for semiconductor industrial purposes, a material gradient is unintentionally formed in the thin film where the initial material deposited does not have the desired stoichiometry. This requires longer formation times to reach the desired or required stoichiometric levels and c          lead to films that are not as uniform as desired. Such problems increase and exaggerated as the physical characteristics of the different source materials become increasingly divergent.  
         SUMMARY  
         [0005]    This invention provides a method for improving the stoichiometric character of a thermal-vapor-deposited material layer formed materials having different physical (e.g., melting and boiling points) and chemical properties. An inert medium is added to the source materials with evaporation container (e.g., a crucible) that are to form a binary (or greater upon vaporization and condensation. By this method, films of increased uniformity and maintained stoichiometry are achievable.  
           [0006]    These and other advantages and features of the invention           be more clearly understood from the following detailed description which is provided in conjunction with the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]    [0007]FIG. 1 is a cut-away illustration showing source material           prior art techniques;  
         [0008]    [0008]FIG. 2 is a cut-away illustration of materials used for evaporative deposition of a thin film in accordance with an embodiment of           invention;  
         [0009]    [0009]FIG. 3 is an illustration of a technique of thin film deposit in accordance with an embodiment of the invention;  
         [0010]    [0010]FIG. 4 is an illustration of a thin film deposited by prior a          techniques;  
         [0011]    [0011]FIG. 5 is an illustration of a thin film deposited in accord          with an embodiment of the invention; and  
         [0012]    [0012]FIG. 6 is an illustration relating to an example of a thin fil          produced in accordance with an embodiment of the invention.  
     
    
     DETAILED DESCRIPTION  
       [0013]    The invention relates to thin films that are at least binary i          nature and their deposition by evaporative techniques. In the semiconduct          industry it is often important to maintain both the stoichiometry in thin fil          and as well as the uniformity of the films. Thermal evaporation is an inexpe          and commonly used method of forming such films. This invention utilizes; method of increasing the surface area of an evaporation container, preferabl          using an inert medium added to source materials held by the container that to form the binary (or greater) film. By this method, films of increased uniformity and maintained stoichiometry are achievable.  
         [0014]    In the following detailed description, reference is made to various specific embodiments in which the invention may be practiced. The embodiments are described with sufficient detail to enable those skilled in t          to practice the invention, and it is to be understood that other embodiment may be employed, and that structural and electrical changes may be made without departing from the spirit or scope of the present invention.  
         [0015]    The terms “substrate” and “wafer” can be used interchangeably in the following description and may include any foundatio          surface, but preferably a semiconductor-based structure. The structure sho          be understood to include silicon, silicon-on insulator (SOI), silicon-on-sapp          (SOS), doped and undoped semiconductors, epitaxial layers of silicon supp          by a base semiconductor foundation, and other semiconductor structures.           semiconductor need not be silicon-based. The semiconductor could be sili          germanium, germanium, or gallium arsenide. When reference is made to t          substrate in the following description, previous process steps may have been utilized to form regions or junctions in or over the base semiconductor or foundation.  
         [0016]    Now referring to the figures, where like reference number denote like features, FIG. 1 shows an example of how evaporative depositio          techniques in the prior art utilized source material. Prior art binary films w          produced by thermal evaporation by applying thermal energy to source: mat          until they vaporized and then condensed on the desired target (e.g., a semiconductor wafer). As is shown, to form a binary film, source materials comprising a first source material  14  and a second source material  16  are ad          to an evaporation container  10 , such as a crucible or boat. These two sourc          materials  14  and  16 , generally in the form of solid pellets shaped like marble          pebbles, are the two components that are desired to physically or chemical          combine to form the binary film. The source materials  14  and  16  can be in form of two sets of pellets, each respective set comprising one of the first or second source materials  14  and  16  as shown in FIGS. 1 and 2. Alternatively two source materials can be preliminarily combined in a desired stoichiome          form one set of pellets. As another alternative, the source materials  14  and can be in the form of a single solid entity comprising the entire mass of sou          material. In the prior art, the two source materials  14  and  16 , once added           evaporation container  10 , were subjected to thermal energy from a heat so           12 , typically a resistive heating coil, laser, or electron beam. Upon applicat          enough thermal energy, the materials  12  and  16  melt and then vaporize to the thin filn upon condensing. However, because the source materials  14             16  often have very divergent physical characteristics (e.g., melting and boili          points), one of the materials  14  typically melts and vaporizes, and subseque condenses on the target before the other of the source materials  16 , leading undesirable film stoichiometric distribution and uniformity. These diverger          physical characteristics can also lead to dissociation (the separation of chemi          components into simpler fragments) during evaporation, also negatively impacting film quality.  
         [0017]    In accordance with the invention, the problems associate          the prior art techniques can be mitigated, as shown in FIG. 2, by the additi          an inert medium  18  to the source materials  14  and  16  (be them in any of t          alternative forms) prior to the addition of thermal energy. The inert mediu          is preferably a material that has a high melting temperature (above that of e          source material  14  and  16 ), and is non-reactive in general, and particularly           the source materials  14  and  16 . The inert medium  18 , for instance, can be silicon or a ceramic based material.  
         [0018]    Typically the inert medium  18  consists of solid material si          in shape and size to the source materials  14  and  16  (e.g., pellets); however, will be readily apparent to those of skill in the art that a multitude of variati          size and shape of the inert medium  18  are possible and, depending on the circumstances, desirable. Though the shape of the inert medium  18  can va          generally spherical shapes are preferred because such a design achieves the maximum relative surface area without interfering with the evaporation pro (because of folds, sharp corners, etc.). Further, the added inert medium  18  preferably large enough to effectively maximize evaporation container  10  su          area by contacting the container  10  itself, as well as the source materials  14             16 . However, the size of the inert medium  18  should not be so large as to interfere with the evaporation process (e.g., by blocking the evaporation container  10  opening).  
         [0019]    As shown in FIG. 2, the inert medium  18  is dispersed throughout the source material  14  and  16  within the evaporation container Preferably, enough inert medium  18  is added to the source materials  14  an          so that the thermal energy used for evaporation can be efficiently transferre          from the evaporation container  10  to the source materials  14  and  16  as equ          as possible.  
         [0020]    As shown in FIG. 3, The added inert medium  18  of the invention serves to increase the heating area during the evaporation process The addition of the inert medium  18  also reduces the amount of power nee          to melt the source material  14  and  16 , even towards the middle of the evaporation container  10 , which typically in the prior art required additiona          energy. When heat is applied by the heat source  12 , preferably in a vacuum chamber  11 , the source material  14  and  16  in the evaporation container me form a liquefied source material  24 , which upon continued application of thermal energy becomes a vaporized source material  26 . This vaporized so          material  26  condenses upon contacting the cooler wafer  20 , which is positi          in proximity to the evaporation container (preferably within a vacuum evaporation chamber  11 , positioned above and facing the source material). Upon condensing, the vaporized source material  26  forms a thin film  22  comprising a combination of source materials  14  and  16 , desirably in the sa          stoichiometric ratio as initially present in the evaporation container. Typical film of about 25 Å to about 5 μm is desired as useful in the semiconductor industry, which can be produced using the invention.  
         [0021]    The uneven heating, melting, and evaporation of the sour          materials  14  and  16  found in the prior art is diminished so that the two sou          materials  14  and  16  melt and vaporize more quickly and more synchronous          The result is that the resultant film deposits in less time, leading to more un          films, and has a more desirable stoichiometry due, in part, to less dissociatio           
         [0022]    As illustrated in FIG. 4, because of the uneven heating, melting, evaporation, and dissociation of components found in the prior art first portion  28  of the thin film  22  was, in general, predominantly comprise          whichever of the source materials  14  and  16  has the lowest melting and boil points, wherein the second portion  30  of the thin film  22  has closer to the desired stoichiometry, being deposited once the second of the source mater           14  and  16  reaches its boiling point. It is also possible that under the circumstances of the prior art that the outermost portion of the thin film  22  would have an undesirably high amount of the second source material  14  o          to vaporize, which would continue to be deposited even after the first sourc          material is exhausted. Thus, a gradient  32  would be created in the thin film          where the proportional amounts of source material  14  and  16  shifts from o          extreme to the other through the thickness of the film  22 . Additionally, un          such circumstances, an uneven surface  34  could develop on the thin film  22  shown in FIG. 5, when compared to the thin film  22  of the prior art, the invention can achieve a thinner, more uniform thin film  22  of a more consis          desired stoichiometry.  
         [0023]    Though this invention has been described primarily with reference to binary films utilizing two source materials  14  and  16 , it can als          achieve thin films  22  of desired uniformity and stoichiometry utilizing three more source materials.  
       EXAMPLE  
       [0024]    The following supporting data was obtained in experiment          using actual embodiments of the invention. Table I below shows experime          results. The experiments are explained in reference to FIG. 6.  
                                                                 TABLE I                                   Inert   Source   Power   Film Silver   Film Sele           Medium   Material   (% maximum)   (mole %)   (mole %)                                    Control   None   Ag 2 Se   11%   59.60   40.4           added       Run 1   Si added   Ag 2 Se   13%   64.80   35.2       Run 2   Si added   Ag 2 Se   16%   68.90   31.1                  
 
         [0025]    Each experimental run was conducted in a vacuum chamb and used a standard ceramic crucible  108  as an evaporation container  10  an          standard resistive heating coils  110  for a heat source  12 , as is known in the           As a deposition target, a 3500 Å layer of TEOS oxide over a 200 mm silico          wafer having a ( 111 ) crystalline orientation served as a substrate  104  upon           to condense the thin film. The source material used in all runs were pellets formed of silver and selenium (Ag 2 Se), manufactured on site to be of know          stoichiometry. The target stoichiometry for the deposited thin films was Ag 66 Se 33  and the initial stoichiometry of the source material reflected this de          film stoichiometry in a 2:1 ratio (with Ag being no greater than 2). For eac          run, thermal energy was applied to the crucible  108  and its contents by the resistive heating coils  110  as a function of the % total power. The Ag 2 Se so          pellets  100  were heated for a minimum of 60 seconds to vaporize. Time to boiling was subjective and a function of the % power used. The desired thi          for each deposited experimental film was 500 Å.  
         [0026]    For the Control Run (reflecting prior art techniques), no medium was added to the Ag 2 Se source pellets  100 . The power used was a          11% of total power. As is shown in Table I, the resulting stoichiometry of t          deposited film did not achieve the target 2:1 Ag to Se ratio, but the resultin          ratio did reflect results common to techniques used in the prior art. The undesired stoichiometry was due to the dissimilar physical characteristics of silver and selenium, uneven heating, and dissociation, resulting in uneven deposition rates and amounts between the source materials.  
         [0027]    As shown in Table 1, Run 1 utilized the same Ag 2 Se sour          pellets  100 , but inert silicon (Si) media  102  was added in accordance with t          invention. Thermal energy was applied by the resistive heating coils at abo          13% total power. The 500 Å film was deposited and determined by su          analysis to have close to target stoichiometry. Run 2 also utilized inert          (Si) media  102  in accordance with the invention. For Run 2, thermal e          was applied at about 16% total power. The resulting film was not as do target stoichiometry as with Run 1, but was still closer than the Control which used no inert media.  
         [0028]    The above description, examples, and accompanying d          are only illustrative of exemplary embodiments, which can achieve the fe          and advantages of the present invention. It is not intended that the inve          limited to the embodiments shown and described in detail herein. The invention can be modified to incorporate any number of variations, alter          substitutions or equivalent arrangements not heretofore described, but w          commensurate with the spirit and scope of the invention. Accordingly, t          invention is not to be considered as being limited by the foregoing descri          but is only limited by the scope of the appended claims.