Abstract:
Silicon and metal are coevaporated onto a silicon substrate in a molecular beam epitaxy system with a larger than stoichiometric amount of silicon so as to epitaxially grow columns of metal silicide embedded in a matrix of single crystal, epitaxially grown silicon. Higher substrate temperatures and lower deposition rates yield larger columns that are farther apart while more silicon produces smaller columns. Column shapes and locations are selected by seeding the substrate with metal silicide starting regions. A variety of 3-dimensional, exemplary electronic devices are disclosed.

Description:
ORIGIN OF THE INVENTION 
     The invention described herein was made in the performance of work under a NASA contract, and is subject to the provisions of Public Law 96-517 (35 USC 202) in which the Contractor has elected not to retain title. 
     TECHNICAL FIELD 
     The present invention relates to the fabrication of electronic devices and integrated circuit devices by the deposition of circuit elements on a substrate such as silicon using the techniques of molecular beam epitaxy. More specifically, a new process is disclosed that allows the formation of circuit elements in three dimensions, rather than as planar layers, thus providing an entirely new class of structures. 
     BACKGROUND OF THE INVENTION 
     The prior art recognizes molecular beam epitaxy (MBE) as the best process for depositing very thin layers of metal and semiconductor compounds onto substrates such as silicon. The MBE process uses an ultra-high vacuum chamber containing the substrate and one or more evaporation crucibles. The material to be deposited is heated in a crucible until the material vaporizes. Molecules of the vaporized material travel unimpeded through the vacuum in straight lines to the surface of the substrate. Because of their straight flight paths, the molecules are easily collimated into a controllable beam, by suitable apertures, so as to impinge on the substrate at a selected rate and from a selected direction. Shutters may be interposed in the beam to block the beam for periods of time. Varying the heating of the crucible controls the rate of free molecule production. 
     The substrate is usually heated so that the arriving molecules remain mobile on the surface for a short time. Thus, each molecule has time to locate a preferred site upon which to attach so that a regular crystal growth is facilitated. In this way, very thin layers of single crystal or monocrystalline material may be deposited that are on the order of nanometers thick. 
     To enhance single crystal growth, the deposited layer should have a natural crystal structure similar in shape and size characteristics to the crystal structure of the substrate so that epitaxial growth takes place. In other words, the regular crystal lattice of the substrate provides a template upon which the arriving atoms of deposited material are organized into a similar, regular, single crystal structure. 
     Any substrate adaptable to the above outlined principles could profit from the process of this invention. For example, substrates may comprise silicon, germanium, or compound semiconductors such as gallium arsenide, indium arsenide and indium antimonide. However, the discussion herein is oriented to the most common and best understood substrate material which is silicon. 
     The deposited conductor is usually selected to be chemically compatible with the chosen substrate and may be a metal and semiconductor compound or even a combination of two semiconductors such as silicon and germanium. A conductor for an indium antimonide substrate, for example, might be a compound of nickel and antimony. Once again, however, the discussion herein is focused on metal silicide deposits which are also well known and characterized with respect to their structure and properties. 
     Metal silicides, combinations of metals such as cobalt, platinum, chromium, nickel, tantalum, or iridium with silicon, are good choices for the deposited layer on silicon substrates since they are chemically compatible with the silicon substrate. To deposit metal silicides, the metal and the silicon are coevaporated in separate crucibles at rates so as to impinge on the silicon substrate in correct stoichiometric ratios to form the desired single crystal compound in a thin layer. For example, cobalt disilicide (CoSi 2 ) is a well studied metal silicide conductor that is produced by MBE methods in which cobalt and silicon are coevaporated in a ratio of one cobalt atom for every two silicon atoms. 
     Prior art MBE methods control the thickness of the deposited layer by the length and rate of deposition. This affects only the dimension perpendicular to the substrate. Lateral dimensions, those parallel to the substrate surface, are controlled by lithographic techniques and limited to relatively large dimensions. The present invention, by contrast, provides a means whereby both vertical and lateral dimensions are controllable so as to permit the creation of a whole new class of three-dimensional MBE deposited devices not heretofore possible. 
     STATEMENT OF THE PRIOR ART 
     U.S. Pat. No. 4,171,234 to Negata et al. discloses a process that avoids the use of masks during MBE by forming irregular shapes in the substrate. These shapes are used to shadow an incident molecular beam, at various angles, so as to modify the characteristics of the deposited crystalline layer. The kinds of epitaxial structures that can be created this way are obviously quite limited. Beyond the height of the substrate mesas the deposited layer would revert to a planar layer of no distinction. The instant invention, however can begin with an ordinary planar substrate and develop a wide variety of three dimensional structures of almost any desired configuration. 
     U.S. Pat. No. 4,099,305 to Cho et al. discloses a process similar to Negata above and subject to the same limitations. 
     SUMMARY OF THE INVENTION 
     The present invention contemplates a new MBE type process that produces column like structures that grow epitaxially from the substrate surface in a direction generally perpendicular thereto. The height, width, shape, and spacing of the columns are all selectable by modification of the processing parameters rather than by masking. A large variety of desired three dimensional shapes may be generated to make available an entire new set of electronic devices. Some of these new devices are described, by way of example, in order to emphasize the potential of this inventive technique. 
     Briefly, the new MBE process involves coevaporating metal and silicon in ratios well removed from stoichiometric with a large excess of silicon. Given the correct growth environment, vertical columns of single crystal metal silicide epitaxially form upward from the silicon substrate surface. The columns are embedded in a surrounding matrix of single crystal silicon. It has been determined that the spacing, thickness, and height of the columns may be chosen by varying the process parameters. In addition, the location and shape of the columns may be selected by seeding the substrate in the places where columns are desired. 
     The principles of the invention are applicable to many substrate materials such as silicon, germanium, gallium arsenide, indium arsenide, or indium antimonide. For the selected substrate, a chemically compatible conductor is chosen comprising a combination deposit of metal and semiconductor or a combination deposit of two semiconductors, once again using an excess quantity of the semiconductor that forms a matrix on the substrate. Thus, although the conductive structures are described as metal compounds in this specification, for ease of explanation, the word metal is intended to include electrically conductive materials in general, especially semiconductors. 
     These and many other features and attendant advantages of the invention will become apparent as the invention becomes better understood by reference to the following detailed description when considered in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1 and 2 illustrate the basic steps in the development of vertical columns of conductive material on a substrate. 
     FIGS. 3-5 illustrate the subsequent steps of a first embodiment of the invention for seeding the shape and location of the columns. 
     FIGS. 6-8 illustrate a second method to seed columns. 
     FIGS. 9-11 illustrate a third seed method that creates a planar substrate upon which to grow column structures. 
     FIG. 12 depicts an example of a three dimensional device that can be made with the instant invention, namely, a photoemission sensor using columns that start and stop in the vertical direction. 
     FIG. 13 illustrates how vertical columns may be tied together to create a three dimensional device, in this case, an infrared sensor. 
     FIG. 14 depicts another possibility, a permeable base transistor. 
     FIG. 15 depicts the development of a column so thin that quantum effects may be studied. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIGS. 1 and 2 demonstrate the basic process of the invention. FIG. 1 shows a small section of a silicon substrate  10  which is processed in an ultra-high vacuum MBE system chamber. Substrate  10  is cleaned by standard techniques well known to the art so as to produce an atomically clean surface  12 . Substrate  10  is heated to a temperature such that arriving atoms of the deposition material have an interval of movement on surface  12  sufficient to locate the desired bonding sites. For the metal silicides, temperatures in the range of 640 to 800 degrees Centigrade have been successfully used. Cobalt disilicides will be utilized as the deposited material for purposes of illustrating and describing the invention. Of course, the subject of this invention is applicable to metal silicides generally. Also all other combinations of metal with semiconductor, or semiconductor with semiconductor, like silicon with germanium, are suitable for deposition as well depending on the selected substrate material. 
     For cobalt disilicide, cobalt and silicon are coevaporated in the MBE chamber and allowed to shower onto surface  12  as suggested by arrows in FIG.  1 . The ratio of cobalt to silicon is intentionally made non-stoichiometric with an excess of silicon. For example, ten silicon atoms may be directed to surface  12  for every cobalt atom. This silicon rich ratio produces an unexpected and unique effect as shown in FIG.  2 . 
     In FIG. 2, the cobalt atoms, being temporarily mobile on surface  12 , preferentially seek out and coalesce with other cobalt disilicide molecules at locations  13 . At first, locations  13  are random, but soon establish growth centers upon which columns  16  of single crystal cobalt silicide grow. The excess silicon forms a single crystal matrix  14  between and around columns  16 . Matrix  14  is depicted as transparent with dashed lines  14  in the drawings to enhance clarity. Both the matrix  14  and the columns  16 , once begun, grow generally vertically upward from surface  12  by epitaxial crystal formation. Sometimes, for different substrate crystallographic orientations, the columns grow upward at a non-perpendicular angle. 
     The height of columns  16  is, in principle, unlimited. The shape and locations of the columns are selectable as well. By raising the substrate temperature or decreasing the rate of deposition, the interval of mobility on surface  12  is lengthened so that columns are produced which are farther apart and have larger diameters. Alternatively, the ratio of silicon to metal may be changed. Less metal results in columns of the same spacing but lesser diameter. Shuttering the cobalt beam stops column growth when desired. New columns can then be initiated by unblocking the cobalt beam. Furthermore, locations  13  may be deliberately chosen and shaped as described in FIGS. 3-11. 
     In FIG. 3, a fragment of silicon substrate  20  is shown. A planar layer of single crystal cobalt silicide  22  is formed on substrate  20  by conventional MBE methods. A standard photoresist layer  24  is patterned with electron beam lithography and selectively removed, as at  26 , by conventional procedures. The exposed parts of the silicide, not protected by resist, are removed by wet or dry etch techniques so as to produce the structure of FIG.  4 . Finally, the remaining resist is dissolved away, as in FIG. 5, to leave behind regions  28  of cobalt silicide. Regions  28  have the desired shape and position to serve as nucleation sites upon which columns of cobalt silicide may be grown using the procedures described with respect to FIGS. 1 and 2. 
     Another method for initiating column growth in selected locations is shown in FIGS. 6-8. A room temperature substrate  30  receives a stoichiometric deposit of cobalt disilicide. The atoms freeze immediately to the surface to create an amorphous mixture of cobalt and silicon in a layer  32  (FIG.  6 ). An electron beam is used to write a desired pattern of crystallized regions  34  that epitaxially align with substrate  30  as in FIG.  7 . An acid etch step preferentially removes the amorphous material to once again leave behind regions  34  of single crystal silicide upon which vertical structures can nucleate as in FIG.  8 . 
     FIGS. 9-11 show a third and perhaps preferred embodiment of the invention. A layer of pure cobalt (or other metal)  42  is deposited on a substrate  40 . Again using well known electron beam lithography techniques, a resist  44  protects selected areas of cobalt layer  42  while exposed areas  46  are removed to produce islands of cobalt  42  as shown in FIG.  10 . The FIG. 10 structure is heated to a temperature in the range of 400-600 degrees C. This causes the cobalt to diffuse into the substrate surface to form areas of cobalt disilicide  48  upon which nucleation can be initiated. The advantage of this method is that the final structure, shown in FIG. 11, is planar since the cobalt atoms do not substantially change the physical size of the silicon substrate crystal structure. The planar surface promotes more cleanly defined column structures when the process of FIGS. 1 and 2 is used to grow epitaxial formations thereon. 
     Even free standing column structures may be produced by plasma etching the matrix embedded columns with CF 4  which attacks silicon at a rate about 100 times greater than silicide compounds. Hence, the silicon matrix ( 14  in FIG. 2) could be removed from about the columns leaving behind free standing structures. 
     Numerous new electronic devices are made possible by the advent of three dimensional epitaxially grown structures. A few examples are described in FIGS. 12-15. 
     Metal silicides are good optical and infrared sensing substances since they produce charge carriers in response to incident photons. The metal layer must be quite thin, however, for these charge carriers to reach the semiconductor and affect the current flow therethrough. Unfortunately, thin layers do not absorb very many photons. Prior art detectors which use planar layers of metal silicide on silicon are limited in sensitivity because of this tradeoff. The instant invention increases this sensitivity by allowing third dimensional expansion of the silicide material for a given cross section of incident photon flux. FIG. 12 shows a substrate  50  upon which a matrix  52  of silicon is grown using MBE methods. Shuttering the cobalt beam on causes layers of growth  54  which include many short columns of silicide  58  suspended in matrix  52 . Shuttering the cobalt beam off causes layers of growth  56  with no silicide. The three dimensional cloud of silicide particles is much more likely to intercept a photon in a given cross sectional flux of photons than a single layer as in the prior art. Thus, a layered internal photo-emission sensor may be built in which the current flow between contacts  60  is very sensitive to radiation. Each element of silicide  58  is very thin giving a good internal yield of charge carriers, yet absorption is increased by a massive increase in the area of silicide per unit volume of the sensor. 
     FIG. 13 shows another possibility, where columns  64  of silicide are grown in a matrix  68  deposited on a substrate  70  so as to present increased absorption surface to incident radiation. Columns  64  are joined together at the top by a layer of silicide  66  which can be deposited by conventional techniques. 
     In FIG. 14, a possible three dimensional permeable base transistor is disclosed wherein metal silicide column conductors  74  connected to a silicide base  76  mediate current flow between emitter and collector contacts  78 . 
     FIG. 15 demonstrates an experimental device with a substrate  80 , a matrix  82 , a layer of silicon oxide insulator  84 , and a metal contact  86 . A column of silicide  88  grown up from substrate  80  carries current between contacts  90 . With the process of the present invention, columns having diameters of 10 to 250 nanometers have been produced. Both larger and smaller diameters are well within the capabilities of the process. Columns of only a few nanometers in diameter are primarily subject to quantum effects in the carrying of current. Thus, column  88  in FIG. 15 comprises a quantum wire and these quantum effects can be studied. 
     Three dimensional structures this small are two orders of magnitude smaller than has been possible in the prior art. Clearly, myriad new electronic devices and structures are possible using the principles of the instant invention. 
     It is to be realized that only preferred embodiments of the invention have been described and that numerous substitutions, modifications and alterations are permissible without departing from the spirit and scope of the invention as defined in the following claims.