Abstract:
A method of forming a suspended semiconductor film is provided comprising providing a semiconductor structure including a layer of semiconductor film over a sacrificial layer, the semiconductor film secured to a substrate; depositing a film of material over the semiconductor film that has a tensile or compressive strain with respect to the semiconductor film patterning the deposited film to leave opposed segments spaced from each other by a central portion of the semiconductor film; patterning the semiconductor film and removing the sacrificial layer beneath the semiconductor film to leave a semiconductor film section anchored to the substrate at at least two anchor positions, with the film segments remaining on the semiconductor film adjacent to the anchor positions and spaced from each other by the central position of the suspended semiconductor film such that the film segments apply a tensile or compressive stress to the suspended semiconductor film.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a divisional of application Ser. No. 10/302,777, filed Nov. 22, 2002 now U.S. Pat. No. 6,858,888, which claims priority to provisional patent application Ser. No. 60/333,331, filed Nov. 26, 2001, the disclosures of which are incorporated herein by reference. 
    
    
     REFERENCE TO GOVERNMENT RIGHTS 
     This invention was made with United States government support awarded by the following agency: NSF 0079983. The United States government has certain rights in this invention. 
    
    
     FIELD OF THE INVENTION 
     This invention pertains generally to the field of semiconductor processing and particularly to the growth of thin films of semiconductor in a controlled manner. 
     BACKGROUND OF THE INVENTION 
     Significant improvements in the functionality of integrated circuits may be obtained by integrating different materials with specialized properties onto a base semiconductor such as crystalline silicon. For example, semiconductor structures incorporating thin films of gallium nitride, germanium, germanium-silicon, etc. on a silicon base would enable the development of transistors with integrated optics capabilities or the capability of operation at much higher frequencies than presently possible. A major obstacle to growing thin films of one semiconductor material on another (such as germanium on silicon) is the lattice mismatch and consequent strain-induced film morphology. This obstacle controls and limits the film morphology and, in turn, the electrical and optical properties of the film. 
     Strain induced thin film morphology limitations are encountered, for example, in the heteroepitaxial growth of silicon-germanium on crystalline silicon substrates. It is found that heteroepitaxial growth of Si 1-x Ge x  with x&gt;0.2 typically results in the formation of three-dimensional islands which can act as quantum dots (QDs) because they localize charge. These coherently strained QDs form naturally as a strain reduction mechanism for the film. If x is less than 0.2, a strained film is formed which does not have the 3D islands. Heterojunction bipolar transistors (HBTs) have been made using such defect-free epitaxial films of silicon-germanium and have shown dramatically improved performance relative to silicon HBTs. However, it would be desirable to be able to increase the germanium concentration in such films beyond that which has been possible in the prior art because of the occurrence of the quantum dot islands at higher germanium concentrations. In addition, the thickness of a heteroepitaxial silicon-germanium film of a given germanium concentration is limited by the critical thickness at which dislocations form because of the 4.2% lattice mismatch. While it would be desirable to be able to produce thicker defect-free films for many applications such as HBTs, for certain applications, the quantum dots that are formed in strained films are desirable because of potential applications in quantum computation and communication, light detectors, and lasers. It would be preferable for many of these applications that the arrangement of quantum dots be regular, rather than random, and with a narrow quantum dot size distribution. 
     SUMMARY OF THE INVENTION 
     The micromachined structures of the present invention provide a selected surface stress level in a semiconductor film, such as silicon, that allows the growth of an epitaxial film on the semiconductor film in a controlled manner to result in desired properties. Such structures can be produced by lithography and scaled to sub-micron dimensions. Parallel batch fabrication of multiple devices on a silicon wafer can be carried out with subsequent dicing of the structures after fabrication of transistors or other devices. 
     In the present invention, a biaxial or uniaxial tensile or compressive stress is applied to a suspended semiconductor film (e.g., crystalline silicon) and a heteroepitaxial film of semiconductor material is grown on the stressed semiconductor film. The stress in the base semiconductor film is introduced by utilizing thin film segments deposited on the semiconductor film which have strain with respect to the semiconductor film as deposited, applying either tensile or compressive stress to the semiconductor film. Utilizing the induced stress on the semiconductor film, the natural lattice mismatch strain and/or thermal expansion strain can be enhanced or countered, as desired, allowing growth morphologies to be controlled to allow applications such as the production of specific arrays of quantum dots, high germanium concentration films, and arrays of quantum dots with controlled size distributions. 
     The semiconductor microstructures of the invention include a suspended semiconductor film anchored to a substrate at at least two opposed anchor positions, and strain inducing thin film segments deposited on the semiconductor film adjacent to the anchor positions to apply either compressive or tensile stress to the semiconductor film between the film segments. Crystalline silicon may be utilized as the semiconductor film, although it is understood that other semiconductors (e.g., germanium, gallium arsenide, etc.), or other forms of the semiconductor (e.g., polycrystalline silicon) may constitute the semiconductor film. For silicon thin films, the film segments may comprise layers of, e.g., silicon dioxide and silicon nitride which are particularly suited to apply tensile stress to the semiconductor film. The semiconductor film may be formed as a beam which is anchored to the substrate at two opposed positions and is suspended from the substrate between the two opposed anchor positions. The semiconductor film may also be formed with arms anchored to the substrate at multiple pairs of opposed anchor positions to apply stress in multiple directions to a central portion of the semiconductor film. A layer of material such as silicon-germanium may be deposited on the central region of the semiconductor film, with the characteristics of the deposited layer affected by the stress in the underlying semiconductor film. For example, in accordance with the invention, the number of quantum dots in silicon-germanium deposited on a silicon semiconductor film is inversely related to the tensile stress imposed on the underlying semiconductor film. 
     The semiconductor microstructures in accordance with the invention may be formed by providing a semiconductor structure including at least a layer of semiconductor film over a sacrificial layer, with the semiconductor film secured to a substrate. A film of material is then deposited over the semiconductor film that is in tensile or compressive strain with respect to the semiconductor film. The deposited film is patterned to leave opposed segments spaced from each other by a central region of the semiconductor film. The semiconductor film is then patterned and the sacrificial layer is removed beneath the patterned semiconductor film to leave a semiconductor film section anchored to the substrate at opposed anchor positions. The film segments remain on the semiconductor film adjacent to the anchor positions and spaced from each other by the central region of the suspended semiconductor film such that the film segments apply a tensile or compressive stress to the suspended semiconductor film. 
     Further objects, features and advantages of the invention will be apparent from the following detailed description when taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the drawings: 
         FIG. 1  is a plan view of multiple semiconductor microstructures in accordance with the invention having a semiconductor film anchored to the substrate at two opposed anchor positions. 
         FIG. 2  is a more detailed plan view of a portion of a suspended semiconductor film having notches formed therein to provide stress concentration. 
         FIG. 3  is a photomicrograph illustrating formation of quantum dots in a silicon-germanium layer deposited on the microstructure of  FIG. 2  at a low stress position. 
         FIG. 4  is a photomicrograph as in  FIG. 3  illustrating the formation of quantum dots in the deposited layer surface at an intermediate stress position. 
         FIG. 5  is a photomicrograph as in  FIG. 3  illustrating the layer surface at a highly strained position. 
         FIG. 6  is an illustrative side view of a silicon-on-insulator structure that may be utilized in forming semiconductor microstructures in accordance with the invention. 
         FIG. 7  is a view as in  FIG. 6  at a further processing step. 
         FIG. 8  is a view as in  FIG. 7  after a further processing step. 
         FIG. 9  is a view as in  FIG. 8  after a further processing step. 
         FIG. 10  is a view as in  FIG. 9  after a further processing step. 
         FIG. 11  is a view as in  FIG. 10  after a final processing step leaving a suspended semiconductor film with film segments thereon. 
         FIG. 12  is a view similar to that of  FIG. 9  after an alternative processing step. 
         FIG. 13  is a view as in  FIG. 12  after a further processing step. 
         FIG. 14  is a view as in  FIG. 13  after a final processing step leaving the suspended semiconductor film with film segments thereon. 
         FIG. 15  is a photomicrograph showing a large quantum dot formed in the deposited layer at a high stress area of the semiconductor structure as in  FIG. 2 . 
         FIG. 16  is a photomicrograph illustrating a perspective view of a large quantum dot as in  FIG. 15 . 
         FIG. 17  is a plan view of another semiconductor microstructure in accordance with the invention having a semiconductor film with arms anchored to the substrate at two pairs of opposed anchor positions that are diagonally oriented with respect to each other. 
         FIG. 18  is a plan view of a semiconductor microstructure having a semiconductor film with arms anchored to the substrate at four pairs of opposed anchor positions. 
         FIG. 19  is a plan view of another semiconductor microstructure with a semiconductor film also having arms anchored to the substrate at four pairs of opposed anchor positions. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     With reference to  FIG. 1 , an example of the invention in which the semiconductor film is formed as a beam anchored at two ends to form a bridge is illustrated generally at  30 . A plurality of different semiconductor film bridge beams  31  of various lengths and configurations are shown. Each of the semiconductor film beams  31  is anchored to a substrate  32  at opposed anchor positions  33  and  34 . The anchored beams  31  are suspended over an opening or depression  35  in the substrate. Thin film segments  38  and  39  are deposited on the semiconductor film beams  31  adjacent to the anchor positions  33  and  34 , respectively. As illustrated in  FIG. 1 , the thin film segments  38  and  39  may be deposited both over a portion of the suspended beam  31  as well as over the surface of the substrate  32 . The film segments  38  and  39  are spaced from each other to leave a central position  40  of each of the beams  31  that is not covered by the films  38  and  39  and which is available to be used for other purposes, e.g., formation of devices thereon, and particularly for the deposit of a heteroepitaxial film of another semiconductor. The deposited film segments  38  and  39  have a strain as deposited on the semiconductor film beams  31  to apply either compressive or tensile stress to the beams  31 . Commonly, the film segments  38  and  39  will be deposited to provide tensile strain, thereby applying tensile stress to the thin film beams  31 , with the tensile stress induced in a direction parallel to the length of the beams  31 . The stress in the beams may be concentrated by utilizing notches or cutouts as illustrated at  41  in  FIG. 1  or by forming openings (not shown) in the beams. 
       FIG. 2  illustrates an example of the beam structure of  FIG. 1  having notches  41  therein for concentration of stress. An exemplary structure of this type was formed, as described further below, to provide a crystalline silicon thin film bridge  31  and deposited film segments  38  and  39  formed of layers of silicon dioxide and silicon nitride. A layer of silicon-germanium was then deposited on the central portion  40  of the beam and annealed. Atomic force microscope images taken from the position marked A, B and C in  FIG. 2  are shown in  FIG. 3 ,  FIG. 4 , and  FIG. 5 , respectively. Due to the shape of the bridge structure, the highest strain occurs near the notches  41  at the position C and the lowest strain occurs near the deposited film segments  38  and  39 , such as at the position marked A in  FIG. 2 .  FIG. 3 , taken from the position marked A in  FIG. 2 , shows a quantum dot morphology similar to that which would be seen for silicon-germanium growth on an unstrained silicon substrate.  FIG. 4 , at an area of greater tensile stress in the beam  31 , shows a significant decrease in the density of quantum dots, indicating that coarsening throughout the anneal period has resulted in a lower density of small quantum dots. In the image of  FIG. 5 , from the highest stress area of the beam, the density of small quantum dots is extremely low (none are visible in the scan area; the black spot is a minor defect in the silicon buffer layer). 
     An exemplary process for forming the stressed thin film structures of the invention is illustrated with respect to  FIGS. 6–11 . In this exemplary process, the device fabrication begins with a commercial silicon-on-insulator wafer  45  composed of a base substrate layer of crystalline silicon  46 , a silicon dioxide layer  47 , and a top crystalline silicon layer  48 , (e.g., a 4 inch &lt;100&gt; SOI (Si/SiO 2 /Si=10/1/550 μm). The wafer  45  is initially oxidized with a 0.8 μm thermal oxide (at 1050° C.) coating  50  as shown in  FIG. 7 . The oxide coating  50  serves to protect silicon areas where quantum dot growth occurs after micromechanical or microelectronic structures are fabricated. The oxide layer on the front side of the wafer is then etched to form openings  51  and to leave a central oxide layer  52  at positions at which the thin film segments are to be formed, as shown in  FIG. 8 , and a layer  53  of silicon nitride is then deposited over the wafer including the open areas  51 , as shown in  FIG. 9 . The front side silicon nitride layers are then removed by deep reactive ion etching (RIE (UW-STS)) to open the areas  51  leaving the silicon dioxide layer  52  and the silicon nitride layer  53  as the thin film segments over the silicon layer  48 , as shown in  FIG. 10 . RIE etching of the silicon layer  48  is followed by etching of the buried oxide layer  47 , acting as a sacrificial layer, e.g., by HF:HCl (1:1), as illustrated in  FIG. 11 , to release a suspended silicon beam  31  formed from the silicon layer  48 . The overlying film segments composed of the layers  52  and  53  have built-in strain to stress the suspended beam  31  in, e.g., tensile, stress. As illustrated in  FIG. 11 , the thin film beam  31  is suspended above a surface  56  of the base substrate  46  in the area of the opening  58  which was opened by the etching processes. It is understood that the beam  31  is anchored at opposed anchor positions to the silicon substrate  46  by the oxide layer  47  and the silicon layer  48  (which, in this case, is integral with the silicon beam  31 ). 
     An alternative procedure for freeing the beam is illustrated with respect to  FIGS. 12–14 . With the front side silicon nitride layer removed, as was shown in  FIG. 10 , an RIE etching is carried out through the oxide layer  48  to isolate the thin film beam  31  on the oxide layer  47 , as shown in  FIG. 12 . The backside silicon nitride layer  53  is then removed, e.g., by RIE, followed by a backside etching of the substrate silicon layer  46  that terminates on the oxide layer  47  to leave an opening  60  in the backside of the wafer, as shown in  FIG. 13 . The oxide layer  47  in the exposed area  60  is then etched away to release the beam  31  with the thin film segments  33  or  34 , composed of the layers  52  and  53 , over portions of the beam  31 . 
     After patterning and releasing of the stressed suspended silicon film beams  31 , a thin film of another semiconductor may be deposited on the stressed films  31 . For example, silicon-germanium films may be grown on the stressed films  31 . As an example, silicon-germanium films at various silicon to germanium ratios were formed on the film beams  31  of  FIG. 1 . After these beams were patterned and released, they were chemically cleaned and loaded into a UHV Molecular Beam Epitaxy (MBE) chamber where growths of Si Ge were performed between 450° C. and 550° C. At a high flux rate of Ge, the film growth is kinetically limited, which causes a high density of small quantum dots (less than 30 nm) to form, regardless of the stress on the beams  31 . By subjecting the samples to a 30 minute post-growth anneal at the growth temperature, however, clear differences in morphology on the strained and unstrained areas of the substrate were found.  FIGS. 3–5 , discussed above, illustrate a density gradient of small quantum dots which is inversely proportional to the underlying substrate strain. Following a 30 minute anneal at the growth temperature of 550° C., a few extremely large islands (about 0.5 μm) were found, as illustrated in  FIGS. 15 and 16 . As shown in  FIG. 15 , small quantum dot islands were seen surrounding the large island denoted at  62  in  FIGS. 15 and 16 . In the highly strained areas of the silicon beam, the densities of these large islands is as much as 50 times greater than in the unstrained areas. The lower density of small quantum dots seen in  FIGS. 4 and 5  can be attributed to the relatively high density of nearby large islands. 
     The semiconductor film can be anchored to the substrate at more than two positions so that the film can be stressed in more than one direction.  FIG. 17  illustrates a semiconductor film  69  formed as a cross with two pairs of arms  70  which are anchored to the substrate  71  at two pairs of anchor positions  72  and  73 , which are diagonally oriented with respect to each other. The cross-shaped film  69  is suspended above an opening or depression  75  in the substrate  71 . Film segments  76  are deposited on the arms  70 , as described above, adjacent to the opposed pairs of anchor positions  72 , and film segments  78  are deposited over the arms  70  of the semiconductor film  69  at positions adjacent to the anchor positions  73 . The film segments  76  and  78  strain the semiconductor film  69  in diagonally opposed directions so that the center portion  80  of the semiconductor film is stressed in two perpendicular directions. As shown in  FIG. 18 , a semiconductor film  85  may be anchored to a substrate  86  at four pairs of anchor positions  88 ,  89 ,  90 , and  91 . A series of arms  94  extend away from a center portion  95  of the semiconductor film  85  to be anchored to the substrate at the anchor positions  88 – 91 . Portions of the arms  94  have film segments  98  deposited thereon to stress the arms and thereby provide stress extending in four symmetrically distributed directions to the center portion  95  of the semiconductor film  85 .  FIG. 19  illustrates a variation on the structure of  FIG. 18  in which the film segments  98  essentially entirely cover the arms  94  of the suspended semiconductor film, again providing symmetrical stress to the center portion  95  of the semiconductor film, but with greater stress levels than are imposed with the structure of  FIG. 18 . 
     It is understood that the invention is not confined to the particular embodiments set forth herein as illustrative, but embraces all such forms thereof as come within the scope of the following claims.