Abstract:
A method and structure for preventing film delamination provide for forming a thick film then partitioning the thick film into a plurality of discrete portions prior to subsequent thermal processing operations. The partitioning alleviates the effects of film stress at the interface between the film and the underlying material and prevents delamination during the subsequent thermal cycling operations, that take place subsequent to the formation of the film. The partitioned film includes a pattern density of at least about 80 percent and the discrete portions do not individually serve as device structures.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates most generally to the prevention of delamination of deposited films such as used in semiconductor manufacturing and solar cells.  
       BACKGROUND  
       [0002]     Film delamination due to subsequent thermal processing is a problem that plagues the semiconductor manufacturing industry and other applications in which film deposition is utilized. Commonly, the problem is due to a poor adhesion force between the deposited film and the material upon which the film is deposited. This problem can occur when the film is the same as the material upon which it is being deposited or different than the subjacent material. The poor adhesion between the film and the material upon which it is deposited, causes delamination such as peeling, cracking and/or blistering of the film during subsequent thermal processing such that takes place at an elevated temperature in the vicinity of 300° C. or greater. For example, this phenomena is experienced when a stack of amorphous silicon films are formed. Amorphous silicon (a-Si) films may be hydrogenated to passivate dangling bonds and the hydrogenated amorphous silicon is designated a-Si:H. The aforementioned adhesion force is particularly weak at the a-Si:H/a-Si:H interface formed between two films. The upper a-Si layer will delaminate after a subsequent thermal cycle due to residual stress (compressive) formed at the interface due to the deposition characteristics and conditions of the upper a-Si:H film and also due to hydrogen diffusion at the interface. This problem is particularly egregious when the film is relatively thick, such as 1 micron or greater. The problematic subsequent thermal cycling is difficult to avoid since semiconductor devices typically undergo a number of thermal cycling processes during their manufacture. For example, the formation of a passivating film such as silicon nitride typically takes place at an elevated temperature and for a sufficient time to effectuate delamination.  
         [0003]     Although described in conjunction with an a-Si film, the delamination problem occurs in various films used in semiconductor device and solar cell fabrication.  
         [0004]     There have been various approaches that address this delamination problem. These efforts include fine-tuning the film deposition conditions to minimize the compressive stress of the deposited film, instituting various cleaning procedures prior to the film deposition process, de-gassing prior to the film deposition process, instituting a sticking layer prior to the film deposition process, and roughening the surface upon which the film will be deposited. The effectiveness of these conventional procedures is limited and many of the aforementioned procedures for addressing the problem, are unsuitable for various manufacturing environments.  
         [0005]     Even when only partial delamination occurs, contamination from the cracking, peeling, blistering delaminated film degrades the quality of the entire semiconductor device, even in areas where the film does not delaminate. It would therefore be advantageous to provide a process and structure for preventing film delamination during subsequent thermal processing operations.  
       SUMMARY OF THE INVENTION  
       [0006]     To address these and other needs, and in view of its purposes, the present invention provides a method for preventing delamination of a deposited film. The method includes providing a substructure with a first material thereon, depositing a thick film having a thickness of at least 1 micron, on the first material, then partitioning the thick film into a plurality of discrete portions prior to a subsequent thermal treatment, wherein the thick film does not delaminate during the subsequent thermal treatment.  
         [0007]     The film may be uniformly partitioned into discrete portions that are regularly arranged and spaced, include a pattern density of at least 80% and are not individually functioning elements of a semiconductor device.  
         [0008]     According to another aspect to the invention, provided is a solar cell formed on a substrate and including a film stack thereon. The film stack includes a subjacent amorphous silicon material and a superjacent amorphous silicon film disposed directly on the subjacent amorphous silicon material. The superjacent amorphous silicon film includes a thickness of at least 1.5 microns and is patterned into a plurality of discrete portions. The portions are regularly arranged and spaced and include a pattern density of at least 90 percent. The discrete portions are not separately functioning elements of the solar cell. 
     
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0009]     The present invention is best understood from the following detailed description when read in conjunction with the accompanying drawing. It is emphasized that, according to common practice, the various features of the drawing are not necessarily to scale. On the contrary, the dimensions of the various features are arbitrarily expanded or reduced for clarity. Like numerals denote like features throughout the specification and drawing.  
         [0010]      FIGS. 1A, 2A ,  3 A, and  4 A are cross-sectional views showing a sequence of processing operations according to one exemplary embodiment of the invention;  
         [0011]      FIGS. 1B, 2B , and  4 B are top views of the structure formed in corresponding  FIGS. 1A, 2A  and  4 A; and  
         [0012]      FIG. 5  is a plan view showing portions of the substrate that includes both a blanket film formed according to the prior art and a patterned film according to the invention. 
     
    
     DETAILED DESCRIPTION  
       [0013]     The present invention provides a method and structure for preventing delamination of films during subsequent thermal cycling. The invention finds application in various films that are prone to delamination such as thick films, i.e., film having a thickness greater than 1 micron. In one embodiment, the thick films may be used as sacrificial layers in semiconductor devices such as MEMS (microelectromechanical structure). In other exemplary embodiments, the films may be used in other applications in other semiconductor devices. In one particular exemplary embodiment, the films may be used in solar cells devices formed on semiconductor substrates.  
         [0014]      FIGS. 1A and 1B  respectively show corresponding cross sectional and top views of a deposited film that, according to the invention, is resistant to delamination. Film  7  is formed on lower material  1 . Lower material  1  may be a film having a thickness  3  that may range from an absolutely nominal thickness to a thickness  3  in the range of about 1.5 microns. According to the exemplary embodiment in which lower material  1  is a film, the film includes bottom surface  2  which may be formed on or over various substrates such as used in solar cell and other semiconductor device manufacturing. In another exemplary embodiment, lower material  1  may be a substrate. Lower material  1  includes upper surface  5 . Film  7  is formed over lower material  1  and includes thickness  9  of about 1.0 microns or greater although film  7  is most prone to delamination when thickness  9  is greater. In one exemplary embodiment, thickness  9  may be about 4 microns or greater but other thicknesses may be used in other exemplary embodiments. Film  7  may be a permanent, structural film or it may be a sacrificial layer such as used in an MEMS device. Film  7  includes top surface  11 . In one exemplary embodiment, film  7  may be amorphous silicon, and in one exemplary embodiment may be a-Si:H but film  7  may be formed of any of various different materials. In yet another exemplary embodiment, such as in a solar cell, each of film  7  and lower material  1  may be amorphous silicon or a-Si:H. Film  7  may be the same as lower material  1  or it may differ in various exemplary embodiments. Film  7  may be formed using various conventional formation methods such as chemical vapor deposition, CVD, which includes PECVD, plasma enhanced chemical vapor deposition and LPCVD, low pressure chemical vapor deposition. According to conventional methods, film  7  may be formed to include a compressive stress that results in a residual stress at the interface between film  7  and lower material  1 . In addition to the aforementioned deposition techniques, film  7  may be formed using other deposition techniques and the invention finds particular application in embodiments in which film  7  is formed to include a compressive film stress.  
         [0015]     After film  7  is formed, it is partitioned into a plurality of discrete portions such as shown in  FIGS. 2A and 2B , each of which illustrate discrete portions  15  separated by separating portions  13 . Conventional photolithography and etching techniques may be used to form discrete sections  15 . In the illustrated embodiment, film  7  is completely etched though exposing top surface  5  of lower material  1  but in other exemplary embodiments, film  7  need not be completely etched and separating sections  13  may include a thickness of film  7  over top surface  5  of lower material  1 .  
         [0016]      FIG. 2B  shows a plurality of squares, but it should be understood that in other exemplary embodiments, discrete portions  15  may be diamond-shaped, other rectangular shapes, parallelograms, trapezoids, or they may be round, oval or elliptical in other exemplary embodiments. In still other exemplary embodiments, discrete portions  15  may have irregular shapes. Discrete portions  15  may be the same size and shape throughout, such as shown in  FIG. 2B  or they may be differently shaped, differently sized, or both. Discrete portions  15  may be uniformly separated by a uniform spacing as in the illustrated embodiment, or they may separated by non-uniform separating portions  13 . In one embodiment, the discrete portions  15  are regularly arranged and spaced. The individual discrete portions are not individually functioning elements of a semiconductor device; rather, they include a high pattern density and essentially form a film divided into tightly packed sections. In one exemplary embodiment, the pattern density may be 80%-90% or greater. Pattern density represents the percentage of the original film layer still occupied by discrete portions  15 , as opposed to spacing portions  13 .  
         [0017]     For the illustrated embodiment in which discrete sections  15  are rectangles, the orthogonal sides  19  and  20  of each rectangle, may each include a length ranging from about 10 to about 100 microns. The area of each discrete portion  15  may be about 100 to 10000 μm 2 , but other areas may be used in other exemplary embodiments.  FIG. 2B  also points out that the discrete portions are in close proximity to one another. In the illustrated embodiment, spacing portions  13  include a spacing of about one-third of the width of each discrete portion  15 . The spacing, i.e., width of spacing portions  13 , may be as low as 1 micron in one exemplary embodiment. In various exemplary embodiments, the spacing may represent 0.1 to 0.01 of the width of the corresponding discrete portion  15 . The invention provides for generally forming a plurality of the discrete portions in close proximity and the aggregate area of the discrete portions  15  may be at least four times as great as the aggregate area of the separating portions  13  which represent exposed portions of upper surface  5  in the illustrated embodiment. Other relative dimensions of the discrete portions  15  versus separating portions  13  may be used in other exemplary embodiments. For example, if the spacing is 1 micron and the area of each discrete portion  15  is 10000 μm 2 , the area of the discrete portions will be considerably greater than the area of the separating portions for the various configurations of discrete portions that may be used. It is an aspect and advantage of the invention that a majority of film  7  remains over lower material  5  after film  7  is partitioned. Alternatively stated, film  7  still retains its structural integrity on a global basis after partitioning.  
         [0018]     After film  7  is partitioned such as shown in  FIG. 2B , the structure is resistant to delamination during subsequent thermal cycling operations. After the structure in  FIG. 2B  is formed, various thermal cycling operations may take place such as thermal operations that take place at temperatures of 400° C. or greater and for a time of 3 minutes or longer. The thermal cycling operations may be thermal treatments such as annealing or other diffusion operations that are used to condition all or portions of the substructure. Other heating operations including heating operations with considerably longer times and higher temperatures, may be used in other exemplary embodiments. In still other exemplary embodiments, the subsequent thermal operation may be the deposition of a further film over film  7  and lower material  1 .  
         [0019]      FIG. 3A  shows further film  18  formed over film  7  and including plugs  21  that fill separating sections  13  that were shown in  FIGS. 2A and 2B . In one exemplary embodiment, further film  18  may be a SiN film. Further film  18  may be the same material or a different material than film  7 . Various film deposition techniques may be used to form further film  18  and the thermal conditions may include a temperature of at least 400° C. and for a deposition time of at least 3 minutes. Higher deposition temperatures and longer processing times may be used in other exemplary embodiments. According to the exemplary embodiment in which further film  18  is SiN, the film may include a thickness of about 4500 angstroms and may be formed at a deposition temperature of about 400° C. Other film thicknesses may be used in other exemplary embodiments. An aspect of the invention is that, during the subsequent thermal cycling operation such as the deposition of further film  18 , Applicants have found that the delamination of film  7  is prevented due to the partitioning of film  7  into discrete portions  15 . According to the embodiment in which further film  18  is a SiN film, the SiN film may be a passivation layer. In other exemplary embodiments such as when film  11  and lower material  1  form a solar cell, various other films such as contact layers and electrodes may be formed over the structure.  
         [0020]     According to one exemplary embodiment such as shown in  FIGS. 4A and 4B , a planarization process may follow the formation of further film  18  to produce a planarized surface  23  formed of upper surface  11  of discrete portions  15  and the top surfaces of plug materials  21 .  FIGS. 4A and 4B  show the structure after the planarization process. In one exemplary embodiment, further film  18  and film  7  may be formed of the same material thereby producing, as illustrated in  FIGS. 4A and 4B  a continuous upper film formed of the same material and which is resistant to delamination.  
         [0021]      FIG. 5  is a top view that illustrates an example contrasting conventional techniques and the inventive technique performed on the same film formed on a substrate. The same film is formed in sections  30  and  40  of the substrate then, only in section  30 , the film is patterned according to the invention to produce discrete portions  32  separated by separating portions  34 . Blanket film  43  remains in prior art, conventional section  40 . After the film is patterned in section  30 , the entire substrate undergoes a thermal processing operation and  FIG. 5  illustrates that, in portion  40 , delamination occurs at delamination locations  45 . The delamination may include blistering, cracking or peeling of the film. The film patterned according to the invention, and which appears in section  30 , does not exhibit any delamination.  
         [0022]     The preceding merely illustrates the principles of the invention. It will thus be appreciated that those skilled in the art will be able to devise various arrangements which, although not explicitly described or shown herein, embody the principles of the invention and are included within its spirit and scope. Furthermore, all examples and conditional language recited herein are principally intended expressly to be only for pedagogical purposes and to aid the reader in understanding the principles of the invention and the concepts contributed by the inventors to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions. Moreover, all statements herein reciting principles, aspects, and embodiments of the invention, as well as specific examples thereof, are intended to encompass both structural and functional equivalents thereof. Additionally, it is intended that such equivalents include both currently known equivalents and equivalents developed in the future, i.e., any elements developed that perform the same function, regardless of structure.  
         [0023]     This description of the exemplary embodiments is intended to be read in connection with the figures of the accompanying drawing, which are to be considered part of the entire written description. In the description, relative terms such as “lower,” “upper,” “horizontal,” “vertical,” “above,” “below,” “up,” “down,” “top” and “bottom” as well as derivatives thereof (e.g., “horizontally,” “downwardly,” “upwardly,” etc.) should be construed to refer to the orientation as then described or as shown in the drawing under discussion. These relative terms are for convenience of description.  
         [0024]     Although the invention has been described in terms of exemplary embodiments, it is not limited thereto. Rather, the appended claims should be construed broadly, to include other variants and embodiments of the invention, which may be made by those skilled in the art without departing from the scope and range of equivalents of the invention.