Abstract:
A planar diode and method of making the same employing only one mask. The diode is formed by coating a substrate with an oxide, removing a central portion of the oxide to define a window through which dopants are diffused. The substrate is given a Ni/Au plating to provide ohmic contact surfaces, and the oxide on the periphery of the window is coated with a polyimide passivating agent overlying the P/N junction.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The present invention relates to a wafer-level process for fabricating semiconductor planar devices such as diodes and, more particularly, concerns a simplified wafer level process for fabricating planar diodes from semiconductor.  
         [0002]     Diodes are mass produced in great numbers every year. As a high volume component found in virtually every electrical device of any complexity, the market for diodes is very large, competitive, and sensitive to pricing pressure. Manufacturers must produce these devices both with sufficient reliability and low cost to satisfy the competitive demands of the marketplace. Currently, one known process for manufacturing diodes from semiconductor chips entails the use of several photolithographic steps employing a mask, each of which increases the cost of manufacture.  
         [0003]     A specific example of one such known process is a standard glass passivated pallet process employing three masking steps, and is illustrated in the process flow diagram of  FIG. 1 . Beginning in step  1  with a wafer of silicon  10 , the wafer is doped in step  2  so as to provide P+, N, and N+ regions (respectively indicated as  12 ,  14 , and  16  in  FIG. 1 ). Prior to the first photolithographic step  3 , a protective oxide layer is provided in the form of an oxide coating  18 . After baking, developing, and hard baking, a first mask is used to generate the structure shown in the figure after step  3 , in which windows  19  are opened up in the oxide layer  18 . In step  4  these windows are etched to form grids in the silicon wafer, defining the intended boundaries of each diode.  
         [0004]     In step  5 , a layer  20  of polynitride is deposited to prepare the surface for step  6 , the second photolithographic step in which a mask is employed. Here, a layer of glass powder  22  is deposited along the grid as indicated, baked under high pressure and fired (step  7 ).  
         [0005]     In step  8 , a low temperature oxide chemical vapor deposition process is used to overlay an oxide (silicon dioxide) on the surface to protect the glass for step  9 , the third photolithographic step in which a mask is employed (here to deposit a polymer coating).  
         [0006]     Contact etching and photoresist etching is performed in step  10 , exposing the P+ and N+ surfaces, which are then coated with nickel plating in step  11  to provide ohmic contacts.  
         [0007]     This known process requires three photolithographic steps in which a precision mask is employed. The repeated use of masks and the care with which they must be used is a substantial component in the cost of the finished product produced by this process.  
         [0008]     Demand persists for diodes that can be manufactured using simpler and cheaper processes  
       SUMMARY OF THE INVENTION  
       [0009]     The present invention includes embodiments that provide a planar diode and methods for its manufacture. The device comprises: (a) a substrate of a first conductivity type (preferably an N-type conductivity silicon substrate) and a doped, centrally located region of a second conductivity type (preferably a P+-type region) defining a P/N junction therebetween; (b) a nickel plating on the underside of the substrate and along the doped region of the substrate which corresponds to the second conductivity type; (c) an oxide coating on the peripheral portion of the top side of the substrate adjacent the doped central portion; and (d) a coating of a passivating material such as polyimide on top of the oxide on the top side of the substrate, the passivating material extending partially over the P/N junction. Boron may be employed as a dopant in this invention.  
         [0010]     According to another embodiment of the invention a method of forming a planar diode is provided. The method comprises: (a) providing a substrate of a first conductivity type; (b) depositing a layer of oxide over the substrate; (c) using a mask to expose the central portion of the oxide layer for etching; (d) removing the central portion of the oxide via etching; (e) forming a P/N junction in the substrate via window diffusion; (f) plating nickel onto the window and on the opposite side of the substrate; and (g) coating the remaining oxide and a portion of the plating on a side of the substrate with a passivating agent, such as a polyimide  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]     The present invention will become more readily apparent from the following detailed description taken in conjunction with the accompanying drawings in which:  
         [0012]      FIG. 1  is a schematic view of a known process for manufacturing planar diodes.  
         [0013]      FIG. 2  is a schematic view of a first embodiment of a process for the manufacture of diodes according to the principles of the invention.  
         [0014]      FIG. 3  is a schematic view of a second embodiment of a process for the manufacture of diodes according to the principles of the invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0015]     The present invention is directed both to a process for manufacturing planar diodes and the diodes so manufactured. One advantage associated with the present invention is that it requires the use of only one mask in contrast to other approaches which require more, thereby resulting in a more economical and reliable process for manufacturing planar diodes. Not only does the use of a single mask simplify the process, but it requires less equipment as well, as only a single conventional photolithographic aligner is required by this process. According to one aspect of the invention, a coating is provided of passivating material such as polyimide in order to enhance the resistance of the device to mechanical and environment stresses and moisture. This results in a more economical and reliable process for manufacturing planar diodes.  
         [0016]      FIG. 2  illustrates a process for such manufacture in accordance with an embodiment of the present invention. The process begins at step  101  with a wafer  100 , typically made of silicone (although the process can be employed with other semiconductive materials).  
         [0017]     In step  102 , the upper surface is oxidized in a known manner to provide an oxide layer  110  of silicon dioxide. (The lower surface may optionally be oxidized as well.)  
         [0018]     Next, in step  103 , a photoresist  120  is developed for contact etching. This is the only step in the process in which a mask is employed.  
         [0019]     In step  104 , contact etching exposes a window  112  in the oxide layer. Through this layer, a P/N junction is formed in step  105  via window diffusion, thus creating P+, N, and N+ type regions  114 ,  116 , and  118  respectively, as is known in the art. Alternatively, different impurities can be employed to create a N/P junction comprising N+, P, and P+ type regions.  
         [0020]     Ohmic contacts  132  and  134  are provided in step  106  via nickel plating, as is known in the art. The oxide  110  acts as a mask for the deposition of the metal contact  132  and is self-aligned to the P/N junction without need of an additional mask for metallization along the window  112 .  
         [0021]     By the end of step  106 , a functional diode has been produced. However, in order to passivate the surface and thus provide a more reliable and durable device, a polyimide coating  140  is added in step  107  via the screen printing method (which is less expensive than the use of a mask). The polyimide coating serves to protect the device, and in particular, the P/N junction, against contamination and moisture. Optionally, and to further protect the nickel surface against corrosion, a gold plating may be applied to the exposed nickel surfaces.  
         [0022]     By reducing the number of masks that are employed to one, the resulting planar diodes are cheaper to manufacture.  
         [0023]     According to one example, the dimensions of the diode may be as follows:  
                                                                 Layer   Thickness (approx., and in microns)                                        Oxide   2.0           photoresist   5.0           P+   50.0           N+   100.0           Polyimide   10.0           Nickel/Gold   2.0                      
 
         [0024]      FIG. 3  illustrates an alternative embodiment of the invention, in which steps  201 - 206  are identical to steps  101 - 106 , both in process and in resulting intermediate structures. This process begins at  201  with a wafer. In step  202 , the upper surface is oxidized in a known manner to provide an oxidized layer. In step  203 , a photoresist  120  is developed for contact etching. This is the only step in the process in which a mask is employed. In step  204 , contact etching exposes a window in the oxide layer. Through this layer, a P/N junction is formed in step  205  via window diffusion, thus creating P+, N, and N+ type regions. Ohmic contacts are provided in step  206  via nickel plating.  
         [0025]     The process diverges from that shown in  FIG. 2  in step  207 , in which the remaining oxide layer  110  is stripped off. Removing the layer of oxide at this point has the advantage of enabling the removal of contaminants that my have infiltrated between the oxide and the silicon, thereby providing a cleaner P/N junction. Then, in step  208 , a passivating layer of polyimide  240  is applied directly to the exposed silicon, overlapping some with the upper contact  132 .  
         [0026]     Although various embodiments are specifically illustrated and described herein, it will be appreciated that modifications and variations of the present invention are covered by the above teachings and are within the purview of the appended claims without departing from the spirit and intended scope of the invention. For example, this approach could be applied to the manufacture of different kinds of semiconductor devices such as transient voltage suppressors, thyristors, and transistors.