Abstract:
A trench for isolating active devices on a semiconductor substrate, formed by creating a trench which has a peripheral edge, and disposing an isolating material in the trench. The isolating material extends over the peripheral edge of the trench, thereby covering at least a portion of the substrate surrounding the trench, and substantially limiting leakage of the active devices disposed on the substrate.

Description:
This is a continuation of application Ser. No. 07,988,613 filed on Dec. 10, 1992, now U.S. Pat. No. 5,433,794. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to semiconductor manufacturing, and more particularly to the formation of trenches useful in isolating active areas on a semiconductor substrate. 
     BACKGROUND OF THE INVENTION 
     As device dimensions get smaller, and device density increases, it becomes more and more difficult to build an efficient and reliable isolation process to separate active devices. The limits of the standard LOCOS process have motivated the search for and the development of new isolation schemes, and trench isolation is a promising candidate as it uses a fully recessed oxide, has no birds&#39;s beaks, is fully planar, and does not suffer from the field oxide thinning effect. 
     However, the trench isolation process still suffers from problems such as &#34;corners&#34; effects at the trench edge that can increase device leakage current, especially when the trench is recessed. This problem has been mentioned in a scientific paper entitled, &#34;A New Three-Dimensional MOSFET Gate-Induced Drain Leakage Effect in Narrow Deep Submicron Devices,&#34; by Geissler, Porth, Lasky, Johnson and Voldman, and published in the 1991 IEEE IEDM Technical Digest in which it is described as a key problem of isolation trench technology. The present invention substantially eliminates this problem by creating a smooth trench profile with a self-aligned cap or dome. 
     SUMMARY OF THE INVENTION 
     The process of the present invention solves the corner effects problem found with trench isolated structures and smoothes the wafers&#39;surface by adding spacer forming steps to the process. 
     The process of the present invention is begun with the standard trench forming steps, but further forms spacers around the periphery of the trench. These spacers then combine with the isolation material disposed in the trench to form a dome-like structure over the trench. The cover extends over the peripheral edges of the trench thereby limiting the corners effect. The problem of the recessed regions in the trench is substantially avoided. Hence, it is possible to avoid the forming of sharp corners by creating a small rounded cap over the trench. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be better understood from reading the following description of nonlimitative embodiments, with reference to the attached drawings, wherein below: 
     FIG. 1 is a schematic cross-section of a semiconductor substrate having a filled trench, illustrating the &#34;corners&#34; effect at the trench edge; 
     FIG. 2 is a schematic cross-section of a semiconductor substrate having a filled trench, illustrating a smooth trench profile and a self-aligned cap according to the present invention; 
     FIG. 3 is a schematic cross-section of a trench in a semiconductor substrate on which has been grown or deposited a pad oxide layer and a mask layer according to the present invention; 
     FIG. 4 is a schematic cross-section of the substrate of FIG. 3, after a thermal oxide has been grown in the trench and a blanket layer of isolation material has been deposited, according to the present invention; 
     FIG. 5 is a schematic cross-section of the substrate of FIG. 4, in which the isolating material has been removed to expose the masking layer, but remains in the trench, according to the present invention; 
     FIG. 6 is a schematic cross-section of the substrate of FIG. 5, in which the masking layer is removed, according to the present invention; 
     FIG. 7 is a schematic cross-section of the substrate of FIG. 6, on which has been deposited an insulating layer, according to the present invention; 
     FIG. 8 is a schematic cross-section of the substrate of FIG. 7, illustrating remnants of the insulating layer which protect the edges of the trench, according to the present invention; and 
     FIG. 9 is a schematic cross-section of the substrate of FIG. 8, after the remnants of the insulating layer have been partially combined with the isolating material, according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 illustrates one of the problems currently confronting semiconductor manufacturers in the formation of isolation trenches T. When the isolation material 4&#39; is etched, an uneven profile frequently results wherein little or no isolating material 4&#39; remains at the corners of the trench T. The exposed corners are potential points of current leakage between regions of the active areas (not shown), thus the phrase &#34;corners&#34; effect. 
     Applicants&#39; solution to the problem of the &#34;corners&#34; effect is illustrated by the isolation trench T of FIG. 2. The isolation trench T of the present invention has a smooth profile. The material 4 which is disposed in the trench T extends over the peripheral edges of the trench T, thereby covering the corners, and minimizing the problem of leakage between regions of the active areas, such as transistors and source/drain regions of transistors. 
     The isolation trench T of the present invention is made by the process described below. Referring to FIG. 3, a semiconductor substrate 1 is illustrated in which a trench T has been formed. The trench T is between approximately 0.2μ and 0.8μ in depth, the preferred depth being 0.4μ. Hence, the term &#34;shallow&#34; trench isolation. 
     The trench T has sidewalls which can be substantially vertical to the substrate, or slightly sloped, i.e., having an angle of approximately 90° to 70°. If the trench sidewalls are sloped too much, it becomes difficult to etch a trench T to the desired depth. 
     On the surface of the substrate are disposed layers 2 and 3. In the preferred embodiment, insulating layer 2 is an oxide, frequently called a pad oxide. However, any suitable insulating material known in the art could also be used. 
     Layer 3 is preferably a nitride layer which has been deposited by any of the methods well known in the art. Although a nitride mask layer has insulative properties, a conductive layer can also be used to form the mask layer 3. For example, a polysilicon mask or a polysilicon/nitride mask could also be used to form layer 3. The active areas of the semiconductor wafer are patterned and etched substantially concurrent with the trench T formation process. These active areas are frequently the location of transistors or other components of integrated circuits. 
     A thermal oxide 2A can be grown or deposited in the trench T, thereby lining the trench T and providing greater resistance to cracking, and for passivating the trench sidewalls. The trench T is then filled with a material 4 which functions as an insulator, thereby providing isolation of the various electrical devices which may be disposed on the substrate 1o The isolation material 4 is frequently a thick layer of oxide, such as (tetraethyl orthosilicate) TEOS, CVD-oxide, BPSG, nitride, or a combination thereof, or a similar insulating material, which is preferably deposited. Other materials have also been found to be effective for use in isolation trenches T, such as, for example, a combination of nitride and oxide, or polysilicon which has been oxidized. 
     The isolation material 4 fills the trench T and covers the surface of the substrate 1 having layers 2 and 3 disposed thereon. 
     FIG. 5 illustrates the present invention after a planarization step has been performed. The planarization can be accomplished by any suitable method, such as chemical-mechanical polishing (CMP), or a dry etch, or a combination thereof. When the overfill of the isolation material 4 is planarized, a portion of the isolation material 4 remains which extends beyond the surface of the substrate 1. 
     The structure of FIG. 6 results after layer 3 is removed, preferably via a wet etch. However, other suitable methods can also be used. The portion of the isolation material 4 which protrudes above the substrate 1 surface becomes more evident at this stage. The protruding portion of the isolation material 4 has substantially vertical sides. 
     If a pad oxide etch is performed at this point, the uneven profile, characterized by recessed isolating material 4&#39;, of the structure in FIG. 1 often results. The structure of FIG. 1 is prone to reliability defects and defective endproducts due to the inverse narrow width effect. Consequently, leakage along the isolation trench T&#39; at the corners results. 
     However, in the process of the present invention, another insulating material 5 is deposited. The insulating material 5 is, preferably a material having similar chemical properties to isolating material 4. Therefore, material 5 is likewise an oxide, such as, TEOS or nitride with TEOS or (borophosphosilicate glass) BPSG. The preferred embodiment employs CVD-oxide. Other suitable deposited dielectrics can be used, as well as polysilicon which is subsequently oxidized. Borophosphosilicate glass (BPSG) is another possibility for use as the insulating material 5. The insulating material 5 usually conforms to the profile of the structure, as shown in FIG. 7. The conformal properties of the insulating material 5 enable the self-alignment of the spacers 5. 
     The structure of FIG. 8 results after a partial spacer etch is performed. The partial spacer etch is preferred because such a dry etch tends not to damage the active area regions which may also be disposed on the silicon 1. The spacers 5 formed from the conformal layer 5 have similar chemical properties to the isolation material 4. In the preferred embodiment, the conformal material 5 is the same as the isolating material 4. The spacers 5 are located on the surface of the substrate 1 at the corners of the trench T, and are self-aligned to the trenches T. 
     Preferably, a wet pad oxide etch is then performed which results in the structure of FIG. 9. The isolating material 4 combines with the spacers 5 to form an isolation trench T having a dome or cap-like covering 4a, and a smooth profile. The dome 4a extends over the peripheral edges of the trench T, thereby substantially overcoming the &#34;corners&#34; effect, and consequent leakage between active areas on the substrate 1. 
     In FIG. 2 a gate oxide layer 2B and a gate layer 6 have been grown. The gate oxide 2B can also be deposited. At this stage, gates or other microminiature structures can be fabricated on the substrate 1. 
     Attention is called to U.S. Pat. No. 4,729,006, entitled &#34;Sidewall Spacers for CMOS Circuit Stress Relief/Isolation and Method for Making,&#34; to Daily, et al. for further details of the trench fabrication process. 
     All of the U.S. Patents cited herein are hereby incorporated by reference herein as if set forth in their entirety. 
     While the particular process as herein shown and disclosed in detail is fully capable of obtaining the objects and advantages herein before stated, it is to be understood that it is merely illustrative of the presently preferred embodiments of the invention and that no limitations are intended to the details of construction or design herein shown other than as described in the appended claims. For example, one having ordinary skill in the art will realize that the trench of the present invention and the process of making the same can be used to isolate a variety of active microminiature structures, such as, for example field emitters.