Abstract:
A semiconductor structure that includes a first gate structure, second gate structure and a nitrogen-containing etch-stop layer. The first gate structure whose sidewalls are bounded by at least one first spacer is formed on a semiconductor substrate. The second gate structure whose sidewalls are bounded by at least one second spacer is formed on the semiconductor substrate, wherein the second gate structure is adjacent to the first gate structure. The nitrogen-containing etch-stop layer is formed over the first and second gate structures, having a thickness substantially the same over the semiconductor substrate, thereby improving a step coverage of a subsequent layer formed on the nitrogen-containing etch-stop layer between the first and second gate structures.

Description:
BACKGROUND  
       [0001]     The present invention relates generally to integrated circuits, and more particularly to an etch-stop layer that avoids the formation of the voids in semiconductor structures.  
         [0002]     As integrated circuit (IC) structures are designed to be ever smaller, they are also placed closer together. This allows the IC chips to continue to become smaller. However, density and proximity of IC structures may raise certain manufacturing issues. For example, an etch-stop layer formed on two closely adjacent gate structures may cause difficulties in a subsequent formation of an inter-level dielectric layer thereon. An etch-stop layer is one that stops the progress of the etching of an overlying layer in some local areas so that the etching in other local areas can be completed. Such etch-stop layer is usually deposited on two closely adjacent gate structures before forming a contact structure therebetween. Conventionally, the thickness of the etch-stop layer varies substantially over the surfaces of the gate structures. This often causes formation of a void between the two adjacent gate structures in a subsequent inter-level dielectric layer deposition step.  
         [0003]     Typically, the spaces between two adjacent gate structures have shapes of channels that are narrow and deep. After an etch-stop layer is formed on the gate structures, these channels do not always have simple shapes with strictly vertical, parallel walls. Since the thickness of the etch-stop layer varies, the so called overhead portions are often formed on the sides of the gate structures, thereby providing the channel with a neck portion. In a subsequent step of forming an inter-level dielectric layer, the dielectric material would first close the neck portion of the channel before it fills up its bottom portion. This may create a void in the inter-level dielectric layer at the bottom portion of the channel.  
         [0004]     This void may not be harmful if it is never again opened. However, if succeeding pattern etches intrude into the void, it is a cavity that can retain contaminants. The void can also receive metal deposition if a conductive material penetrates into it. The trapped conductive material may not be easily removed from the unfortunately shaped void space by any conventional method. Then, or later, the trapped conductive material may function as an electrical shorting line between adjacent interconnect lines.  
         [0005]     As such, desirable in the art of integrated circuit structure designs are designs and methods for avoiding the formation of voids in semiconductor structures caused by etch-stop layers of non-conformal thickness.  
       SUMMARY  
       [0006]     The present invention provides a semiconductor structure that includes a first gate structure, second gate structure and a nitrogen-containing etch-stop layer. The first gate structure whose sidewalls are bounded by at least one first spacer is formed on a semiconductor substrate. The second gate structure whose sidewalls are bounded by at least one second spacer is formed on the semiconductor substrate, wherein the second gate structure is adjacent to the first gate structure. The nitrogen-containing etch-stop layer is formed over the first and second gate structures, having a thickness substantially the same over the semiconductor substrate, thereby improving step coverage of a subsequent layer formed on the nitrogen-containing etch-stop layer between the first and second gate structures.  
         [0007]     The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]      FIG. 1  illustrates a cross-sectional view of a semiconductor structure with a non-conformal etch-stop layer formed thereon.  
         [0009]      FIG. 2  illustrates a cross-sectional view of a semiconductor structure with a conformal etch-stop layer formed thereon in accordance with one embodiment of the present invention.  
         [0010]      FIG. 3  illustrates a cross-sectional view of the semiconductor structure having an inter-level dielectric layer formed on the etch-stop layer in accordance with the embodiment of the present invention. 
     
    
     DESCRIPTION  
       [0011]      FIG. 1  illustrates a cross-sectional view of a semiconductor structure  100  with a conventional non-conformal etch-stop layer  105  deposited on a semiconductor substrate  102 . A substrate  102 , on which various structures are formed, has a surface  104 . Two adjacent gate structures  106  and  106 ′ of MOS transistors are formed on the semiconductor substrate  102 . Spacers  108  and  108 ′ are formed on the sidewalls of the gate structures  106  and  106 ′, respectively. The gate structures  106  and  106 ′, together with the spacers  108  and  108 ′, are coated with an etch-stop layer  105 . The thickness of the etch-stop layer  105  varies substantially over the vertical surfaces and corners of the gate structures  106  and  106 ′. The etch-stop layer  105  has sections  110  covering the surface  104  of the semiconductor substrate  102  and sections  112  covering the top surfaces of the gate structures  106  and  106 ′. The etch-stop layer  105  also has sections  114 , which have a severely diminished thickness covering the outside corners of the gate structures  106  and  106 ′. The etch-stop layer  105  also has sections  116 , which are of a diminished thickness covering the nearly vertical surfaces of the spacers  108  and  108 ′. The etch-stop layer  105  also has sections  118 , which are of a severely diminished thickness covering the inside corner where the sidewall spacers  108  and  108 ′ meet the surface  104 . The etch-stop layer  105  also has a section  120  covering the bottom of the narrow vertical channel  128  between the spacers  108  and  108 ′ that are on the two neighboring and proximate gate structures  106  and  106 ′. The etch-stop layer  105  has therefore assumed a slightly “mushroomed” shape over the gate structures  106  and  106 ′. As illustrated in  FIG. 1 , the etch-stop layer  105  has diminished thickness at the bases of the mushroom shapes.  
         [0012]     An interlayer dielectric (ILD) layer  107  is deposited to cover the entire etch-stop layer  105 . The ILD is planarized, typically by a technique such as chemical-mechanical-polish (CMP), to produce a flat surface  124 . However, the deposition of the ILD layer  107  in the narrow vertical channel  128 , between the spacers  108  and  108 ′ that are on the two neighboring and proximate gate structures  106  and  106 ′, is non-conformal. Not only is this vertical channel  128  narrow and deep, but, because of the neighboring mushroom shaped etch-stop layer  105 , it may also be wider at the bottom portion than at the upper portion. It is difficult to deliver enough ILD material into the bottom of the vertical channel  128  to completely fill it before the upper portion is covered by ILD material. This often causes formation of a void  126  in a narrow vertical channel  128  during the ILD deposition.  
         [0013]     Among the contaminants that may eventually and permanently be trapped in the void  126  during succeeding processing is, metal from electrical interconnections for example. A further example of metal that may penetrate into the void  126  from an interconnection process can be seen in the contact tungsten plug deposition. It is difficult to remove all metal material from the narrow vertical channel  128 . Any metal left in the channel can function as a shorting path between metal interconnect patterns. Also, thin sections of the etch-stop layer  105  can causes over-etching of its underlying material in an etching process subsequent to the formation of the ILD layer  107 . These conditions can cause serious reliability problems.  
         [0014]      FIG. 2  illustrates a cross-sectional view of a semiconductor structure  200  having an etch-stop layer  205  of conformal thickness in accordance with one embodiment of the present invention. A substrate  201 , on which various structures are formed, has a surface  203 . Two adjacent gate structures  207  and  207 ′ of MOS transistors are formed on the semiconductor substrate  201 . The gate structures  207  and  207 ′ are separated from the substrate  201  by gate dielectric layers  240  and  240 ′, respectively. In one example, the maximum thickness of the gate dielectric layers  240  and  240 ′ are about 26 angstroms. The gate structures  207  and  207 ′ have a space less than 200 nm. The gate structures  207  and  207 ′ may include metal or silicide layers  250  and  250 ′, respectively. The metal or silicide layers  250  and  250 ′ may be made of a material including refractory metals such as TiSi 2 , CoSi 2 , NiSi, Ptsi, W, WSi 2 , TiN, TiW, TaN, or combinations thereof.  
         [0015]     The gate structures  207  and  207 ′ also have sidewall spacers  209  and  209 ′, which are typically made of dielectric oxide of less than 350 angstroms. The sidewall spacers  209  and  209 ′ can be made of single or multiple layers made of materials including SiON, Si 3 N 4 , LPTEOS oxide, high temperature oxide (HTO), furnace oxide, Hf containing oxide, Ta containing oxide, Al containing oxide, high K dielectric (e.g. K&gt;5), oxygen content dielectric, nitrogen content dielectric, or a combination thereof. The gate structures  207  and  207 ′, together with the sidewall spacers  209  and  209 ′, are coated with a conformal etch-stop layer  205 , typically a Si 3 N 4  or silicon oxynitride (SiON) layer, which is formed using a plasma-free low pressure chemical vapor deposition (LPCVD) process at a temperature below about 520 degrees Celsius. This process provides the etch-stop layer  205  with a thickness substantially the same over the gate structures  207  and  207 ′ and over the substrate  201 . Other nitrogen containing etch-stop layers may be formed in other exemplary embodiments. Various high dielectric constant (K) dielectric layers may be formed in still other exemplary embodiments. The conformal etch-stop layer  205  formed by the LPCVD process has tensile stress no less than 1.1 Gpa. Since the deposition of the conformal layer is a non-plasma-enhanced process, plasma damage to the semiconductor structure  200  can be avoided.  
         [0016]     To illustrate, the conformal etch-stop layer  205  is divided into various sections. Sections  202  of the conformal layer cover the top surfaces of the gate structures  207  and  207 ′. Sections  204  of the conformal etch-stop layer  205  cover the nearly vertical surfaces of the sidewall spacers  209  and  209 ′. A section  206  of the conformal etch-stop layer  205  covers the narrow portion of the surface  203  that lies between the sidewall spacers  209  and  209 ′ of the two neighboring and proximate gate structures  207  and  207 ′. Sections  208  of the conformal etch-stop layer  205  cover the broad exposed areas of the surface  203 . In one exemplary embodiment, a plurality of layers may be formed, as above, to form the conformal etch-stop layer  205 . A narrow vertical channel  210  is defined between the portions of the conformal etch-stop layers  205  on two adjacent sidewall spacers  209  and  209 ′ of the two neighboring and proximate gate structures  207  and  207 ′. Since the thickness of the etch-stop layer  205  is substantially the same over the gate structures  207  and  207 ′, the vertical channel  210  is narrower at the bottom portion than at upper portion. Thus, no mushroom shapes are produced. This avoids the formation of a void at the vertical channel  210  during a subsequent inter-level dielectric layer deposition process.  
         [0017]      FIG. 3  illustrates a cross-sectional view of the semiconductor structure  300  having an ILD layer  302  formed on the etch-stop layer  205  in accordance with one embodiment of the present invention. An ILD layer  302  is deposited to cover all exposed IC structural surfaces. The ILD layer  302  is planarized, typically by a technique such as CMP, to produce a flat surface  304 . The step coverage of the ILD layer  302  deposited at the vertical channel  210  is improved. In this embodiment, the step coverage of the ILD layer  302  at the vertical channel  210  is about 90%. Since the vertical channel  210  has a wider upper portion and a narrower bottom portion, the formation of a void in the ILD layer  302  at the vertical channel  210  can be avoided. Thus, the formation of tungsten-stringer, or any other metal short is also eliminated. With less hazard of producing voids and metal shorts, transistors and other components can be placed more closely together and greater integrated circuit density can be achieved. The size of memory cells can, therefore, be reduced.  
         [0018]     After the formation of the ILD layer  302 , a series steps of lithography, etching and deposition can be performed to form a contact structure at the channel  210  between two neighboring gate structures  207  and  207 ′. Because the conformal etch-stop layer  205  has uniform thickness, the etch results are more uniform. This means that a thinner etch-stop layer can be used. In one embodiment, the thickness of the etch-stop layer  205  can be less than 600 angstroms. This also means that there is a greater etch margin by using the conformal etch-stop layer  205 .  
         [0019]     This method and conformal etch-stop layer can be applied to dynamic random access memory (DRAM), static random access memory (SRAM), flash, non-volatile memory cells, and volatile memory cells. For example, the conformal etch-stop layer can be used in forming contacts in an SRAM cell that has a ratio of a longitudinal pitch and a transverse pitch in a range of about 1.7 to 8.  
         [0020]     The above illustration provides many different embodiments or embodiments for implementing different features of the invention. Specific embodiments of components and processes are described to help clarify the invention. These are, of course, merely embodiments and are not intended to limit the invention from that described in the claims.  
         [0021]     Although the invention is illustrated and described herein as embodied in one or more specific examples, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims. Accordingly, it is appropriate that the appended claims be construed broadly and in a manner consistent with the scope of the invention, as set forth in the following claims.