Gate structures with increased etch margin for self-aligned contact and the method of forming the same

A novel gate structure and a method of forming the same for a self-aligned contact on a semiconductor substrate. The method includes forming a gate oxide layer on the semiconductor substrate. Then a first conductive layer is formed on the gate oxide layer. Next, a second conductive layer, preferably a refractory metal silicide (e.g. WSix), is formed overlying the first conductive layer. A capping layer is formed overlying the second conductive layer. Then the capping layer is etched to form a patterned capping layer having a lower outside corner. An upper portion of the second conductive layer is selectively dry etched laterally to form a lateral recess under the capping layer to increase etch margin. A lower portion of the second conductive layer is then etched anisotropically down to the first conductive layer along a sidewall approximately vertically aligned with the lower outside corner of the patterned capping layer. The recess has an inward extant from the lower outside corner in a range of between 100-300 Angstroms. Increased etch margin is provided in the gate structure to prevent shorts between contact plugs and gate structures during contact formation.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to a method of manufacturing semiconductor devices and, more particularly, to contact formation.

2. Description of the Related Art

In conventional semiconductor manufacturing, after source/drain regions are formed along a side of gate structures, an interlevel insulating layer is deposited over the resultant structure. Contact holes are then formed in the interlevel insulating layer down to the source/drain regions in order to electrically connect with the source/drain regions. Subsequently, metal is deposited to fill the contact holes for interconnection. During formation, shorts between the contact fillings and the gate structures must be prevented to avoid device failures.

In recent years, aligning contact holes between closely spaced interconnects has become a dominant impediment to ever increasing density of ULSI (ultra large scale integration) circuits. Furthermore, as modern contact lithography technology approaches its limit, forming small contact holes within alignment tolerances has become quite difficult.

To alleviate these problems, SAC (self-aligned contact) technology has been developed. U.S. Pat. No. 5,831,899 assigned to Integrated Device Technology, Inc., the disclosure of which is incorporated herein by reference, illustrates such SAC technology.

SAC technology uses a selective etching process to form contact holes. SAC technology utilizes the fact that various materials used for semiconductor fabrication, such as silicon nitride, silicon dioxide, polysilicon and photoresist each have different etch removal rates during etching. An example of conventional SAC structure is shown in- FIG. 1 A. The material used for spacers 12 and/or a capping layer 14 , e.g., silicon nitride, has a slower etch rate than the material used for an interlevel insulating layer 16 , e.g., silicon oxide. As a result, the contact holes 18 can be defined by using an etchant that etches away predetermined regions of the interlevel insulating layer between the gate structures at a faster rate than it etches the spacers 12 and/or the capping layers 14 . SAC technology thus generally enables the accurate formation of the contact holes even when the photoresist pattern defining the contact holes is not precisely aligned. During formation, the remaining amount of shoulder (shoulder remaining) or etch margin indicated as x and y in FIG. 1A , prevents shorts from occurring between contact plugs 20 and gate structures 22 .

Prior SAC technology is not flawless. For example, accidental shorts can still occur between gate structures and contact plugs. Due to the limits of current selective etching processes, some amount of unwanted etching can occur in the spacers or the capping layers. When this happens, portions of the spacers or the capping layer encapsulating the gate structures are undesirably etched away. As a result, the gate structure becomes exposed to the contact plug, causing shorts between the contact plugs and the gate structures. Thus, as can be inferred from FIG. 1A , securing a sufficient shoulder remaining is critical to preventing inadvertent shorts between adjacent gate structures and contact plugs.

With the prior art gate structures and the method of manufacturing the same, however, it has not been easy to fabricate a self-aligned contact with a sufficient shoulder remaining or etch margin without using complicated and costly process steps. This is especially true today when design rules permit features sizes below 0.18 micrometers ( m).

The invention disclosed in U.S. Pat. No. 5,989,987 attempts to improve the basic SAC process by chemically etching the silicide layer using a wet etchant that is selective against a silicide to constrict the silicide layer 13 laterally. The resulting structure is shown in FIG. 1 B. However, using the wet etching approach of U.S. Pat. No. 5,989,987 is subject to many process control variables and contamination, requiring a long etching time as well as subsequent cleaning steps.

Accordingly, a need remains for improved SAC technology and gate structures that provide a sufficient shoulder remaining or etch margin but use fewer and simplified process steps.

SUMMARY OF THE INVENTION

The present invention includes a novel gate structure that has an increased etch margin.

Further, the present invention provides an improved method of manufacturing a self-aligned contact with simpler and less costly process steps compared to the conventional contact processing.

In accordance with one aspect of the present invention, a method is provided for fabricating a semiconductor device on a semiconductor substrate. The method includes forming a gate oxide layer on the semiconductor substrate. Then a first conductive layer is formed overlying the gate oxide layer. Next, a second conductive layer, preferably a refractory metal silicide (e.g. WSi x ), is formed overlying the first conductive layer. A capping layer is formed overlying the second conductive layer. Then the capping layer is etched to form a patterned capping layer having a lower outside corner. An upper portion of the second conductive layer is selectively dry etched laterally to form a lateral recess under the capping layer to increase etch margin. A lower portion of the second conductive layer is etched anisotropically down to the first conductive layer along sidewall approximately vertically aligned with the corner.

In the present invention, a lateral recess for increasing etch margin can be formed in a portion of the second conductive layer while the gate structure is constructed. Preferably, the lateral recess is formed by dry etching, thus avoiding potentially expensive and time-consuming wet etching processes, the latter of which causes more contamination and requires additional process steps. Therefore, the present invention is simple and requires fewer process steps taking shorter process time compared to conventional processes. Manufacturing cost is reduced and yield is increased.

DETAILED DESCRIPTION

FIGS. 2-11 illustrate the preferred embodiments of the present invention to increase etch margin during contact formation. This invention may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. In the drawings, it should be understood that when a layer is referred to as being on another layer or substrate, it can be directly on the other layer or substrate, or intervening layers can also be present. Well-known process steps such as forming a gate oxide layer have not been described in detail in order not to obscure the present invention.

As shown in FIG. 2 , a gate oxide layer 32 with a thickness of 25-80 Angstroms is formed on a semiconductor substrate 31 such as a silicon substrate. A first conductive layer 34 is then formed with a thickness of 500-2000 Angstroms on the gate oxide layer 32 . The first conductive layer 34 is preferably a doped polysilicon layer. Next, a second conductive layer 36 with a thickness of 400-1500 Angstroms is formed on the first conductive layer 34 . The second conductive layer 36 is preferably composed of a refractory metal silicide, e.g., WSi x . The second conductive layer 36 preferably comprises an upper portion or selected portion 36 A and a lower portion or unselected portion 36 B. Next, a capping layer 38 with a thickness of 1000-2500 Angstroms is deposited on the second conductive layer 36 . Preferably, the capping layer 38 is formed of SiON, a nitride, an oxide or combinations thereof.

FIG. 3 shows that the capping layer 38 is etched using a photoresist pattern 40 , (See FIG. 2 ), to form a patterned capping layer 38 . Patterned capping layer 38 serves as a hardmask to define a gate structure to be formed. The patterned capping layer 38 has a lower outside corner 44 .

Referring to FIG. 4A , the upper portion 36 A of the second conductive layer 36 is then removed, as further described below, to form a lateral recess 46 therein. Preferably, the recess 46 has an inward extent E from the lower outside corner 44 of patterned capping layer 38 in a range of between 100-300 Angstroms.

According to one embodiment of the present invention, the lateral recess 46 is formed beneath the lower outside corner 44 and extends only between the lower outside corner 44 and the lower portion 36 B of the second conductive layer 36 , indicated as height A in FIG. 4 A.

Preferably, the recess 46 , in cross section, has predefined contours such as rectangular or triangular in shape, as illustrated in FIGS. 4A and 4B , respectively.

The upper portion of the second conductive layer could be removed using wet etching to form a recess therein. However, wet etching has many disadvantages such as higher cost of etchants and DI water, much longer process time, and unnecessary rinse and dry steps. Also, attempts to form a recess using wet etching during definition of a gate structure highly complicates the gate formation process. In addition, potential contamination resulting from wet etching can significantly increase the incidence of defects.

For these reasons, removing the upper portion 36 A of the second conductive layer 36 to form recess 46 therein is preferably performed by dry etching such as plasma etching. Thus, the present invention using dry etching is simple with fewer process steps, less costly and thus reduces device failure probability. Example etch chemistries for producing recess 46 construction includes SF 6 or CF 4 , HBr(or N 2 , O 2 , or HeO 2 ), and Ar. Example power is 50-500 watts, with a pressure of 2-200 mTorr. More preferably, plasma etching is performed using SF 6 or CF 4 flow of approximately 40 standard cubic centimeters per minute (sccm), at a pressure of between approximately 150 and 200 mTorr, and at approximately 50-100 Watts.

As shown in FIGS. 5A and 5B , following the formation of lateral recess 46 , the lower portion 36 B of the second conductive layer 36 is etched to form a sidewall 47 therein. The sidewall 47 is vertically aligned with the lower outside corner 44 of the patterned capping layer. Preferably, etching the lower portion 36 B of the second conductive layer 36 is done dry etching. More preferably, dry etching is performed using SF 6 flow of approximately 30 sccm and HBr flow of approximately 30 sccm, at a pressure of approximately 4-30 mTorr, and at approximately 250-350 Watts.

According to one embodiment of the present invention, the process of etching the upper portion of the second conductive layer can be performed in situ with the process of etching the capping layer. Alternatively, the process of etching the upper portion of the second conductive layer is performed in situ with the process of etching the first conductive layer. Also, the process of etching the upper portion of the second conductive layer can be performed in situ with the process of etching the lower portion of the second conductive layer 36 .

Referring to FIGS. 6A and 6B , the first conductive layer 34 is then etched to form one or more gate structures 49 with separate conductive areas 34 of the first conductive layer atop gate oxide layer 32 . The lower outside corner 53 of the patterned second conductive layer can be contiguous with an upper outside corner 48 of the patterned first conductive layer 34 .

As shown in FIG. 7 , sidewall spacers 50 are formed along opposite sides of the gate structure 49 . In detail, an insulative film, formed of a material such as silicon nitride, is deposited and etched back to form the sidewall spacers 50 . The sidewall spacers 50 substantially fill the recess 46 for electrical isolation between gate structures 49 and contact plugs to be formed. Further, the sidewall spacers 50 are used to protect the gate structure 49 from the etchants used to expose contact regions (not shown).

Referring to FIG. 8 , a liner 54 can be formed overlying the gate structure 49 including spacers 50 prior to depositing an interlevel dielectric layer. Preferably, the liner 54 is formed of a material selected from the group consisting of nitride, SiON, oxide and combinations thereof.

Next, as shown in FIG. 9 , an interlevel dielectric layer 56 is deposited over liner 54 overlying the gate structure 49 . Particularly, an insulating layer, preferably formed of a material such as a BPTEOS film, is deposited and planarized using conventional techniques, such as chemical mechanical polishing (CMP). Layer 56 has a thickness Z of 100-400 Angstroms over the gate structure 49 . Then, a photoresist pattern 58 with an opening 59 for forming a contact hole is formed by photolithography. In detail, a photoresist. layer is deposited and patterned to form photoresist pattern 58 exposing the dielectric layer 56 to define a contact hole to be formed.

Referring to FIG. 10 , the underlying interlevel insulating layer 56 is selectively etched using the photoresist pattern 58 with opening 59 to form a contact hole 60 in the interlevel insulating layer 56 . Preferably, a dry etch process is performed to etch the contact hole 60 down to the silicon substrate 31 . Etchants used in this process must be selective to nitrides and silicon.

Referring to FIG. 11 , following the formation of contact hole 60 , metallization steps such as the formation of a contact plug 62 and conductive metal line 64 overlying contact plugs are performed using conventional techniques. FIG. 11 illustrates the contact plug 62 formed as the result of depositing conductive material in the contact hole 60 .

In accordance with the present invention, the thickness or distance of shoulder remaining or etch margin is substantially increased from x and y ( FIG. 1 ) to x and y , where x >x and y >y. Such an increase in etch margin, for example, in accordance with the present invention, can be approximately 100 300 Angstroms in the lateral direction. Also, in the present invention, a lateral recess to increase etch margin is formed in a portion of the second conductive layer while the gate structure is constructed without additional steps being required to form the recess after the gate structures themselves are formed. In addition, the lateral recess of the present invention to increase etch margin IS formed by dry etching during gate formation, thus avoiding additional, more expensive and time-consuming wet etching and thus preventing contamination.

Having described and illustrated the principles of the invention in a preferred embodiment thereof, it should be apparent that the invention can be modified in arrangement and detail without departing from such principles. We claim all modifications and variation coming within the spirit and scope of the following claims.