Semiconductor device having saddle fin transistor and method for fabricating the same

A method for fabricating a semiconductor device includes forming a pad nitride layer that exposes an isolation region over a cell region of a semiconductor substrate; forming a trench in the isolation region of the semiconductor substrate; forming an isolation layer within the trench; etching an active region of the semiconductor substrate by a certain depth to form a recessed isolation region; etching the isolation layer by a certain depth to form a recessed isolation region; depositing a gate metal layer in the recessed active region and the recessed isolation region to form a gate of a cell transistor; forming an insulation layer over an upper portion of the gate; removing the pad nitride layer to expose a region of the semiconductor substrate to be formed with a contact plug; and depositing a conductive layer in the region of the semiconductor substrate to form a contact plug.

CROSS-REFERENCE TO RELATED APPLICATIONS

Priority to Korean patent application number 10-2008-0134817, filed on Dec. 26, 2008, the disclosure of which is incorporated by reference in its entirety, is claimed.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to a semiconductor device and a method for fabricating the same, and more particularly, to a semiconductor device capable of preventing contact defect and overcoming a limit in a process margin by employing a damascene process and a saddle fin transistor structure, and a method for fabricating the same.

2. Brief Description of Related Technology

Recently, as a design rule of a semiconductor device is sharply reduced to sub-40 nm technology, an area of an active region in which the device can be formed is also reduced. Accordingly, various limitations in a process margin are present. Particularly, abnormal shape of a gate line, i.e., leaning of the gate line or roughness of the gate line, is a serious problem. Also, in a landing plug contact process, contact defects are present in a great level due to the influence of the abnormal shape of the gate line and limitation in space.

With employment of a recently developed saddle fin transistor, an off-current margin property of a cell transistor is improved and thus the limitation in device properties can be overcome to some level. The saddle fin transistor can ensure a stable refresh property in a recess gate structure and, at the same time, enhance a cell driving current property by forming a bottom face of the recess gate in a fin transistor structure to enlarge a channel width. In the saddle fin transistor, an isolation layer formed in the semiconductor substrate is etched to protrude an active region, so that both side faces and an upper face of the active region are exposed. A gate is then formed so as to cover the exposed portion of the active region. Consequently, a channel is formed in the exposed three faces of the active region, and the driving current property through the channel thus can be enhanced.

However, patterning for a linear type recess structure and increase in the leaning of the gate line and the roughness of the gate line cause an increase in the contact defect in a self aligned contact (SAC) process. Also, due to reduction in an area to be formed with a device together with such contact defect, it is actually impossible to ensure a margin of SAC process. Therefore, a novel process capable of overcoming the contact defect and the limitation in the process margin is necessary.

SUMMARY OF THE INVENTION

Disclosed herein is a semiconductor device capable of overcoming SAC defect and limitation in a process margin resulting from reduction in a design rule. The semiconductor device includes a landing plug contact formed without using a SAC process, and a method for fabricating the same.

In one embodiment, a semiconductor device includes a trench having a selected depth in an active region of a semiconductor substrate; an isolation layer surrounding the active region and having a recessed isolation region that forms a saddle type fin structure in the active region; a saddle fin gate of a cell transistor buried in the trench and the recessed isolation region; an insulation layer over saddle fin gate, the insulation layer exposing a region of the semiconductor substrate; and a contact plug in the region exposed by the insulation layer.

In another embodiment, a semiconductor device includes a semiconductor substrate having a cell region and a peripheral circuit region; a saddle fin gate disposed in the cell region under a surface of the semiconductor substrate; a stack type gate disposed over the peripheral circuit region of the semiconductor substrate; an insulation layer covering the saddle fin gate, the insulation layer exposing a contact region in the cell region; and a contact plug in the region exposed by the insulation layer in the cell region.

The saddle fin gate includes an inner wall of a trench having a certain depth in an active region of the cell region; a gate insulation layer along a surface of a recessed isolation region that forms a saddle fin structure; and a gate metal layer in the trench and the recessed isolation region.

In another embodiment, a method for fabricating a semiconductor device includes forming a pad nitride layer that exposes an isolation region over a cell region of a semiconductor substrate; forming a trench in the isolation region of the semiconductor substrate; forming an isolation layer within the trench; etching an active region of the semiconductor substrate by a selected depth to form a recessed active region; etching the isolation layer by a selected depth to form a recessed isolation region; depositing a gate metal layer in the recessed active region and the recessed isolation region to form a gate of a cell transistor; forming an insulation layer over an upper portion of the gate; removing the pad nitride layer to expose a region of the semiconductor substrate to be formed with a contact plug; and depositing a conductive layer in the region of the semiconductor substrate to form a contact plug.

In still another embodiment, a method for fabricating a semiconductor device includes forming a pad nitride layer that exposes an isolation region over a cell region and a peripheral circuit region; forming a trench isolation layer in the isolation region of the semiconductor substrate; recessing a portion of an active region to be formed with a channel by a selected depth; recessing a portion of the trench isolation layer to form the recessed active region in a saddle fin shape; forming a saddle fin gate buried under a surface of the semiconductor substrate in the recessed active region; forming an insulation layer over an upper portion of the saddle fin gate to isolate the saddle fin gate; removing the pad nitride layer to expose a portion of the semiconductor substrate in the cell region to be formed with a contact plug and the peripheral circuit region of the semiconductor substrate; forming a gate insulation layer over the peripheral circuit region of the semiconductor substrate; forming a conductive layer over the cell region; forming a conductive layer over the peripheral circuit region of the semiconductor substrate; and etching the conductive layers to form a contact plug in the cell region and a stack type gate in the peripheral circuit region.

While the disclosed method is susceptible of embodiments in various forms, specific embodiments are illustrated in the drawings (and will hereafter be described), with the understanding that the disclosure is intended to be illustrative, and is not intended to limit the invention to the specific embodiments described herein.

DESCRIPTION OF SPECIFIC EMBODIMENTS

Disclosed herein is a method capable of preventing a contact defect and ensuring a process margin by forming a contact plug with a cell transistor of a saddle fin structure and a non-SAC using a damascene process.

FIGS. 1 through 14Bare cross-sectional views illustrating a method for forming a semiconductor device having a saddle fin transistor in accordance with an embodiment and resulting intermediate and final structures.FIGS. 1,3,4,5,7,9and11are plan views,FIGS. 2A,4A,6A,8A,10A,12A and14A are cross-sectional views taken along a line A-A′ in the plan views, respectively, andFIGS. 2B,4B,6B,8B,10B,12B and14B are cross-sectional views taken along a line B-B′ in the plan views, respectively.

A structure of a semiconductor device having a saddle fin transistor will be described first.FIGS. 14A and 14Bare cross-sectional views in a direction of a bit line and a direction of a word line, respectively, illustrating a semiconductor device having a saddle fin transistor in accordance with an embodiment.

Referring toFIGS. 13,14A, and14B, a semiconductor device includes a semiconductor substrate100having a cell region and a peripheral circuit region. The device further includes a trench310formed to a selected depth in an active region300of the cell region and an isolation layer130surrounding the active region300and having a recessed isolation region320that forms a saddle fin structure. The device includes a saddle fin gate of a cell transistor in the trench310and the recessed isolation region320. In the peripheral region of the semiconductor substrate is a stack type gate. The device further includes an insulation layer180over the saddle fin gate, the insulation layer180exposing a contact region of the cell region, and a contact plug210aformed between the insulation layer180.

The saddle fin gate disposed in the cell region includes a gate insulation layer150formed on a surface of the trench310, a polysilicon layer160over the gate insulation layer150, and a gate metal layer170buried within the trench. If necessary, the polysilicon layer160can be omitted.

The stack type gate includes a gate insulation layer200, a gate conductive layer210b, a metal electrode230, and a hard mask240, which are sequentially stacked over the peripheral circuit region of the semiconductor substrate.

The contact plug210aand the gate conductive layer210bof the stack type gate formed in the peripheral circuit region can be formed of the same material, for example, a polysilicon layer or a metal layer. In addition, a nitride layer spacer191can be provided on a side face of the insulation layer180and a spacer193can be provided on a side face of the stack type gate formed in the peripheral circuit region. An interlayer insulation layer250can be formed over the resulting structure.

Next, a method for fabricating a semiconductor device having a saddle fin transistor in accordance with an embodiment will be described.

Referring toFIGS. 1,2A and2B, a pad oxide layer110and a pad nitride layer120are sequentially formed over a semiconductor substrate100, such as, for example, a silicon (Si) substrate. The pad oxide layer110functions to relieve stress applied to the semiconductor substrate100by attraction of the pad nitride layer120, and is preferably formed to a thickness in a range of 50 to 150 Å. The pad nitride layer120functions as a hard mask in a subsequent etch process for forming an isolation trench330, and is preferably formed to a thickness in a range of 800 to 1,500 Å.

Next, a photoresist pattern (not shown) for delimiting an isolation region over the pad nitride layer120is formed and the pad nitride layer120and the pad oxide layer110are then anisotropically etched using the photoresist pattern as a mask to expose the isolation region of the semiconductor substrate. The photoresist pattern is removed and the exposed region of the semiconductor substrate is then preferably anisotropically etched to a thickness in a range of 2,000 to 3,500 Å to form an isolation trench330for forming an isolation layer.

Next, an insulation layer, e.g., a high density plasma (HDP) oxide layer or a spin on dielectric (SOD), is deposited over an entire face of the semiconductor substrate to bury the isolation trench330. A planarization process, such as, for example, etch back or chemical mechanical polishing (CMP), is performed on the deposited insulation layer to form an isolation layer130in the isolation trench330. The insulation layer can be deposited as a multi-layer of two or more layers. Also, the etch back or CMP process on the insulation layer can be performed until the pad nitride layer120is exposed using the pad nitride layer120as an etch stop layer. Though not shown, before the insulation layer is deposited, an inner wall oxide layer or a liner nitride layer can be formed on an inner wall of the isolation trench330. The active region is delimited by the isolation layer130.

Referring toFIGS. 3,4A and4B, an amorphous carbon layer, for example, is preferably formed to a thickness in a range of 1,500 to 2,500 Å over the resulting product of the semiconductor substrate formed with the isolation layer130inFIGS. 1,2A, and2B. The region of the amorphous carbon layer to be formed with a channel of a cell transistor is etched to form a hard mask140, which exposes the region to be formed with the channel of the cell transistor. The pad nitride layer120and the pad oxide layer110in the cell region are etched using the hard mask140as a mask to expose the region of the semiconductor substrate to be formed with the channel, and the exposed portion of the semiconductor substrate is then etched to a certain depth to form a channel trench310. The channel trench is formed in a region of the cell region to be formed with the channel310of a cell transistor.

Subsequently, the portion of the isolation layer130adjacent to the channel trench310is recessed to form a recessed isolation region320, which forms a saddle type fin. In other words, the portion of the isolation layer130in contact with both ends of the isolation trench330is etched to a certain depth to form the fin shape as shown.

The depth to which the isolation layer130is recessed is preferably in a range of 1,500 to 2,000 Å to ensure a surface resistance (Rs) required in a device. Also, the etch using the hard mask140and the etch on the semiconductor substrate100and the isolation layer130for forming the saddle fin structure can be performed in situ.

Referring toFIGS. 5,6A and6B, a gate insulation layer150is formed over surfaces of the channel trench and the saddle fin. The hard mask (140inFIG. 4A) is optionally removed. The gate insulation layer is preferably formed of an oxide layer with a preferred thickness in a range of 50 to 70 Å. A polysilicon layer160with a preferred thickness in a range of 30 to 60 Å is formed over the gate insulation layer150. The polysilicon layer can be formed over the entire structure. Next, a gate metal layer170is deposited over the polysilicon layer160and is preferably formed of a hybrid tungsten (W) or tungsten silicide (WSi) with a preferred thickness in a range of 300 to 500 Å. The gate metal layer170is, as shown, formed to fill the channel trench310and resulting structure and over the recessed isolation region320. If necessary, the polysilicon layer160can be omitted and the gate metal layer170can be directly formed over the gate insulation layer150.

Referring toFIGS. 7,8A and8B, an etch back or a CMP followed by an etch back is performed to reduce the gate metal layer170to a height of the surface340of the active region300. Therefore, the gate metal layer170remains only in the channel trench310and the recessed isolation region320in the cell region. Meanwhile, the etch back can be performed so that the upper surface of the gate metal layer170can be lower than that of the surface340of the active region300.

Next, to isolate the gate metal layer170from the other conductive layers, an insulation layer180, for example, such as a HDP oxide layer or boron phosphorus silicate glass (BPSG), is formed to a certain thickness over the entire face of the resulting product. An etch back or CMP process is performed on the insulation layer180to expose the pad nitride layer120so that the gate metal layer170is buried and isolated from the other conductive layers. Therefore, a gate, which is buried within the trench in a direction of a major axis (B-B′) and forms a saddle fin structure in a direction of a minor axis (A-A′), is formed in the cell region.

Referring toFIGS. 9,10A and10B, all the remaining pad nitride layer120in the cell region and the peripheral circuit region is removed, for example, using a phosphoric acid solution. Next, to prevent the cell region from being oxidated when subsequently forming the gate oxide layer in the peripheral circuit region, a buffer nitride layer190with a preferred thickness in a range of 30 to 50 Å is formed over the resulting product. The buffer nitride layer and the pad oxide layer110in the peripheral circuit region are removed and an oxide layer with a preferred thickness in a range of 20 to 50 Å is then formed over the resulting product to form a gate insulation layer200in the peripheral circuit region. At this time, an additional process can be performed in a case of forming a dual gate structure.

Referring toFIGS. 11,12A and12B, the buffer nitride layer190and the pad oxide layer110that remains in the cell region are etched to expose the region of the semiconductor substrate to be formed with the contact plug. The buffer nitride layer190in the cell region can be left, as shown, on the side wall of the insulation layer180to form a spacer191. A contact plug210ain the cell region and a gate conductive layer210bin the peripheral circuit region are preferably formed by depositing a doped polysilicon layer or metal layer with a preferred thickness in a range of 2000 to 3000 Å over the resulting structure. The gate conductive layer210bformed in the peripheral circuit region is used as a gate electrode.

Next, a photoresist pattern220that exposes the peripheral circuit region is formed to ensure a certain thickness of the polysilicon layer required in the peripheral circuit region, and the gate conductive layer210bin the peripheral circuit region is etched by a predetermined thickness using the photoresist pattern as a mask to leave the gate conductive layer210bwith a preferred thickness in a range of 500 to 800 Å.

Referring toFIGS. 13,14A and14B, the photoresist pattern (220inFIG. 6A) is removed and a metal electrode layer and a hard mask layer are then sequentially formed over an entire face of the resulting product. The metal electrode layer is preferably formed of a tungsten (W) or tungsten silicide (WSi) layer and is preferably formed to a thickness in a range of 300 to 500 Å. The hard mask layer is preferably formed of a nitride layer and is preferably formed to a thickness in a range of 2,000 to 3,000 Å. The metal electrode layer preferably reduces resistance of a gate formed in the peripheral circuit region. The hard mask patterns the gate formed in the peripheral circuit region.

A photoresist pattern (not shown) that delimits a region to be formed with the gate in the peripheral circuit region is formed, and the hard mask layer, the metal electrode layer, and the gate conductive layer210bare then etched using the photoresist pattern as a mask to form a gate stack including a metal electrode230and a hard mask240in the peripheral circuit region. The etch preferably is also performed in the cell region, and the hard mask layer and the metal electrode layer in the cell region are entirely removed and the doped polysilicon (or metal) layer is etched until the insulation layer180is exposed, thereby accomplishing isolation between the contact plugs210a.

Next, an interlayer insulation layer250is deposited over the resulting product formed with the gate in the peripheral circuit region. The interlayer insulation layer250is preferably formed of an oxide layer with a preferred thickness of 3,000 to 5,000 Å. After that, processes of forming a storage node contact and a capacitor are performed according to a conventional manner.

The disclosed semiconductor substrate and a method for fabricating the same preferably have one or more advantages. For example, it is possible to fundamentally prevent defects resulting in the SAC process since there is no need for a photolithography process for forming a landing plug by forming a cell transistor of a saddle fin structure and a non-SAC process using a damascene process. It is also possible to largely improve turn around time required to produce a product by reducing a number of processes. In addition, by employing a buried word line and a saddle fin structure, it is possible to ensure superior device properties. It is possible to enhance operation speed of a device with reduction of a parasitic capacitance of the word line and the bit line.