Semiconductor device with trench isolation structure

1. Field of the Invention

The present invention relates to a semiconductor device with a trench isolation structure, and more particularly to a MOSFET having a trench isolation structure therein.

2. Description of the Related Art

A significant requirement in sub-micron high-performance semiconductor technologies is junction depth reduction in, for example, MOS (metal-oxide-semiconductor) source and drains and bipolar emitters and bases. Reducing the junction depth suppresses MOS (metal-oxide-semiconductor) transistor punch-through leakage and minimizes short channel effects such as drain-induced barrier lowering (DIBL) in CMOS devices. Devices junctions with relatively high dopant concentrations, ultra-shallow depths, low contact sheet resistances, and low junction leakage currents will be critical for advanced deep submicron technologies.

ULSI integrated Circuits having submicron channel lengths require scaling down of the geometries of devices, including the channel length of metal oxide semiconductor field effect transistors (MOSFETS or MISFET's), which is the ubiquitous workhorse in ULSI circuits. The driving force behind scaling down channel length has been an increase of packing density of devices on a typical chip which has concomitant advantages of increased functionality as well as higher throughput for the process. However, the reduction of channel lengths has resulted in severe technological problems in terms of device performance and reliability.

The internal electric fields near the drain end of small geometry MOSFET's are very high, resulting in carrier heating effects and hot electron degradation of devices. To reduce the electric fields, conventionally a lightly doped drain (LDD) is used in MOSFET's where the doping level in the drain adjacent to the channel is reduced so that the electric field near the drain-channel junction can be lowered. However, the LDD approach has severe problems for deep submicron channel lengths since there is an unacceptable real estate penalty for the LDD regions. Further, the fields cannot be reduced sufficiently and the trapped hot electrons in the gate oxide can have serious depletion effects in the LDD region. Source/drain junction depths, especially the LDD junction depths, have to be made very shallow to avoid charge sharing effects such as threshold voltage, V T , lowering with reduction of the channel length, punch-through and drain induced barrier lowering. Additionally, there are source/drain series resistance problems associated with the shallow LDD region, and to a lesser degree with the shallow LDD regions. Also, there can be enhanced leakage in the LDD region due to band-to-band tunneling or gate induced drain leakage (GIDL). Further, there is a source/drain and gate overlap Miller capacitance and a large source/drain junction capacitance because of the high tank doping required for punch-through protection. All of these factors pose severe technological challenges in fabricating the devices, slow down circuit switching speeds, and increase active power dissipation.

In view of the drawbacks mentioned above, there is a continued need to develop new semiconductor device that overcome the disadvantages associated with prior art.

It is therefore an object of the invention to provide a semiconductor device with a trench isolation structure that can prevent drain induced barrier lowering (DIBL), punch-through leakage and spiking leakage without degrading the performance and the reliability of the semiconductor device.

It is another object of this invention to provide a semiconductor device with a trench isolation structure that can solve the poor electrical property of the conventional semiconductor device with a shallow junction depth and a short channel resulting from the shrink of design rules.

It is a further object of this invention to provide a new structure of semiconductor devices without the problems resulting from the shrink of design rules.

To achieve these objects, and in accordance with the purpose of the invention, the invention provides a semiconductor device with a trench isolation structure, said semiconductor device comprising: a substrate having a trench isolation, a source and a drain region therein, wherein said trench isolation isolates and is between said source and said drain region; a crystalline silicon layer over said substrate; a gate oxide layer on said crystalline silicon layer and above said trench isolation; and a gate electrode on said gate oxide layer.

The invention also provides a semiconductor device with a trench isolation structure, said semiconductor device comprising: a substrate having a trench isolation structure, a source region and a drain region therein, wherein said trench isolation structure isolates and is between said source and said drain region; a crystalline silicon layer over said substrate having a lightly doped drain region therein; a gate oxide layer on said crystalline silicon layer and directly above said trench isolation structure; and a gate electrode on said gate oxide layer, wherein said gate electrode has a spacer adjacent the sidewalls of said gate electrode.

It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed.

The foregoing aspects and many of the attendant advantages of this invention will become more readily appreciated as the same becomes better understood by reference to the following detailed description, when taken in conjunction with the accompanying drawings, wherein:

FIG. 1A shows a cross-sectional diagram of a substrate having a trench isolation structure therein;

FIG. 1B shows a result of forming a crystalline silicon layer over the substrate shown in FIG. 1A;

FIG. 1C shows a result of sequentially forming a gate oxide layer and a gate electrode on the crystalline silicon layer shown in FIG. 1B;

FIG. 1D shows a result of forming a LDD region in the crystalline silicon layer and a spacer adjacent the sidewall of the gate electrode; and

FIG. 1E shows a result of forming source and drain regions in the substrate and salicide layers on the crystalline silicon layer and the gate electrode.

It is to be understood and appreciated that the process steps and structures described below do not cover a complete process flow. The present invention can be practiced in conjunction with various integrated circuit fabrication techniques that are used in the art, and only so much of the commonly practiced process steps are included herein as are necessary to provide an understanding of the present invention.

The present invention will be described in detail with reference to the accompanying drawings. It should be noted that the drawings are in greatly simplified form and they are not drawn to scale. Moreover, dimensions have been exaggerated in order to provide a clear illustration and understanding of the present invention.

Referring to FIG. 1A, a substrate 100 having a trench isolation structure 102 therein is shown. The substrate 100 preferably comprises, but is not limited to: a silicon substrate with a < 100 > crystallographic orientation. The substrate can also comprise other semiconductor substrate such as a silicon-on-insulator (SOI) substrate. The substrate 100 can have N well or P well therein. The trench isolation structure 102 can be formed by conventional methods such as the following process steps. Firstly, a trench is formed into the substrate 100 by using conventional photolithography and etching methods that can be used to form conventional shallow trench isolations. The depth of the trench is from about 800 angstrom to about 2000 angstrom. Then a dielectric layer is formed over the substrate 100 to fill the trench by conventional deposition methods such as chemical vapor deposition processes. Next the dielectric layer is planarized by conventional methods such as chemical mechanical polishing methods to expose the substrate 100 . The dielectric layer comprises, but is not limited to: a silicon dioxide layer. Other dielectric material such as silicon nitride should not be excluded.

Referring to FIG. 1B, a crystalline silicon layer 104 is formed over the substrate 100 . The crystalline silicon layer 104 comprises, but is not limited to: an in-situ doped epitaxial silicon layer formed by conventional deposition methods. The thickness of the crystalline silicon layer 104 is from about 100 angstrom to about 500 angstrom, and is preferably about 250 angstrom.

Referring to FIG. 1C, a gate oxide layer 106 and a gate electrode 108 are sequentially formed on the crystalline silicon layer 104 . The gate oxide layer 106 can be formed by using conventional methods such as dry oxidation methods to oxidize a portion of the crystalline silicon layer 104 and form an oxide layer. The gate electrode 108 comprises, but is not limited to: a polysilicon gate electrode. The gate electrode 108 can be formed by the following process steps. First of all, a conductive layer is formed over the oxide layer by conventional chemical or physical vapor depositions. Then a gate electrode pattern is formed by conventional photolithography methods. Next the conductive layer is etched by conventional etching methods to expose the oxide layer and form the gate electrode 108 . The oxide layer then is etched by conventional etching methods to expose the remaining crystalline silicon layer 104 .

Referring to FIG. 1D, a lightly doped drain (LDD) region 110 is formed in the crystalline silicon layer 104 by using conventional ion implantation processes and a spacer 112 is formed adjacent the sidewall of the gate electrode 108 . The spacer 112 can be formed by forming a dielectric layer such as a silicon dioxide layer or a silicon nitride layer over the substrate 100 by conventional chemical vapor deposition processes and anisotropically etching the dielectric layer. The LDD region 110 as well as the spacer 112 can also be omitted if they are not necessary.

Referring to FIG. 1E, source and drain regions 114 a and 114 b are formed in the substrate 100 by conventional ion implantation processes and salicide layers 116 a , 116 b and 116 c are formed on the LDD region 110 and the gate electrode 108 by using conventional methods. The dopants used to form the source and drain regions 114 a and 114 b can be N type such as phosphorus and arsenic or P type such as boron and BF 2 . The salicide layers 116 a , 116 b and 116 c can be WSi 2 , TiSi 2 and MoSi 2 formed by conventional processes. The salicide layers 116 a , 116 b and 116 c such as TiSi 2 layers, can be formed by forming a Ti layer over the substrate 100 by a conventional physical vapor deposition process, heating the substrate 100 to form the TiSi 2 layers by a rapid thermal processing method and removing the remaining Ti layer by conventional etching processes.

If the substrate 100 is a P− type substrate, for example, and the dopants are N type, the semiconductor device set forth is an NMOS transistor. If the dopants are P type, then the semiconductor device is a PMOS transistor and the trench isolation structure 102 is formed in an N well. The invention can also apply to various CMOS transistors such as N well CMOS transistors, P well CMOS transistors, twin well CMOS transistors and BiCMOS transistors.

As the junction depth and the channel length are further reduced, the problems of drain induced barrier lowering (DIBL), punch-through leakage and spiking leakage will be more crucial. The invention uses a trench isolation that can be formed by using conventional methods to prevent the problems mentioned above. Thus the poor electrical property of the conventional semiconductor device with a shallow junction depth and a short channel resulting from the shrink of design rules can be solved.

Other embodiments of the invention will appear to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. It is intended that the specification and examples to be considered as exemplary only, with a true scope and spirit of the invention being indicated by the following claims.