Patent Publication Number: US-6656853-B2

Title: Enhanced deposition control in fabricating devices in a semiconductor wafer

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method for an enhanced deposition control in fabricating devices in a semiconductor wafer. 
     2. Description of the Related Art 
     A continuing trend in semiconductor technology is to build integrated circuits with more and/or faster semiconductor devices. The drive toward this ultra large-scale integration has resulted in continued shrinking of device and circuit features. Among such trend, fabrication of semiconductor devices involves deposition of a silicon nitride layer over a semiconductor wafer to protect underlying structure. 
     Various deposition techniques of a silicon nitride layer are known. One such technique is disclosed in U.S. Pat. No. 6,060,393 wherein a silicon oxynitride layer is deposited using plasma enhanced chemical vapor deposition (PECVD) process and used as an etch-stop layer for a local interconnect. Another technique is disclosed in U.S. Pat. No. 5,997,757 wherein a silicon nitride layer is deposited using low-pressure chemical vapor deposition (LPCVD) process over 1 hour. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a method for enhanced deposition control over the prior art deposition techniques of silicon nitride layer in terms of time and/or improving electron mobility within underlying structure. 
     According to the present invention, there is provided a method for an enhanced deposition control, which comprises forming at least one device within a substrate of a semiconductor wafer, and depositing a silicon nitride layer over the wafer in a reactor at a pressure of at least approximately 10 4  Pa. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of exemplary embodiments of the invention as illustrated in the accompanying drawings. The drawings are not necessarily scale, emphasis instead being placed upon illustrating the principles of the invention. 
     FIGS. 1A to  1 C depict a cross section of a portion of a semiconductor wafer during the formation of a silicon nitride layer using an enhanced deposition control and then an overlying dielectric layer. 
     FIGS. 2A to  2 C depict the portion of FIG. 1C following the formation of local interconnects that extend through the dielectric layer and the silicon nitride layer. 
     FIG. 3 depicts ON drive current against OFF current characteristic of n-channel metal-oxide-semiconductor field effect transistor. 
     FIG. 4 depicts ON drive current against OFF current characteristic of p-channel metal-oxide-semiconductor field effect transistor. 
     FIG. 5 depicts the varying of ON drive current with different stress within overlying silicon nitride layer. 
     FIG. 6 depicts the varying of [(transconductance)×(gate length Lg)] with different gate lengths. 
     FIGS. 7A to  7 C depict a cross section of a portion of a semiconductor wafer during the formation of a silicon nitride layer using an enhanced deposition control and then an overlying dielectric layer. 
     FIG. 8 depicts the varying of gate capacitance with different gate voltages for a n-channel metal-oxide-semiconductor field effect transistor without the silicon nitride layer. 
     FIG. 9 depicts a cross-section of a portion of a semiconductor wafer according to a first comparative example. 
     FIGS. 10A to  10 C depicts a cross section of a portion of a semiconductor wafer according to a second comparative example. 
     FIG. 11 depicts ON drive current against OFF current characteristic of n-channel metal-oxide-semiconductor field effect transistor according to the first comparative example. 
     FIG. 12 depicts the varying of gate capacitance with different gate voltages for a n-channel metal-oxide-semiconductor field effect transistor according to the second comparative example. 
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     The process steps and structures described below do not form a complete process flow for fabricating integrated circuits in a semiconductor wafer. The present invention can be practiced in conjunction with integrated circuit fabrication techniques currently used in the art, and only so much of the commonly practiced process steps are included as are necessary for an understanding of the present invention. The figures representing cross sections of portions of a device during fabrication are not drawn to scale, but instead are drawn to illustrate the features of the present invention. 
     In accordance with an exemplary embodiment of the present invention, there is provided an enhanced deposition process for use in fabricating a dual-layer dielectric in a manner to improve performance of the underlying device or devices. The dual-layer dielectric includes a silicon nitride layer  9  and an overlying dielectric layer  10  (see FIG.  1 C). For example, the dual-layer dielectric includes a thick dielectric layer  10  over a thin silicon nitride layer  9 . In accordance with embodiments of the present invention, the deposition pressure, which is monitored and controlled during the deposition of silicon nitride layer  9 , has been maintained at least approximately 10 4  Pa. 
     FIG. 1A depicts a cross-section of a portion of a semiconductor wafer before forming dual-layer dielectric including silicon nitride layer  9 . As shown, the portion includes a silicon substrate  1  in which one or more devices have been formed. Within substrate  1  is formed a field oxide region  2  used to isolate the devices. The portion also includes a gate  6  that is part of a metal-oxide-semiconductor field effect transistor (MOSFET) having a source region  8   a  and a drain region  8   b  formed within substrate  1 . As shown, gate  6  is formed on a gate oxide layer  3  under which a channel is formed between source and drain regions  8   a  and  8   b . Gate oxide layer  3  is formed on substrate  1 . In an exemplary embodiment, gate oxide layer  3  is a film of silicon oxynitride (SiO x N y ) having a thickness of 2 nm. Gate  6  includes a layer  4  of doped polycrystalline silicon (referred to hereinafter as polysilicon). In the embodiment, polysilicon layer  4  is approximately 150 nm thick, and gate  6  also includes an optional conductive silicide layer  5  formed on polysilicon layer  4 . Further, in the embodiment, gate  6  has a gate length Lg of 0.1 μm. The gate length Lg is the dimension of gate  6  measured in a direction in which source and drain regions  8   a  and  8   b  are separated from each other across the channel. Oxide spacers  7  of, for example, silicon dioxide (SiO 2 ), are formed on the vertical side surfaces or walls of gate  6 . 
     The materials of polysilicon layer  4  and silicide layer  5  are described. For an n-channel MOSFET (NMOSFET), polysilicon layer  4  is doped with a dopant of the n-type conductivity, such as, phosphorous and arsenic. For a p-channel MOSFET (PMOSFET), polysilicon layer  4  is doped with a dopant of the p-type conductivity, such as, boron. The doped ions are activated by a rapid-thermal-anneal (RTA) at a temperature of 1000° C. over 10 seconds. The material of silicide layer  5  includes cobalt silicide (CoSi 2 ) and nickel silicide (NiSi 2 ). Silicide layer  5  is also formed on the source and drain regions  8   a  and  8   b . In other words, the top surface of gate  6 , the surfaces of source and drain regions  8   a  and  8   b  are formed with silicide layer  5 . 
     FIG. 1B depicts an exemplary cross-section of an improved portion of semiconductor wafer during the deposition of silicon nitride layer  9 . Silicon nitride layer  9  includes dielectric material or materials including silicon nitride (Si x N y ). In the embodiment, silicon nitride layer  9  is deposited within a reactor chamber, which is schematically indicated by dashed lines and reference numeral  40 . Silicon nitride layer  9  is deposited over the entire surface of semiconductor wafer by a LPCVD process. The thickness of silicon nitride layer  9  is approximately 50 nm. In the deposition process within reactor chamber  40 , the reactants including silane (SiH 4 ) and ammonia (NH 3 ) are applied by a flow into chamber  40 . Molecular nitrogen (N 2 ) is used as carrier gas. Instead of molecular nitrogen (N 2 ), other inert gas, such as, helium (He) and argon (Ar) may be used as carrier gas. The ammonia (NH 3 ) flow rate is maintained greater than the silane (SiH 4 ) flow rate. A ratio of the ammonia (NH 3 ) flow rate to the silane (SiH 4 ) is not much exceeding 100 although ratios that vary from this value by plus approximately 30 percent or minus approximately 20 percent are suitable. In an exemplary embodiment, the silane (SiH 4 ) flow rates from 30 sccm to 50 sccm are suitable. The ammonia (NH 3 ) flow rates from 2000 sccm to 4000 sccm are suitable. The molecular nitrogen (N 2 ) flow rates from 2000 sccm to 7000 sccm are suitable. According to the embodiment, the deposited layer  9  of silicon nitride over gate  6  and oxide spacers  7  provides conformal step coverage. Step coverage is enhanced when pressure within reactor chamber  40  is maintained at approximately 4×10 4  Pa. In embodiments according to the present invention, pressures within reactor chamber  40  from 1×10 4  Pa to 6×10 4  Pa are suitable. Under this pressure condition, the deposition rate exceeds 50 nm/minute so that a silicon nitride layer 9 of 50 nm thick is deposited in less than 1 minute. In reactor chamber  40 , temperatures range from 600° C. to 800° C. 
     Prior art LPCVD processes tend to maintain deposition pressures from 30 Pa to 250 Pa (see S. M. Sze “VLSI TECHNOLOGY” Second Edition, published in 1988 by McGraw-Hill Book Company). All these pressures are far lower than the pressure range from 1×10 4  Pa to 6×10 4  Pa according to embodiments of the present invention. The pressure range in embodiments of the present invention is 102 to 103 times as much as the deposition pressures used by the prior art LPCVD processes. 
     With reference to FIG. 1C, after the deposition of silicon nitride layer  9 , a dielectric layer  10  is formed over silicon nitride layer  9 . The dielectric layer  10  includes any suitable dielectric material or materials, including borophoshosilicate glass (BPSG) or any suitable silsesquioxane that includes hydrogen silsesquioxene, methyl silsesquioxane, methylated hydrogen silsesquioxane, or furuorinated silsesquioxane. Dielectric layer  10  may be formed to any suitable thickness using any suitable technique that may depend, for example, on the material or materials used. In the embodiment, dielectric layer  10  of BPSG is formed to about 500 nm by deposition using plasma enhanced chemical vapor deposition (PECVD) process followed by planarization using chemical-mechanical polish (CMP) process. The BPSG layer  10  includes silicon dioxide (SiO 2 ), about 4 weight % boron, and 4 weight % phosphorous. Temperature during this PECVD process is at approximately 400° C., which is lower than the temperature range from 600° C. to 800° C. during LPCVD for deposition of silicon nitride layer  9 . This temperature difference between the two deposition processes effectively maintains tensile stress created within silicon nitride layer  9 . Because of higher temperature for deposition of silicon nitride layer  9  and lower temperature for deposition of dielectric layer  10 , tensile stress of silicon nitride layer  9  is maintained. It will be appreciated that dielectric layer  10  functions to maintain tensile stress created within silicon nitride layer  9 . 
     The tensile stress maintained within silicon nitride layer  9  has effects on underlying structure, which will be described later. Before describing further on tensile stress within silicon nitride layer  9 , a local interconnect formation process using silicon nitride layer  9  as an etch-stop layer is described below with reference to FIGS. 2A,  2 B and  2 C. 
     In FIG. 2A, dielectric layer  10  has been planarized using CMP process. A patterned resist mask  11  formed with etch-openings  12  has been coated over top of dielectric layer  10 . Dielectric layer  10  exposed by patterned resist mask  11  has been dry-etched within a reactive ion etching (RIE) etcher. Selective portions of dielectric layer  10  have been removed from below etch-openings  12 . In the RIE etcher, feed gas including octaflourobutene (C 4 F 8 ) argon (Ar) and oxygen (O 2 ) has been used. The etching process has been stopped at silicon nitride layer  9 . 
     In FIG. 2B, selective portions of silicon nitride layer  9  have been dry-etched from below etch-openings  12  within the same RIE etcher using new feed gas including Freon®(CHF 3 ). Contact holes  12   a  have been created through dielectric layer  10  and silicon nitride layer  9 . The underlying silicide layer  5  of cobalt silicide is not damaged during the etching process, thus protecting underlying structure including source and drain regions  8   a  and  8   b . In this etching process, the selectivity, i.e., the ratio between (etch rate of silicon nitride layer  9 ) and (etch rate of cobalt silicide layer  5 ) exceeds 50. 
     In FIG. 2C, one or mode conductive materials including tungsten (W) have been deposited to form contact plugs  13  within the etched contact holes  12   a  created through dielectric layer  10  and silicon nitride layer  9 . Contact plugs  13  make electrical connections with metal regions  14  formed on dielectric layer  10 . In the embodiment, contact holes  12   a  are created and contact plugs  13  are deposited to make electrical contacts with source and drain regions  8   a  and  8   b , respectively. In another exemplary embodiment of the present invention, a contact hole may be created through dielectric layer  10  and silicon nitride layer  9  to expose silicide layer  5  of gate  6  and a contact plug may be deposited within such contact hole to make electrical contact with the gate  6 . 
     In order to consider the performance of a NMOSFET fabricated according to the embodiment of the present invention (see FIGS. 1A-1C and  2 A- 2 C), two comparative examples, namely, first and second comparative examples, have been prepared. With reference to FIG. 9, the first comparative example will now be described. With reference to FIGS. 10A-10C, the second comparative example will be later escribed. 
     With reference to FIG. 9, the first comparative example will now be described. 
     FIG. 9 depicts a cross-section of a portion of a semiconductor wafer having a silicon nitride layer  107  and a dielectric layer  108  as prepared for local interconnect processing. The portion includes a silicon substrate  101 . The portion also includes a gate  105  that is part of a NMOSFET having a source region and a drain region (not shown) formed within substrate  101 . Gate  105  includes a polycrystalline silicon layer  103  formed on a gate oxide layer  102  that has been formed on substrate  101 . Gate  105  also includes a conductive silicide  104  formed on top of polysilicon layer  103 . Oxide spacers  106  have been added to the vertical side surfaces or walls of gate  105 . 
     Silicon nitride layer  107  is deposited over the wafer to the thickness of approximately 50 nm at approximately 480° C. in a plasma enhanced chemical vapor deposition (PECVD) system using silane (SiH 4 ), nitrous oxide (N 2 O) and nitrogen (N 2 ). Dielectric layer  108  was a conformal layer of tetraethlorthosilicate (TEOS). The exposed top surface of dielectric layer  108  has been planarized down, using CMP process. 
     Although not shown, a patterned resist mask with an etch-opening has been formed on top surface of dielectric layer  108 . A local interconnect has been formed using damascene techniques wherein materials of dielectric layer  108  and silicon nitride layer  107  have been removed from below the etch-opening using a plasma etching process. Glue layer  110  and plug  111  are deposited within the etched opening  109  created through dielectric layer  108  and silicon nitride layer  107  to make electrical contact with underlying structure. 
     With reference to FIGS. 10A-10C, the second comparative example will now be described. 
     FIG. 10A depicts a cross-section of a portion of a semiconductor wafer having a silicon nitride layer  207  and a dielectric layer  208  as prepared for local interconnect processing. The portion includes a silicon substrate  201 . The portion also includes gates  203 , each of which is part of a MOS having a source/drain region  206  formed within substrate  201 . Source/drain region  206  has an LDD structure. Gate  203  includes a tungsten polysilicide layer formed on a gate oxide layer  202  that has been formed on substrate  201 . Gate  203  also includes an offset oxide  204  formed on top of the tungsten polysilicide. Oxide spacers  205  have been added to the vertical side surfaces or walls of gates  203 . Offset oxide  204  of each gate  203  is a film of silicon dioxide (SiO 2 ). Oxide spaces  205  are made of silicon dioxide (SiO 2 ). 
     Silicon nitride layer  207  is deposited over the wafer to the thickness of approximately 50 nm at temperature ranging from 750° C. to 800° C. by low pressure chemical vapor deposition (LPCVD) process using ammonia (NH 3 ) and silane (SiH 4 ) or dichlorosilane (SiH 2 Cl 2 ). Molecular nitrogen (N 2 ) is used as carrier gas. Pressure ranges from 10 Pa to 100 Pa. Time required to deposit silicon nitride to a thickness of 50 nm using LPCVD process is approximately one hour. Dielectric layer  208  was a conformal layer of silicon dioxide (SiO 2 ). The exposed top surface of dielectric layer  108  has been planarized down, using CMP process. 
     In FIG. 10B, a patterned resist mask with an etch-opening  210  has been formed on top surface of dielectric layer  208 . Material of dielectric layer  208  has been removed from below etch-opening  210  using a plasma etching process. During this etching process, silicon nitride layer  207  protects underlying structure. 
     In FIG. 10C, material of silicon nitride layer  207  has been removed from below the etch-opening  210  using a plasma etching process. Plug  211  is deposited within the etched opening  210   a  created through dielectric layer  208  and silicon nitride layer  207  to make electrical contact with underlying structure. 
     Varying of performance of a NMOSFET with different deposition techniques of a silicon nitride layer are considered with respect to the first comparative example, second comparative example, and embodiment of the present invention, 
     First, consider performance of a NMOSFET, which has a silicon nitride layer  107 , according to the first comparative example. As mentioned before, the silicon nitride layer  107  is deposited over the NMOSFET at approximately 480° C. using PECVD process. FIG. 11 summarizes the performance by showing ON drive current I ON  against OFF current I OFF  for the NMOSFET according to the first comparative example as well as a NMOSFET not covered by a silicon nitride layer. In FIG. 11, black circles represent experimental data for the NMOSFET according to the first comparative example, and white circles experimental data for the NMOSFET not covered by the silicon nitride layer. The experimental data were obtained by measuring ON drive current and OFF current. ON current was measured under the following conditions: gate voltage Vg=0 V, drain voltage Vd=1.5 V, and source voltage Vs=0 V. OFF current was measured under the following conditions: Vg=Vd=1.5 V and Vs=0 V. FIG. 11 plots the experimental data for different gate lengths Lg. Ten different gate lengths are selected. Some of them are chosen to look into varying of ON current over a range in gate length Lg from 0.08 μm to 0.2 μm, and the other chosen to look into varying of ON current with different gate lengths exceeding this range beyond 0.2 μm. In FIG. 11, the experimental data represented by white circles clearly indicate increasing trend of ON current as the gate length Lg decreases. The same increasing trend of ON current arises from the experimental date represented by black circles as long as the gate length Lg is not less than a certain value. But, different increasing trend of ON current is seen as the gate length Lg becomes less than this certain value. It has been confirmed that this certain value is 0.3 μm. Over the gate lengths Lg less than 0.3 μm, with the same gate length, the NMOSFET according to the first comparative example has lower ON current than ON current of the NMOSFET without silicon nitride layer. This means that the NMOSFET according to the first comparative example exhibits poor performance over the gate lengths less than 0.3 μm. The inventor of this application considers that this poor performance arises from reduced mobility of electrons due to compressive stress created within silicon nitride layer that has been deposited in PECVD process according to the first comparative example. 
     Second, consider also performance of a NMOSFET, which has a silicon nitride layer  207 , according to the second comparative example and that of a NMOSFET, which has a silicon nitride layer  9 , according to the embodiment of the present invention. As mentioned before in the second comparative example depicted by FIG. 10A, silicon nitride layer  207  is deposited over the NMOSFET to the thickness of approximately 50 nm at temperature ranging from 750° C. to 800° C. and at pressure ranging from 10 Pa to 100 Pa using LPCVD process. As mentioned before in the embodiment of the present invention, silicon nitride layer  9  is deposited over the NMOSFET at temperature ranging from 600° C. to 800° C. and at pressure ranging from 1×10 4  Pa to 6×10 4  Pa using PECVD process. Similarly to FIG. 11, FIG. 3 summarizes the effect by showing ON current I ON  against OFF current I OFF  for the NMOSFET according to the second comparative example and for the NMOSFET according to the embodiment of the present invention. In FIG. 3, black squares represent experimental data for the NMOSFET according to the second comparative example, and white squares experimental data for NMOSFET according to the embodiment of the present invention. Throughout FIGS. 3 and 11, black circles represent the same experimental data for the NMOSFET according to the first comparative example. The experimental data shown in FIGS. 3 and 11 were obtained by measuring ON current and OFF current in the same manner. In FIG. 3, the experimental data represented by white squares clearly indicate that, with the same gate length Lg, ON current I ON  of the NMOSFET according to the embodiment of the present invention is higher than those of NMOSFETs according to the first and second comparative examples. This means that the NMOSFET according to the embodiment of the present invention exhibits good performance over almost all of the gate lengths selected to obtain the experimental data. The inventor considers that this good performance exhibited by the NMOSFET according to the embodiment of the present invention arises from enhanced mobility of electrons due to tensile stress created within silicon nitride layer that has been deposited in high pressure LPCVD process according to the embodiment of the present invention. 
     The preceding description has clarified that electron mobility within a NMOSFET is dependent on stress created within overlying silicon nitride layer. Further description on the stress dependent mobility of electrons will be further described later with reference to FIGS. 5 and 6. Before mating such description, consider a PMOSFET wherein holes carry charges. As is well known, hole mobility is lower than electron mobility. Besides, hole mobility is far less dependent on stress created with overlying silicon nitride layer than electron mobility is. FIG. 4 shows ON drive current ION against OFF current I OFF , which is common to three different PMOSFETs, one according to the first comparative example, another according to the second comparative example, and the other according to the embodiment of the present invention. Accordingly, there is no or little varying of ON current with the different PMOSFETs. 
     ON drive current ION of NMOSFET depends on stress created within overlying silicon nitride layer. FIG. 5 shows the relationship between ON current I ON  and stress created within overlying silicon nitride layer. White squares represent experimental data of ON current I ON  measured under the same conditions as in FIG. 3 when OFF current I OFF  is 5 nA/μm (=5×10 −9  A/μm). FIG. 5 states increasing trend of ON current as tensile stress within overlying silicon nitride layer increases. Noticeable increase of ON current is seen when the tensile stress is approximately 1000 MPa (=10 10  dyn/cm 2 ). 
     Increasing of ON current within a NMOSEFT depends on gate length Lg. In FIG. 6, the fully drawn line shows the varying of [(transconductance)×(gate length Lg)] with different gate lengths Lg with respect to a NMOSFET according to the embodiment of the present invention. The bold dashed line shows the varying of [(transconductance)×(gate length Lg)] with different gate lengths Lg with respect to a NMOSFET according to the first comparative example. The normal dashed line shows the varying of [(transconductance)×(gate length Lg)] with different gate lengths Lg with respect to a NMOSFET according to the second comparative example. 
     FIG. 6 clearly states that, when the gate length Lg is less than a threshold value of 0.6 μm, with the same gate length, [(transconductance)×(gate length Lg)] according to the embodiment of the present invention is greater than that according to the second comparative example, which is greater than that according to the first comparative example. This means that the NMOSFET according to the embodiment of the present invention exhibits the best performance when the gate length Lg is less than the threshold value of 0.6 μm. 
     FIG. 6 also states that, when the gate length Lg is not less than the threshold value of 0.6 μm, with the same gate length, [(transconductance)×(gate length Lg)] according to the embodiment of the present invention is less than that according to the second comparative example, which is less than that according to the first comparative example. 
     The inventor considers that the above-mentioned inversion of the relationship across the threshold value of 0.6 μm is due to the degree to which stress applied to the edges of channel affects electron mobility within NMOSFET. According to the embodiment of the present invention, a silicon nitride layer deposited over a wafer coats the top and side surfaces of a gate of a NMOSFET having source/drain regions separated by channel below the gate. The deposited silicon nitride has tensile stress, creating a vertical force component urging the gate against the channel to generate compressive stress therein as well as horizontal force components applied to the edges of the channel to generate tensile stress therein. The compressive stress within the channel becomes predominant when the gate length is large. The tensile stress within the channel becomes predominant when the gate length becomes less than 0.6 μm. The inventor considers that electron mobility increases as the tensile stress within the channel becomes predominant. If the compressive stress within the channel becomes predominant, electron mobility is hampered. 
     With reference to FIGS. 7A-7C, another exemplary embodiment according to the present invention is described. This exemplary embodiment is substantially the same as the first exemplary embodiment as depicted in FIGS. 1A-1C in deposition of a silicon nitride layer  27  (see FIG.  7 B). Silicon nitride layers  27  (FIG. 7B) and  9  (FIG. 1B) are the same in the manner of deposition and composition. Further, silicon nitride layer  27  acts as an etch-stop layer in the same manner as silicon nitride layer  9  does. 
     FIG. 7A depicts a cross-section of a portion of a semiconductor wafer before forming dual-layer dielectric including silicon nitride layer  27 . The portion includes a silicon substrate  21  in which one or more devices have been formed. The portion also includes gates  23 , each of which is part of a MOS having a source/drain region  26  formed within substrate  21 . Source/drain region  26  has an LDD structure. Gate  23  includes a doped tungsten polysilicide layer formed on a gate oxide layer  22  that has been formed on substrate  21 . Gate  23  also includes a cap oxide  24  formed on top of the tungsten polysilicide. Oxide spacers  25  have been added to the vertical side surfaces or walls of gates  23 . 
     FIG. 7B depicts an exemplary cross-section of an improved portion of semiconductor during the deposition of silicon nitride layer  27 . Silicon nitride layer  27  is a thin film of silicon nitride that is approximately 50 nm thick. Silicon nitride layer  27  is deposited within a reactor chamber  40  at temperature ranging from 600° C. to 800° C. by LPCVD process using ammonia (NH 3 ) and silane (SiH 4 ). Molecular nitrogen (N 2 ) is used as carrier gas. Pressure ranges from 1×10 4  Pa to 6×10 4  Pa. Time required to deposit silicon nitride to a thickness of 50 nm is approximately is less than one minute. 
     With reference to FIG. 7C, after depositing a conformal dielectric layer  28  to a thickness of 500 nm, the exposed top surface of the dielectric layer has been planarized down, using CMP process. Temperature during this deposition is at approximately 500° C. Although not shown, a patterned resist mask with an etch-opening has been formed on top surface of dielectric layer  28 . Material of dielectric layer  28  has been removed from below the etch-opening using dry etching process. In the dry etching process, feed gas including octaflourobutene (C 4 F 8 ), argon (Ar) and oxygen (O 2 ) has been used. During this etching process, silicon nitride layer  27  protects underlying structure. The selectivity, i.e., the ratio between (etch rate of dielectric layer  28 ) and (etch rate of silicon nitride layer  27 ), is at approximately 30. Material of silicon nitride layer  27  has been removed from below from below the etch-opening in dry etching process using feed gas including nitrogen trifluoride (NF 3 ) and carbon monoxide (CO). Contact hole  29  has been created through dielectric layer  28  and silicon nitride layer  27 . Although not shown, plug will be deposited within contact hole to make electric contact with underlying structure. 
     Silicon nitride layer  9  or  27  deposited according to the embodiments of the present invention is porous and has sufficiently large number of pores to allow hydrogen to pass toward underlying structure during hydrogen anneal. Silicon nitride layer  207 , deposited using a conventional LPCVD process described as the second comparative example (see FIGS.  10 A- 10 C), is condensed and thus less porous. 
     In this conventional LPCVD process, the doped polysilicon layer of each gate  203  is exposed to temperature of approximately 750° C. over 1 hour until silicon nitride layer  207  is deposited to a thickness of 50 nm. Due to this long exposure time to high temperature, a portion of doped impurity within the polysilicon layer is deactivated, causing an appreciable drop in concentration of activated impurity. This drop in concentration of activated impurity creates depletion within the polysilicon of gate  203 . In the deposition process according to the embodiments of the present invention, exposure time of polysilicon layer of gate  6  (see FIG. 1A) or  23  (see FIG. 7A) is very short and less than 1 minute. The drop in concentration of activated impurity and thus creation of depletion within the polysilicon are suppressed completely or at least to satisfactorily low level. 
     The presence of depletion in the gate has effect on the performance of NMOSFET and PMOSFET formed within a semiconductor wafer. FIG. 12 summarizes the effect by showing the varying of gate capacitance Cg with different gate voltages Vg. A NMOSFET has been considered, which has a gate, measuring 100 μm×100 μm, formed on an oxynitride layer of 2 nm thick that has been formed on a silicon substrate. First test sample has been fabricated according to the second comparative example with reference to FIGS. 10A-10C. Second test sample has been fabricated without such silicon nitride layer. FIG. 8 depicts the varying of gate capacitance Cg with different gate voltages Vg for the second sample. In FIG. 12, the dashed line shows the varying of gate capacitance Cg with different gate voltages Vg for the first test sample, while the fully drawn line shows the varying of gate capacitance Cg shown in FIG.  8 . From FIG. 12, it will be appreciated that there is a considerable deviation in gate capacitance cg if gate voltage exceeds 0.5 V. This deviation is due to the presence of depletion within the gate of the first test sample. Third test sample has been fabricated employing the deposition process of silicon nitride layer according the embodiments of the present invention. FIG. 8 shows the varying of gate capacitance Cg with different gate voltages Vg for the third test sample. The fully drawn line in FIG. 8 is analogous to the fully drawn line of FIG. 12, meaning that there is no depletion in the gate of the third sample fabricated according to the embodiments of the present invention. Accordingly, there is no undesired deviation if the gate voltage exceeds 0.5 V. 
     In the exemplary embodiments of the present invention, the reactants including silane (SiH 4 ) and ammonia (NH 3 ) are applied to reactor chamber  40  for deposition of silicon nitride layer  9 . In another embodiment of the present invention, fluorosilane (SiH x F 4-x ) is used instead of silane (SiH 4 ), where: x=0, 1, 2, 3 or 4. In this embodiment, the reactants including fluorosilane (SiH x F 4-x ) and ammonia (NH 3 ) are applied by a flow into chamber  40 . Molecular nitrogen (N 2 ) is used as carrier gas. Instead of molecular nitrogen (N 2 ), other inert gas, such as, helium (He) and argon (Ar) may be used as carrier gas. 
     In further embodiment of the present invention, disilane (Si 2 H 6 ) is used instead of silane (SiH 4 ). In this case, the deposition temperature should be maintained lower than 600° C. In still further embodiment of the present invention, dichlorosilane (SiH 2 Cl 2 ) is used instead of silane (SiH 4 ). 
     In exemplary embodiments according to the present invention, suitable pressures, within reactor chamber  40 , range from 1×10 4  Pa to 6×10 4  Pa. This pressure range is currently appropriate. If the deposition pressure higher than this range is used, the variation in thickness of silicon nitride layer increases and particles occur frequently. If the deposition pressure is lower than this range, the deposition rate drops. 
     While the present invention has been particularly described, in conjunction with the exemplary embodiments, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art in light of the foregoing description. It is therefore contemplated that the appended claims will embrace any such alternatives, modifications and variations as falling within the true scope and spirit of the present invention.