Abstract:
A method of fabricating a gate dielectric layer. The method comprises: providing a substrate; forming a silicon dioxide layer on a top surface of the substrate; exposing the silicon dioxide layer to a plasma nitridation to convert the silicon dioxide layer into a silicon oxynitride layer; and performing a spiked rapid thermal anneal of the silicon oxynitride layer.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to the manufacture of semiconductor devices; more specifically, it relates to a method of fabricating a nitrided silicon-oxide gate dielectric.  
         BACKGROUND OF THE INVENTION  
         [0002]    The trend in integrated circuits is toward higher performance, higher speed and lower cost. Correspondingly, device dimensions and element sizes are shrinking and gate dielectrics must scale accordingly. As physical gate dielectric thickness has decreased, the need for a higher dielectric constant and less leaky gate dielectric has arisen. In advanced metal oxide semiconductor field effect transistors (MOSFETs) silicon oxynitride (SiO x N y ) layers are used as a gate dielectric. MOSFET transistors include a channel region formed in a silicon substrate, an N or P doped polysilicon gate formed on top of a thin gate dielectric layer and aligned over the channel region and source/drain regions formed in the silicon substrate on either side of the channel region.  
           [0003]    However, there are several problems associated with SiOxN y  layers that affect the performance of devices having a SiO x N y  gate dielectric. These problems are a result of the processes used to fabricate the SiO x N y  layer and the distribution of nitrogen in the layer. SiOxNy layers fabricated using conventional plasma nitridation processes have poor reliability as a result of low time dependent dielectric breakdown (T BD ) and charge-to-breakdown (Q BD ). The degradation in reliability is caused by plasma induced dangling bonds in the dielectric and at the dielectric-silicon interface. Further, the nitrogen concentration in SiO x N y  layers fabricated using conventional plasma and thermal nitridation processes is not uniformly distributed throughout the layer but is concentrated at the SiO x N y /Si interface causing large threshold voltage (V T ) shifts; the shifts of p-channel field effect transistors (PFETs) being larger than that of N-channel field effect transistors (NFETs ). Both mechanisms described above will cause a degradation in the channel mobility. Both mechanisms described above will also cause an increase in negative bias temperature instability (NBTI) which induces V T  and frequency shifts after stressing. Additionally, the relative lack of nitrogen near the surface of conventional SiO x N y  layers results in increased boron penetration from the gate electrode (in PFETs) into the SiO x N y  layer which can degrade Tbd and Qbd as well as influence across-wafer V T  uniformity. Therefore, there is a need for a method of fabricating a SiO x N y  layer having a relatively uniform nitrogen concentration throughout its thickness, high mobility and high Tbd and Qbd, while forming a layer with realtively high nitrogen content to lower leakage current through the gate dielectric when the device is turned off.  
         SUMMARY OF THE INVENTION  
         [0004]    A first aspect of the present invention is A method of fabricating a gate dielectric layer comprising: providing a substrate; forming a silicon dioxide layer on a top surface of the substrate; exposing the silicon dioxide layer to a plasma nitridation to convert the silicon dioxide layer into a silicon oxynitride layer; and performing a spiked rapid thermal anneal of the silicon oxynitride layer.  
           [0005]    A second aspect of the present invention is a method of fabricating a MOSFET, comprising: providing a silicon substrate; forming a silicon dioxide layer on a top surface of the silicon substrate; exposing the silicon dioxide layer to a plasma nitridation to convert the silicon dioxide layer to a silicon oxynitride layer; performing a spiked rapid thermal anneal of the silicon oxynitride layer; forming a polysilicon gate on the annealed silicon oxynitride layer aligned over a channel region in the silicon substrate; and forming source/drain regions in the silicon substrate, the source drain regions aligned to the polysilicon gate. 
       
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0006]    The features of the invention are set forth in the appended claims. The invention itself, however, will be best understood by reference to the following detailed description of an illustrative embodiment when read in conjunction with the accompanying drawings, wherein:  
         [0007]    [0007]FIGS. 1 through 3 are partial cross-sectional views illustrating fabrication of a nitrided gate dielectric layer according to the present invention;  
         [0008]    [0008]FIGS. 4 and 5 are partial cross-sectional views illustrating fabrication of a MOSFET according to the present invention;  
         [0009]    [0009]FIG. 6 is a flowchart of the process steps for fabricating a dielectric layer and the MOSFET illustrated in FIGS. 1 through 4 according to the present invention;  
         [0010]    [0010]FIG. 7 is a schematic illustration of a decoupled plasma system for performing a nitridation step according to the present invention;  
         [0011]    [0011]FIGS. 8 and 9 are plots of temperature versus time illustrating a spike anneal process according to the present invention;  
         [0012]    [0012]FIG. 10 is a secondary ion mass microscopy (SIMs) profile of a gate dielectric fabricated according to the present invention;  
         [0013]    [0013]FIG. 11 is a plot comparing leakage, mobility and electrical thickness at three steps in the fabrication of a gate dielectric according to the present invention; and  
         [0014]    [0014]FIG. 12 is a plot comparing leakage, time to breakdown and charge to breakdown at three steps in the fabrication of a gate dielectric according to the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0015]    The terms nitrided silicon dioxide (nitrided SiO 2 ) and silicon oxynitride (SiO x N y ) are equivalent terms for the purposes of the present invention. The scope of SiO x N y  includes all combinations of integers x and y (or fractions thereof) at which SiO x N y  is stable. A spike rapid thermal anneal (RTA), for the purposes of the present invention, is defined as an anneal wherein the time at maximum temperature (the spike temperature) is about 60 seconds or less.  
         [0016]    [0016]FIGS. 1 through 3 are partial cross-sectional views illustrating fabrication of a nitrided gate dielectric layer according to the present invention. In FIG. 1 a substrate  100  having a top surface  105  is provided. Substrate  100  may be an intrinsic, N-type or P-type bulk silicon substrate or an undoped or an intrinsic, N-type or P-type silicon on insulator (SOI) substrate or a sapphire substrate or a ruby substrate.  
         [0017]    In FIG. 2, a base SiO 2  layer  110  is formed on top surface  105  of substrate  110 . Prior to formation of base SiO 2  layer  110  on surface  105 , the surface is cleaned by any one of a number of cleaning processes well known in the art. For example, surface  105  may be cleaned using a buffered hydrofluoric acid (BHF) clean followed by an NH 4 OH clean followed by an HCl clean. If substrate  100  is a bulk silicon substrate or an SOI substrate, base SiO 2  layer  110  may be formed, in a first example, by a thermal oxidation in a furnace in an oxygen-containing atmosphere at about 600 to 800° C. for about 0.5 to 30 minutes. In a second example, base SiO 2  layer  110  may be formed by a rapid thermal oxidation (RTO) in an oxygen-containing atmosphere at about 800 to 1000° C. for about 5 to 60 seconds. In a third example, base SiO 2  layer  110  may be formed by thermal oxidation in a gaseous environment containing oxygen and either nitric oxide (NO) or nitrous oxide (N 2 O) such that base SiO 2  layer  110  contains from 0 to 5% atomic percent nitrogen. If substrate  100  is a ruby or sapphire substrate, base SiO 2  layer  110  may be formed by a deposition in a chemical vapor deposition (CVD) tool and dielectric layer may be a tetraethoxysilane (TEOS) oxide. TEOS may also be used for a bulk silicon or SOI substrate. TEOS may also be used for bulk silicon or SOI substrates. In one example, base SiO 2  layer  110  is about 10 to 20 Å thick.  
         [0018]    In FIG. 3, a decoupled plasma nitridation (DPN) process followed by a spike RTA process is performed to convert base SiO 2  layer  110  (see FIG. 2) to a nitrided SiO 2  (SiO x N y ) layer  110 A. The plasma nitridation process is described below in reference to FIGS. 6 and 7 and the spike RTA process is described below in reference to FIGS. 6, 8 and  9 . SiO x N y  layer  110 A is about 3 Å thicker than base SiO 2  layer  110  (see FIG. 2) was and contains about 5 to 15% nitrogen atoms relatively distributed throughout the SiO x N y  layer as illustrated in FIG. 10 and described below. In one example, SiO x N y  layer  110 A is about 13 to 23 Å thick.  
         [0019]    [0019]FIGS. 4 and 5 are partial cross-sectional views illustrating fabrication of a MOSFET according to the present invention. FIG. 4 continues from FIG. 3. In FIG. 4, a polysilicon layer  115  is formed on a top surface  120  of SiO x N y  layer  110 A. Polysilicon layer  115  may be formed using one of a number of deposition processes well known in the art, such as low-pressure chemical vapor deposition (LPCVD) or rapid thermal chemical vapor deposition (RTCVD). Polysilicon layer  115  may be undoped or doped N-type or P-type. In one example, polysilicon layer  115  is 1000 to 2000 Å thick.  
         [0020]    In FIG. 5, polysilicon layer  115  (see FIG. 4) is etched; for example, by a reactive ion etch (RIE) processes to form a gate  125 . Spacers  130  are formed on sidewalls  135  of gate  125 . Formation of source/drains  140  (typically by one or more ion-implantation processes) essentially completes fabrication of MOSFET  145 , SiO x N y  layer  110 A being the gate dielectric of the MOSFET. If polysilicon layer  115  (see FIG. 4) was not doped during deposition, gate  125  may be doped N-type or P-type after spacer formation by ion implantation in conjunction with the formation of source/drains  140  or as a separate step.  
         [0021]    [0021]FIG. 6 is a flowchart of the process steps for fabricating a dielectric layer and the MOSFET illustrated in FIGS. 1 through 5 according to the present invention. A silicon substrate will be used as an example. In step  150 , the surface of the silicon substrate is cleaned by any one of a number of cleaning processes well known in the art. In a first example, silicon surface  105  may be cleaned using a buffered hydrofluoric acid (BHF) clean followed by an NH 4 OH clean followed by an HCl clean. Alternatively, in a second example, the silicon surface may be cleaned using BHF followed by an O 3  clean, followed by a dry HCl clean.  
         [0022]    In step  155 , a base SiO 2  layer is formed, for example, by a thermal oxidation in a furnace in an oxygen-containing atmosphere at about 600 to 800° C. for about 0.5 to 30 minutes or by a RTO in an oxygen-containing atmosphere at about 800 to 1000° C. for about 5 to 60 seconds. The base SiO 2  layer is about 10 to 20 Å thick.  
         [0023]    In step  160  a decoupled plasma nitridation process is performed. The decoupled plasma nitridation processes is tuned for the thickness of the base SiO 2  used. A general example and three specific examples are given, for 10 to 20 Å, 12 Å, 15 Å and 18 Å of base SiO 2  respectively, in Chart I.  
                               CHART I                           General   Specific   Specific   Specific       PARAMETER   Example   Example 1   Example 2   Example 3                   Base SiO 2     10-20 Å   12 Å   15 Å   18 Å       He/N 2  MIX   50-95% He   95% He   95% He   95% He       He Flow (sccm)   300-3000   475   475   475       N 2  Flow (sccm)   20-200   25   25   25       Pressure (torr)   50-125   75-125   75-125   75-125       Power (watts)   50-200   100   100   100       Time (sec)   5-60   20   30   40       Substrate Bias (v)   0   0   0   0       Wafer Temp (° C.)    0-200   20   20   20       Chamber Temp (° C.)    0-200   65   65   65       SiO x N y     13-23 Å   15 Å   18 Å   21 Å                  
 
         [0024]    In the examples of Chart I, decoupled plasma nitridation time is used, though any of the parameters, especially N 2  flow, He flow and power may be used to tune the process to the base SiO 2  thickness. The gas mix listed in Chart I is a He/N 2  mix. Other inert gases such as Ne, Ar, Kr and Xe may be used in place of He. The present invention is also applicable to a SiO 2  layer thinner than 10 Å.  
         [0025]    In step  165 , a spike RTA is performed. A spike anneal is used to increase the mobility without driving the nitrogen to the SiO 2 /Si interface. A general example and one specific example are given, for 10 to 20 Å and 15 Å of base SiO 2  respectively, in Chart II.  
                               CHART II                                       General   Specific           PARAMETER   Example   Example 1                           Base SiO 2     10-20 Å   15 Å           Spike Temperature (° C.)   800-1300   1050           Pressure (torr)    1-780   780           N 2  Flow (liters/min)   1-10   10           O 2  Flow (sccm)    0-1000   0           Spike Time (sec)   0-60   0                      
 
         [0026]    The Spike Temperature in Chart II is the maximum temperature reached during the spike anneal. The use of O 2  will increase the thickness of the completed SiO x N y  layer more than if no O 2  is used during the anneal process. In one example, the average concentration of nitrogen in the completed SiO x N y  layer is about 1E21 to 5E21 atm/cm 3  and the equivalent nitrogen dose is about 7E14 to 8E14 atm/cm 2 . This completes fabrication of a nitrided SiO 2  dielectric. The following steps use the nitrided SiO 2  dielectric as a gate dielectric for a MOSFET.  
         [0027]    In step  170 , a polysilicon layer is formed over the nitrided SiO 2  using one of a number of deposition processes well known in the art, such as LPCVD or RTCVD. The polysilicon layer may be undoped or doped N-type of P-type. In one example, the polysilicon layer is 1000 to 2000 Å thick.  
         [0028]    In step  175 , the MOSFET is essentially completed. The polysilicon layer is etched; for example, by a RIE processes to form a gate, spacers are formed on sidewalls of the gate and source/drains are formed in the substrate on either side of the gate (typically by one or more ion-implantation processes). The SiO x N y  layer is the gate dielectric of the MOSFET. If the polysilicon layer was not doped during deposition, the gate may be doped N-type or P-type after spacer formation by ion implantation in conjunction with the formation of the source/drains or as a separate step.  
         [0029]    [0029]FIG. 7 is a schematic illustration of a decoupled plasma system for performing a nitridation process according to the present invention. In FIG. 7, decoupled plasma tool  180  includes a chamber  185  and a wafer chuck  190  (for holding a wafer  195 ) within the chamber. Radio frequency (RF) coils  200  for generating a plasma  205  surround chamber  185 . Gases for plasma  205  are supplied by inlets  210  in sidewalls  215  of chamber  185 . Chamber  185  also includes a vacuum port  220  in a surface  225  of the chamber.  
         [0030]    In use, wafer  195  having a base SiO 2  layer (not shown) on a top surface  230  of the wafer is placed into chamber  185  from a transfer chamber (not shown), a pre-selected gas mixture (in the present example, He/N 2 ) at a pre-selected flow rate is introduced into the chamber via inlets  210  and the chamber maintained at a pre-selected pressure via a pump attached to vacuum port  220 . A pre-selected wattage of RF power is impressed on RF coils  200  to energize and maintain plasma  205 . After a pre-selected time, the RF power is turned off extinguishing plasma  205 , the gas flow is turned off and chamber  185  is brought up to transfer chamber pressure.  
         [0031]    One example of decoupled plasma system is an AME 5200 DPS system manufactured by Applied Materials Corp, Santa Clara, Calif.  
         [0032]    [0032]FIGS. 8 and 9 are plots of temperature versus time illustrating a spike anneal process according to the present invention. In FIG. 8, a wafer is introduced into the RTA tool at a base temperature “A” and a time “T0.” Between time “T1” and time “T2” the wafer temperature is ramped up from base temperature “A” to maximum spike temperature “B.” The slope of the temperature up ramp (S U ) is given by S U =(B-A)/(T2−T1). Between time “T2” and time “T3” the wafer temperature is maintained at maximum temperature “B.” The time (ΔT) at maximum temperature is given by ΔT=(T3−T2). Between time “T3” and time “T4” the wafer temperature is ramped down from maximum temperature “B” to base temperature “A.” The slope of the temperature down ramp (S D ) is given by S D =(A−B)/(T4−T3). If “A,” “B,” S U  and S D  are held constant and “T3” is set equal to “T2” so DT=0, then the plot of temperature versus time illustrated in FIG. 9 results. In FIG. 9, the wafer is raised to a maximum temperature “B” and is held at the maximum temperature “B” for zero time. FIG:  9  illustrates the “sharpest” spike anneal possible. In one example, base temperature “A” is about 200 to 400° C., maximum temperature “B” is about 1050° C., the slope of the up temperature ramp “S U ” is about 75° C./sec, the slope of the down temperature ramp “S D ” is about −75° C./sec and the time at maximum temperature “DT” is about 0 to 60 seconds.  
         [0033]    [0033]FIG. 10 is a secondary ion mass microscopy (SIMs) profile of a gate dielectric fabricated according to the present invention. The base SiO 2  was 15 Å thick and the resultant SiO x N y  layer is 18 Å thick. In FIG. 10, the SiO x N y /Si interface  300  occurs at 18 Å depth. In FIG. 10, the oxygen concentration ranges from about 2E22 atm/cm 3  at a point  305  which is 3 Å from the true surface  310  of the SiO x N y  layer to a maximum of about 3E22 atm/cm 3  at about 7 Å depth to about 2E22 atm/cm 3  at the SiO x N y /Si interface  300 . In FIG. 10, the nitrogen concentration ranges from about 2E21 atm/cm 3  at point  305  of the SiO x N y  layer to a maximum of about 4E21 atm/cm 3  at 10 Å depth to about 1 E21 atm/cm 3  at the SiO x N y /Si interface  300 . In other SIMs profiles the nitrogen concentration reaches about 5E21 atm/cm 2  and the oxygen concentration 5E22 atm/cm 3 . The nitrogen is not concentrated near the SiO x N y /Si interface  300  but relatively uniformly distributed within the SiO x N y  layer at a concentration of about 1E21 atm/cm 3  to 3.5 E21 atm/cm 3  except for the first 3 Å of depth where the SIMs data is not reliable. The present invention produces a SiO x N y  layer having a relatively uniform nitrogen concentration throughout its thickness which results devices having a lower V T  shift compared to devices having a conventional SiO x N y  layers having high nitrogen concentrations near the SiO x N y /Si interface  300 .  
         [0034]    [0034]FIG. 11 is a plot comparing leakage, mobility and electrical thickness at three steps in the fabrication of a gate dielectric according to the present invention. Leakage current thickness and electrical thickness are plotted on the thickness scale on the left of the plot. Leakage current thickness is defined as the equivalent SiO 2  thickness that would generate the leakage current of the identified dielectric. An increase in leakage current thickness corresponds to a decrease in leakage current. Mobility is plotted on the mobility scale on the right of the plot. Leakage, mobility and electric thickness are plotted for 3 cases, a 15 Å base oxide, the 15 Å base oxide after decoupled plasma nitridation (DPN) and the 15 Å base oxide after decoupled plasma nitridation (DPN) and a spike anneal. The DPN process increases the leakage current thickness from about 13 Å to just under 15 Å. A 2 Å increase corresponds to about a 40 times decrease in leakage current density. The spike anneal has no significant effect on the leakage current. The DPN process decreases the mobility from about 237 cm 2 /volt-second to about 230 cm 2 /volt-second. However, the spike anneal restores the mobility to about 237 cm 2 /volt-second. The DPN process increases the electrical thickness by about 0.5 Å. The electrical thickness is unchanged by the spike anneal. Thus, the mobility problem usually associated with SiO x N y  layers has been overcome by the present invention.  
         [0035]    [0035]FIG. 12 is a plot comparing time to breakdown and charge to breakdown at three steps in the fabrication of a gate dielectric according to the present invention. Time to breakdown and charge to breakdown are plotted for a 15 Å base oxide, the 15 Å base oxide after a DPN and the 15 Å base oxide after a DPN and a spike anneal Two samples are plotted for the 15 Å base oxide, one sample for the 15 Å base oxide after DPN and two samples for the 15 Å base oxide after a DPN and a spike anneal. The time to breakdown is about 120 seconds for a 15 Å base oxide and is about 990 seconds for a 15 Å base oxide after a DPN with or without a spike anneal. The charge to breakdown is about 0.75E5 columbs/cm 2  for a 15 Å base oxide and is about the same for 15 Å base oxide after a DPN with or without a spike anneal. Since for conventional plasma nitridation processes the Q BD  is degraded and the T BD  is unchanged, the present invention demonstrates the required reliability by way of sustaining the Q BD  while increasing the T DB  by about 10 fold.  
         [0036]    The description of the embodiments of the present invention is given above for the understanding of the present invention. It will be understood that the invention is not limited to the particular embodiments described herein, but is capable of various modifications, rearrangements and substitutions as will now become apparent to those skilled in the art without departing from the scope of the invention. Therefore, it is intended that the following claims cover all such modifications and changes as fall within the true spirit and scope of the invention.