Patent Publication Number: US-2005130438-A1

Title: Method of fabricating a dielectric layer for a semiconductor structure

Description:
TECHNICAL FIELD  
      This invention relates generally to the field of integrated circuit fabrication and specifically to a method of fabricating a dielectric layer for a semiconductor structure.  
     BACKGROUND OF THE DISCLOSURE  
      Semiconductor devices typically include a dielectric layer. In some devices, the dielectric layer may comprise an oxynitride layer, which may have a larger dielectric constant than that of a silicon dioxide layer. Known techniques for forming an oxynitride layer involve introducing nitrogen into a dielectric layer. These known techniques, however, typically do not effectively or efficiently introduce nitrogen into the dielectric layer. It may be generally desirable to effectively and efficiently introduce nitrogen into the dielectric layer.  
     SUMMARY OF THE DISCLOSURE  
      In accordance with the present invention, disadvantages and problems associated with previous techniques for fabricating a semiconductor structure may be reduced or eliminated.  
      According to one embodiment of the present invention, fabricating a semiconductor structure includes establishing a non-stoichiometry associated with a dielectric layer, where the degree of non-stoichiometry may correspond to a nitrogen profile of the dielectric layer. Deposition of the dielectric layer outwardly from a substrate is controlled to substantially yield the established non-stoichiometry of the dielectric layer. The dielectric layer typically includes a non-stoichiometric portion. Nitrogen is incorporated into the dielectric layer to substantially yield the nitrogen profile.  
      Certain embodiments of the invention may provide one or more technical advantages. A technical advantage of one embodiment may include forming a dielectric layer by depositing silicon oxide outwardly from a substrate, which may allow for formation at relatively low temperatures. Another technical advantage of one embodiment may be that the relative concentration of silicon and oxygen of the dielectric layer may be controlled to provide for specific nitridation. This may allow for a desired nitrogen profile for the dielectric layer.  
      Certain embodiments of the invention may include none, some, or all of the above technical advantages. One or more other technical advantages may be readily apparent to one skilled in the art from the figures, descriptions, and claims included herein.  
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      For a more complete understanding of the present invention and its features and advantages, reference is now made to the following description, taken in conjunction with the accompanying drawings, in which:  
       FIG. 1  illustrates one embodiment of a substrate of a semiconductor structure;  
       FIG. 2  illustrates one embodiment of a dielectric layer deposited outwardly from the substrate;  
       FIG. 3  illustrates example timing diagrams for depositing the dielectric layer;  
       FIG. 4  illustrates one embodiment of nitridation of the dielectric layer; and  
       FIG. 5  is a flowchart illustrating one embodiment of a method of fabricating a semiconductor structure.  
    
    
     DETAILED DESCRIPTION OF THE DRAWINGS  
      Embodiments of the present invention and its advantages are best understood by referring to  FIGS. 1 through 5  of the drawings, like numerals being used for like and corresponding parts of the various drawings.  
       FIGS. 1, 2 , and  4  are a series of schematic cross-sectional diagrams illustrating one embodiment of a method of fabricating a semiconductor structure  10 . Semiconductor structure  10  may be formed for various purposes, for example, for use in connection with a transistor of a p-channel metal oxide semiconductor (PMOS) or an n-channel metal oxide semiconductor (NMOS) device.  
       FIG. 1  illustrates a substrate  12  of semiconductor structure  10 . Substrate  12  may provide, for example, a transistor channel for a transistor, and may comprise silicon or any other suitable semiconductive material. Semiconductor structure  10  may be placed in a chamber during formation.  
       FIG. 2  illustrates a dielectric layer  14  deposited outwardly from substrate  12 . Dielectric layer  14  may provide, for example, a transistor gate insulator for a transistor. Dielectric layer  14  comprises dielectric material and may have any suitable thickness such as between one monolayer to 25 Angstroms, for example, 20 Angstroms. Dielectric layer  14  may comprise a non-stoichiometric portion  16   a  and a stoichiometric portion  16   b . The stoichiometry of dielectric layer  14  may refer to the relative proportion of elements of dielectric layer  14 . As an example, the composition may refer to the proportion of silicon relative to oxygen.  
      Stoichiometric portion  16   b  of dielectric layer  14  may comprise stoichiometric silicon dioxide (SiO 2 ) material. Stoichiometric portion  16   b  may be referred to as stoichiometric because the interface trap density D it  of SiO 2  is relatively low, typically approximately 10 10 /cm2. Non-stoichiometric portion  16   a  may comprise non-stoichiometric silicon oxide (Si z O y ) material, where variables z and y represent the proportion of silicon relative to oxygen and have any suitable values. Non-stoichiometric portion  16   a  may be referred to as non-stoichiometric if the interface trap density D it  of Si z O y  is greater than the interface trap density D it  of SiO 2 .  
      The interface trap density D it  is typically affected by the proportion of silicon relative to oxygen. The interface trap density D it  of Si z O y  may be greater when Si z O y  is silicon-rich as compared to SiO 2 , that is, Si z O y  has excess silicon relative to oxygen, as compared to SiO 2 . According to one embodiment, variables z and y of Si z O y  may have values where the ratio of y/z is less than two. These values indicate that on average throughout the Si z O y  material, certain molecules having an individual silicon atom have only one bonded oxygen atom rather than two atoms as is the case for the SiO 2  material.  
      Non-stoichiometric portion  16   a  may comprise any suitable portion of dielectric layer  14 . For example, non-stoichiometric portion  16   a  may comprise a substantial portion of dielectric layer  14  proximate to substrate  12 . Non-stoichiometric portion  16   a  may comprise at least 45% of the total thickness of dielectric layer  14 . For example if dielectric layer  14  has a thickness of 20 Angstroms, non-stoichiometric portion  16   a  may have a thickness greater than two or three monolayers. As another example, non-stoichiometric portion  16   a  may be thicker than stoichiometric portion  16   b , and may comprise at least 80% of the total thickness of dielectric layer  14 . According to one embodiment, stoichiometric portion  16   b  may be as thin as possible or eliminated altogether.  
      The stoichiometry of dielectric layer  14  affects the nitridation of dielectric layer  14  that may be performed to form an oxynitride layer. Typically, more nitrogen may be introduced in non-stoichiometric layer  16   b  than in stoichiometric layer  16   a . Accordingly, the stoichiometry may be adjusted to achieve nitridation of dielectric layer  14  for a desired nitrogen profile. A nitrogen profile may refer to the distribution of nitrogen within a material. The stoichiometry may be controlled in any suitable manner. As an example, dielectric layer  14  may be deposited in a controlled manner in order to achieve a specific stoichiometry.  
      Dielectric layer  14  may be deposited outwardly from substrate  12  according to any suitable process, for example, a chemical vapor deposition (CVD) process or an atomic layer deposition (ALD). According to the chemical vapor deposition process, substrate  12  is exposed to an appropriate precursor such as SiH 4 , Si[N (C 2 H 5 ) 2 ]  4 , Si[N(CH 3 ) 2 ] 4 , Si(OC 2 H 5 ) 4 , Si 2 H 6 , H 2 SiCl 2 , SiCl 4 , or other suitable precursor and an oxidizer such as O 2 , NO 2 , NO, O 3 , oxygen plasma, or other oxidizer to deposit a silicon dioxide film of the desired stoichiometry outwardly from substrate  12 .  
      According to the atomic layer deposition process, substrate  12  is exposed to a silicon precursor such as SiH 4 , Si[N(C 2 H 5 ) 2 ] 4 , Si[N(CH 3 ) 2 ] 4 , Si(OC 2 H 5 ) 4 , Si 2 H 6 , H 2 SiCl 2 , SiCl 4 , or other suitable precursor and an oxidizer such as O 2 , N 2 O, NO, H 2 O, O 3 , oxygen plasma, or other suitable oxidizer in an alternating fashion. For example, substrate  12  with appropriate surface preparation may be exposed to a gas flow comprising a silicon precursor introduced into the chamber during a first pulse, and then a gas flow comprising an oxidizer introduced during a second pulse. A pulse refers to the act of exposing substrate  12  to a gas flow. An oxidizer may comprise, for example, oxygen (O 2 ), ozone (O 3 ), other oxidizer, or any combination of the preceding. The silicon carrier may comprise, for example, SiH 4 , Si[N(C 2 H 5 ) 2 ] 4 , Si[N(CH 3 ) 2 ] 4 , Si(OC 2 H 5 ) 4 , Si 2 H 6 , H 2 SiCl 2 , SiCl 4 , or other suitable precursor.  
      The duration of an individual pulse or a sequence of pulses, the gas flow, or both the duration and gas flow may be adjusted to deposit a specific amount of atoms outwardly from the surface. The pressure and flow rate may be adjusted to achieve the appropriate film uniformity deposited outwardly from substrate  12 .  
      The amount of atoms deposited outwardly from substrate  12  by a gas flow may be estimated. A gas flow may deposit a certain concentration of atoms at a specific gas flow rate and pressure that deposits a certain amount of atoms during a unit time. Given the deposited amount per unit time and the duration of a pulse or sequence of pulses, the amount deposited during the pulse or sequence of pulses may be estimated. Accordingly, the amount of oxygen and silicon deposited may be controlled by adjusting the duration, number, or both duration and number of pulses. Example timing diagrams for pulses of silicon and oxygen are described in more detail with reference to  FIG. 3 .  
       FIG. 3  illustrates example timing diagrams  30  for pulses of silicon and oxygen. Timing diagram  30   a  illustrates controlling the proportion of silicon relative to oxygen by adjusting the number of pulses. Timing diagram  30   a  includes silicon pulses  32   a , an oxidizer pulse  36   a , and resting time  38   a . Silicon pulses  32   a  represent pulses for depositing silicon, and may have a duration of 0.1 to 60 seconds for a gas flow of 10 sccm to 500 sccm of precursor. Oxidizer pulse  36   a  represents a pulse for depositing oxygen, and may have a duration of 0.1 to 100 seconds for a gas flow of 0.1 sccm to 10,000 sccm. Resting time  38   a  represents a time during which there is no pulse.  
      According to the illustrated embodiment, three silicon pulses  32   a  and one oxidizer pulse  36   a  may be used to deposit a specific proportion of silicon relative to oxygen. Any suitable number of silicon pulses  32   a , any suitable number of oxidizer pulses  36   a , or any sequence of silicon pulses  32   a  and oxidizer pulses  36   a , however, may be used to achieve a desired proportion of silicon relative to oxygen.  
      Timing diagram  30   b  illustrates controlling the proportion of silicon relative to oxygen by adjusting the duration of a silicon pulse  32   b  and an oxidizer pulse  36   b . According to the illustrated embodiment, silicon pulse  32   b  has a longer duration than oxidizer pulse  36   b , which may result in a higher proportion of silicon atoms. Silicon pulse  32   b  may have a duration of five seconds for a gas flow of 100 sccm, and oxidizer pulse  36   b  may have a duration of one second for a gas flow of 1000 sccm. Silicon pulse  32   b  and oxidizer pulse  36   b , however, may have any suitable duration in order to yield a specific proportion of silicon relative to oxygen.  
      Referring back to  FIG. 2 , dielectric layer  14  may be formed such that nitrogen may be substantially uniformly introduced into dielectric layer  14 . According to one embodiment, the composition of dielectric  14  may be controlled in order to yield a specific nitrogen profile and concentration. In some embodiments, non-stoichiometric portion  16   a  may result in a relatively high concentration of nitrogen.  
       FIG. 4  illustrates nitridation  18  of dielectric layer  14 . According to one embodiment, a gate structure may be formed outwardly from dielectric layer  14 . An electrical bias applied to the gate structure may induce a controlled electrical conduction path through a transistor channel of substrate  12 . In this embodiment, nitrogen may be introduced into dielectric layer  14  to convert the layer into silicon oxynitride (SiON), which may provide a larger dielectric constant relative to silicon dioxide layers. According to one embodiment, nitrogen may be incorporated into non-stoichiometric portion  16   a  in a substantially uniform manner with little or no nitrogen reaching substrate  12 .  
      Nitrogen may be introduced into dielectric layer  14  in any suitable manner such as by remote plasma nitridation, immersion plasma nitridation, or thermal nitridation. Plasma nitridation refers to exposing dielectric layer  14  to a nitrogen plasma. The nitrogen plasma includes a nitrogen source such as N 2  and one or more inert gases such as helium, argon, or xenon. Remote plasma nitridation may be performed by forming the plasma in an area away from semiconductor structure  10 . Immersion plasma nitridation may be performed by forming the plasma in the same chamber that houses semiconductor structure  10 .  
      Thermal nitridation may be performed by introducing nitrogen during growth at high temperatures. The nitrogen may be introduced using a primary nitrogen source and a diluent. The primary nitrogen source may comprise, for example, ammonia (NH 3 ) , nitric oxide (NO), or nitrous oxide (N 2 O). The diluent may comprise, for example, nitrogen (N 2 ), helium (He), or argon (Ar). Based on various criteria, one skilled in the art may select an appropriate range for each of time, temperature, and pressure. For example, 60 minutes at greater than 500 degrees Celsius such as 1000 degrees Celsius in one atmosphere of NH 3 . The high temperatures may be achieved by any suitable process, such as by using either a rapid thermal process or a furnace.  
      Alterations or permutations such as modifications, additions, or omissions may be made to semiconductor structure  10  without departing from the scope of the invention. Semiconductor structure  10  may have more, fewer, or other features.  
       FIG. 5  is a flowchart illustrating one embodiment of a method of fabricating semiconductor structure  10 . Semiconductor structure  10  may be formed for various purposes, for example, for use in connection with a transistor. The method begins at step  100 , where substrate  12  is provided. Substrate  12  may provide, for example, a transistor channel for a transistor, and may comprise silicon or any other suitable semiconductive material.  
      Dielectric layer  14  is deposited outwardly from substrate  12  at step  104 . Dielectric layer  14  may provide, for example, a transistor gate insulator for a transistor. Dielectric layer  14  may be deposited in a controlled manner in order to achieve a specific non-stoichiometry, which may affect the nitridation of dielectric layer  14 . Dielectric layer may be deposited by chemical vapor deposition, atomic layer deposition, or other suitable process.  
      Nitridation is performed at step  108 . Nitrogen may be introduced into dielectric layer  14  to convert the layer to silicon oxynitride (SiON). Oxynitride material may provide a larger dielectric constant relative to silicon dioxide material. According to one embodiment, nitrogen may be incorporated into dielectric layer  14  in a substantially uniform manner with little or no nitrogen reaching substrate  12 .  
      Structure  10  is annealed at step  112 . Annealing may be performed in either an inert or oxidizing environment, where an inert ambient may comprise, for example, He, Ar, or N and an oxidizing ambient may comprise any suitable mixture including oxygen. The anneal may be performed under any suitable conditions. For example, the temperature may be in a range of 600 to 1100 degrees Celsius, the pressure may be in a range of one milliTorr to one atmosphere, and time may be in a range of one second to ten minutes. According to one embodiment, dielectric layer  14  may be annealed at a temperature greater than 650 degrees Celsius in a non-oxidizing ambient, and then annealed at a temperature less than 950 degrees Celsius in an oxidizing ambient.  
      Post-processing is performed at step  114 . Post-processing steps may be ascertained by one skilled in the art according to various criteria relating to structure  10  as well as the implementation of structure  10 . For example, if structure  10  is to be used as a transistor gate insulator, then other known transistor fabrication steps may be taken. For example, a gate conductive layer may be formed over the oxynitride and etched to form a gate stack. Either before or after the formation of the gate stack, implants may be formed in substrate  10  such as to form the transistor source and drain, and still others related regions and connections may be formed. After performing post-processing, the method terminates.  
      Alterations or permutations such as modifications, additions, or omissions may be made to the method without departing from the scope of the invention. The method may include more, fewer, or other steps. For example, annealing structure  10  at step  112  may be omitted. Additionally, steps may be performed in any suitable order without departing from the scope of the invention.  
      Certain embodiments of the invention may provide one or more technical advantages. A technical advantage of one embodiment may include forming a dielectric layer by depositing silicon oxide outwardly from a substrate, which may allow for formation at relatively low temperatures. Another technical advantage of one embodiment may be that the relative concentration of silicon and oxygen of the dielectric layer may be controlled to provide for specific nitridation. This may allow for a desired nitrogen profile for the dielectric layer.  
      Although an embodiment of the invention and its advantages are described in detail, a person skilled in the art could make various alterations, additions, and omissions without departing from the spirit and scope of the present invention as defined by the appended claims.