Abstract:
Methods of forming hafnium oxide, zirconium oxide and nanolaminates of hafnium oxide and zirconium oxide are provided. These methods utilize atomic layer deposition techniques incorporating nitrate-based precursors, such as hafnium nitrate and zirconium nitrate. The use of these nitrate based precursors is well suited to forming high dielectric constant materials on hydrogen passivated silicon surfaces.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates generally to integrated circuit (IC) fabrication processes and, more particularly, to methods of forming high dielectric constant materials on silicon. 
     Current Si VLSI technology uses SiO 2  as the gate dielectric in MOS devices. As device dimensions continue to scale down, the thickness of the SiO 2  layer must also decrease to maintain the same capacitance between the gate and channel regions. Thicknesses of less than 2 nanometers (nm) are expected in the future. However, the occurrence of high tunneling current through such thin layers of SiO 2  requires that alternate materials be considered. Materials with high dielectric constants would permit gate dielectric layers to be made thicker, and so alleviate the tunneling current problem. These so-called high-k dielectric films are defined herein as having a high dielectric constant relative to silicon dioxide. Typically, silicon dioxide has a dielectric constant of approximately 4, while it would be desirable to use a gate dielectric material with a dielectric constant of greater than approximately 10. 
     Because of high direct tunneling currents, SiO 2  films thinner than 1.5 nm generally cannot be used as the gate dielectric in CMOS devices. There are currently intense efforts in the search for the replacement of SiO 2 , with TiO 2  and Ta 2 O 5  attracting the greatest attention. However, high temperature post deposition annealing, and the formation of an interfacial SiO 2  layer, make achieving equivalent SiO 2  thicknesses, also known as equivalent oxide thickness (EOT), of less than 1.5 nm very difficult. An EOT of about 1.0 nm, and below, is expected to be used for the 0.07 micrometer device generation. 
     Materials such as hafnium oxide (HfO 2 ) and zirconium oxide (ZrO 2 ) are leading candidates for high-k dielectric materials. The dielectric constant of these materials is about 20 to 25, which is a factor of 5-6 times that of silicon dioxide, meaning that a thickness of about 5-6 nm of these materials could be used to achieve an EOT of about 1.0 nm, assuming that the entire film is essentially composed of the high-k material. One problem with using high-k materials is that an interfacial layer of silicon dioxide, or a silicate layer, with a lower dielectric constant forms during standard processing. 
     Deposition of ZrO 2 , or HfO 2 , using atomic layer deposition (ALD) and tetrachloride precursors has been reported. Substrates heated to between 300° C. and 400° C. have been exposed to ZrCl 4 , or HfCl 4 , precursors alternating with water vapor in an attempt to form ZrO 2  or HfO 2  films respectively. However, it is difficult to initiate deposition on hydrogen terminated silicon surfaces. Hydrogen terminated silicon surfaces result from standard industry cleaning processes. These standard cleaning processes, which are often referred to as HF last clean, typically end in a quick dip of HF. This produces surfaces which are hydrogen terminated, also known as hydrogen passivated. With sufficient exposure of the silicon surface to the reactants, the deposition may eventually be initiated. But, this results in films that are rough with poor uniformity. Another problem with tetrachloride precursors is the incorporation of residual chlorine in the film. The chlorine impurities can result in long term reliability and performance issues. 
     Other precursors use Hf or Zr metal combined with organic ligands such as iso-propoxide, TMHD (2,2,6,6-tetrmethyl-3,5-heptanedionate), or combinations of organic ligands with chlorine. These precursors also have a problem initiating the film deposition on hydrogen terminated silicon surfaces and will incorporate carbon residues in the film. Large ligands may also take up enough space that steric hindrance will prevent the deposition of a uniform monolayer. Up until now, the successful implementation of ALD Zr and Hf oxides have been either on an initial layer of silicon oxide, silicon oxynitride, or in the form of a reduced dielectric constant silicate film, such as ZrSiO 4  or HfSiO 4 . These initial layers may contribute significantly to the overall EOT. 
     SUMMARY OF THE INVENTION 
     Accordingly, a method of forming high dielectric constant materials, ZrO 2  or HfO 2 , is provided. The methods are well suited to forming high dielectric constant materials on hydrogen terminated silicon surfaces, however the methods can be also used to form these materials on a variety of substrates. 
     A method is provided for forming zirconium oxide on a substrate comprises providing a semiconductor substrate within an atomic layer deposition chamber. Heating the substrate to a temperature within the atomic layer deposition regime. Introducing anhydrous zirconium nitrate into the chamber. Purging the chamber with nitrogen. And, introducing water vapor into the chamber, whereby a monolayer of zirconium oxide is deposited. The steps of introducing of anhydrous zirconium nitrate, purging the chamber with nitrogen, and introducing water vapor may each be repeated as necessary to produce a zirconium oxide film of the desired thickness. 
     A method is provided for forming hafnium oxide on a substrate comprises providing a semiconductor substrate within an atomic layer deposition chamber. Heating the substrate to a temperature within the atomic layer deposition regime. Introducing anhydrous hafnium nitrate into the chamber. Purging the chamber with nitrogen. And, introducing water vapor into the chamber, whereby a monolayer of hafnium oxide is deposited. The steps of introducing of anhydrous hafnium nitrate, purging the chamber with nitrogen, and introducing water vapor may each be repeated as necessary to produce a hafnium oxide film of the desired thickness. 
     A method is provided for forming a nanolaminate, which comprises hafnium oxide and zirconium oxide. The method comprises repeating the steps mentioned above with regard to forming zirconium oxide, and repeating the steps mentioned above with regard to forming hafnium oxide, and alternating these steps as desired to produce a nanolaminate, such as HfO 2 /ZrO 2 /HfO 2 /ZrO 2 . 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a flow chart of a process for depositing HfO 2  or ZrO 2 . 
     FIG. 2 is a flow chart of a process for depositing a nanolaminate of HfO 2  and ZrO 2 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 shows a flow chart illustrating the steps of a process of depositing a film of HfO 2  or ZrO 2 . Step  110  provides a semiconductor substrate within an ALD chamber. Commercial ALD tools are now becoming available. Microchemistry. Ltd of Finland (now a division of ASM) manufactures an ALD tool, Model F120, that may be used in connection with the process described herein. In a preferred embodiment, the semiconductor substrate has a silicon surface that is hydrogen terminated. Although the process described herein is well suited to solving the problem of depositing HfO 2  or ZrO 2  on hydrogen terminated silicon surfaces, it is entirely possible to use this process to deposit HfO 2  or ZrO 2  on other surface including, silicon dioxide, silicon oxynitride, silicon germanium, and on silicates, such as ZrSiO 4  and HfSiO 4 . 
     The semiconductor substrate is heated to a temperature for an atomic layer deposition regime. For example, a hydrogen passivated silicon surface was found to be within the atomic layer deposition regime when using anhydrous hafnium nitrate at temperature of approximately 160 to 200° C. 
     Step  120  introduces anhydrous hafnium nitrate (Hf(NO 3 ) 4 ), or anhydrous zirconium nitrate (Zr(NO 3 ) 4 ) into the ALD chamber. The hafnium nitrate, or zirconium nitrate, adsorbs onto the semiconductor substrate surface, even if the substrate surface is hydrogen terminated silicon. 
     Although anhydrous hafnium nitrate and anhydrous zirconium nitrate are not currently commercially available, synthesis and purification techniques for these materials are known. Synthesis of zirconium nitrate was reported in 1962. Due to the similarities between hafnium and zirconium, hafnium nitrate may also be isolated through a similar synthesis process. Hafnium nitrate may be prepared by refluxing hafnium tetrachloride over dinitrogen pentoxide at 30° C., and then purified by sublimation at 100° C./0.1 mmHg for hafnium nitrate. Zirconium nitrate can be similarly purified at 95° C./0.1 mmHg. 
     Step  130  purges the ALD chamber with nitrogen or an inert gas, such as argon, helium or neon, to reduce, or eliminate, any excess anhydrous hafnium nitrate, or anhydrous zirconium nitrate, or undesirable reactants. 
     Step  140  introduces a hydrating gas into the ALD chamber. The hydrating gas provides hydrogen to facilitate removal of nitrogen, including nitrates and nitrogen dioxide. The hydrating gas assists in removing NO 3  ligands, either in the form of NO 3 , or as NO 2  with oxygen atom being used to form a hafnium oxide, or zirconium oxide, film. The hydrating gas may be water vapor, methanol or hydrogen. The exact chemical mechanism is not fully understood, and does not limit the scope of any claim. 
     Step  145  purges the ALD chamber with nitrogen, or an inert gas, to reduce, or eliminate, the hydrating gas and possible undesired reactants within the chamber. 
     Step  150  illustrates the repetition of steps  120 ,  130   140  and  145  to produce a film of the desired thickness. The ALD process is inherently growth rate limited by the number of cycles of alternate exposure to the nitrate, hafnium nitrate or zirconium nitrate, and hydrating gas, with appropriate purging. Step  160  anneals the film to condition the film following completion of the desired number of cycles. 
     For example, a hafnium oxide film was formed on a silicon substrate with a hydrogen terminated silicon surface by placing the substrate into the ALD chamber at 10 millitorr and heating the substrate to approximately 180° C. The substrate was processed using multiple ALD cycles. Each ALD cycle comprised introducing anhydrous hafnium nitrate, purging with nitrogen and introducing water vapor. Samples were produced using approximately 7 cycles, 13 cycles, 17 cycles and 400 cycles. 
     The thickness of each sample was measured using a spectroscopic ellipsometer. The 400-cycle sample had a measure thickness of 128.1 nm, which corresponds to a deposition rate of approximately 3.2 Å/cycle. On the thinner sample, the deposition rate was 3.6 Å/cycle. Considering that the bulk density of hafnium oxide is listed at 9.68 g/cm 3 , the volume of one molecule is 36.1 Å, one monolayer would be expected to be approximately 3.3 Å thick. Accordingly, a deposition rate of between 3.2 Å/cycle and 3.6 Å/cycle corresponds well to a deposition of one monolayer per cycle. It was also determined that the deposition rate is temperature sensitive. Samples run at 170° C. resulted in a deposition rate of 2.8 Å/cycle. 
     Referring now to FIG. 2, a flow chart is shown for producing a nanolaminate, or layered film, comprising layers of hafnium oxide and zirconium oxide. Step  210  provides a semiconductor substrate within an ALD chamber. The semiconductor substrate is heated to a temperature for an atomic layer deposition regime. 
     Step  220  introduces either anhydrous hafnium nitrate (Hf(NO 3 ) 4 ), or anhydrous zirconium nitrate (Zr(NO 3 ) 4 ) into the ALD chamber. Either the hafnium nitrate, or zirconium nitrate, whichever is introduced in this step  220  adsorbs onto the semiconductor substrate surface. 
     Step  230  purges the ALD chamber with nitrogen or an inert gas to reduce, or eliminate, any excess anhydrous hafnium nitrate, or anhydrous zirconium nitrate, or undesirable reactants. 
     Step  240  introduces a hydrating gas into the ALD chamber. The hydrating gas assists in removing NO 3  ligands, either in the form of NO 3 , or as NO 2  with oxygen atom being used to form a hafnium oxide film, or zirconium oxide film. 
     Step  245  purges the ALD chamber with nitrogen or inert gas to reduce, or eliminate, the hydrating gas and possible undesired reactants within the chamber. 
     Step  250  illustrates the repetition of steps  220 ,  230   240  and  245  to produce a layer of material, either hafnium oxide or zirconium oxide, of a desired thickness of the first nitrate. The ALD process is inherently growth rate limited by the number of cycles of alternate exposure to the nitrate, hafnium nitrate or zirconium nitrate, and hydrating gas, with appropriate purging. By repeating through the cycles indicated by step  250  the desired thickness of each layer of material, either hafnium oxide, or zirconium oxide, can be formed. 
     Step  320  introduces which ever of anhydrous hafnium nitrate (Hf(NO 3 ) 4 ), or anhydrous zirconium nitrate (Zr(NO 3 ) 4 ) that was not introduced in step  220  into the ALD chamber. Either the hafnium nitrate, or the zirconium nitrate, whichever is introduced in this step  320  adsorbs onto the semiconductor substrate surface. 
     Step  330  purges the ALD chamber with nitrogen or an inert gas to reduce, or eliminate, any excess anhydrous hafnium nitrate, or anhydrous zirconium nitrate, or undesirable reactants. 
     Step  340  introduces hydrating gas into the ALD chamber. The hydrating gas assists in removing NO 3  ligands, either in the form of NO 3 , or as NO 2  with oxygen atom being used to form either a film of hafnium oxide, or a film of zirconium oxide, whichever was not formed in step  240 . 
     Step  345  purges the ALD chamber with nitrogen or inert gas to reduce, or eliminate, the hydrating gas and possible undesired reactants within the chamber. 
     Step  350  illustrates the repetition of steps  320 ,  330   340  and  345  to produce a layer of material, either hafnium oxide or zirconium oxide, of the desired thickness. In addition, step  350  illustrates the repetition of steps starting again at  220 . This allows a film having multiple alternating layers to be formed for example HfO 2 /ZrO 2 /HfO 2 /ZrO 2  or ZrO 2 /HfO 2 /ZrO 2 /HfO 2 /ZrO 2  with the thickness of each individual layer being separately determined, as well as the overall thickness. 
     Step  360  anneals the film to condition the film and any interfaces between the layers of materials following completion of the desired number of cycles, and sub-cycles.