Abstract:
A process for forming a nanocrystal nanostructure is repeated for growing the nanostructure disposed on an electron beam resist layer that is disposed on a substrate for forming an electron beam shadowmask from the nanostructure on the electron beam resist layer prior to electron beam exposure for patterning the electron beam resist layer in advance of subsequent processing steps. The nanocrystals are semiconductor materials and metals such as silver. The nanostructure enable the creation of ultra-fine nanometer sized electron beam patterned structures for use in the manufacture of submicron devices such as submicron-sized semiconductors and microelectromechanical devices.

Description:
STATEMENT OF GOVERNMENT INTEREST 
     The invention was made with Government support under contract No. F04701-93-C-0094 by the Department of the Air Force. The Government has certain rights in the invention. 
    
    
     FIELD OF THE INVENTION 
     The invention relates to the field of electron beam lithography. More particularly, the present invention relates to the formation of nanostructures using nanocrystals forming shadowmasks for masking electron beam radiation during electron beam lithography. 
     BACKGROUND OF THE INVENTION 
     Low dimensional semiconductor nanostructures are used in electronic, optoelectronic and magnetoelectronic devices. To fabricate semiconductor structures in the nanometer range, it is necessary to develop lithographic techniques with nanometer-scale resolution. Much effort has been made in fabricating nanometer-scale structures using high-energy, highly defined sources such as electron beams and X-rays. The proximity effect, caused by the high-energy sources, limits the resolution of lithography. Scanning techniques with a low energy source, such as scanning tunneling microscope lithography, are not suitable for high throughput. Self-assembly techniques may be utilized to produce shadowmasks that are characterized by periodic, nanometer-scale patterns. Block co-polymers, for example, will phase separate into structures with nanometer-scale periodicities. However, these organic materials result in poor contrasting shadowmasks. Another option is to use higher-contrast inorganic systems. Organically functionalized metal and semiconductor nanocrystals in the two to one hundred nanometer range can assemble into a variety of organized structures, including lamellar wire-like phases. These structures usually consist of self-oriented high-aspect ratio nanocrystals that can be transferred as a Langmuir-Schaeffer film during a Langmuir-Blodgett lift-off process of the film onto substrates of virtually any size. The Langmuir-Blodgett lift-off process has been used to create nanostructures on a substrate bonded to a transfer tool. The Langmuir-Blodgett lift-off process has not been used to create nano sized semiconductor devices. 
     Nanometer sized film structures, such as strands and dots have been created. These strands and dots have been used for testing predictions of quantum confinement and reduced dimensionality as potential building blocks for nanostructure materials. Nanostructure materials have not been used to create nano size semiconductor devices in the electronics industry. In particular, semimetallic bismuth with very small effective mass and high carrier mobilities is a suitable material for studying quantum-confinement effects in one-dimensional systems and is a promising material for thermoelectric applications. However, semimetallic and silicon based materials have not used to form semiconductor devices on substrates during convention photolithographic processing. These and other disadvantages are solved or reduced using the invention. 
     SUMMARY OF THE INVENTION 
     An object of the invention is to provide a method for creating nano size semiconductor structures. 
     Another object of the invention is to provide a method creating nano size electron beam lithography shadowmasks. 
     Yet another object of the invention is to provide a method creating nano size etch masks using electron beam lithography. 
     The method is used for fabricating precisely defined nanometer scale photoresist patterns and semiconductor devices. The Langmuir process is utilized to form high aspect ratio lamellae or wire-patterns of silver nanocrystals on the surface of water. The patterns are transferred onto resist-coated substrates as a Langmuir-Schaeffer for producing a shadowmask. The nanostructure patterns are transferred to the photoresist material by spatially selective electron beam exposure on the silver nanocrystal nanostructure shadowmask. The invention forms nanocrystal structures as resist shadowmasks having a predetermined patterns for blocking electron beam exposures. The combined use of low energy electron beam exposure and self-assembled nanocrystal shadowmasks provide a low-cost fabrication technique for forming semiconductor nanometer scale nanostructures. 
     The low cost and high resolution exposure shadowmask is suitable for low energy electron beam lithography. Nanocrystal assembled dot and wire nanostructures can be used as a shadowmask during low energy electron beam lithography. Oriented nanometer-size dot and strands nanostructures are readily transferred onto a wafer that may be several inches in diameter. Nanocrystal based nanostructures including strands and dots are useful in the processing of nano sized semiconductor devices. The method generates oriented continuous self-assembled nanostructures of semiconductor or metallic materials for used in semiconductor device fabrication. Nanocrystals linked with ligands, such as thiol tails, are fabricated using an organically functionalized nanocrystal solution. Arrays of dots and strands form spontaneously on the surface of water in a Langmuir-Blodgett trough when the nanocrystals in solution are dropped onto the surface of the water in the trough. The nanocrystal structures are then transferred onto electron-beam sensitive photoresist coated substrates by the Langmuir-Blodgett lift-off process, with the modification of firstly depositing an electron beam resist on the substrate. To prepare a thick nanocrystal shadowmask, multiple layers of nanocrystals are added to the previously deposited layer to increase the size of the nanostructure. The nanocrystal patterns are transferred to the photoresist film during electron beam exposure. Spatially selective electron beam exposure on the nanocrystal shadowmask serves to selectively expose the resist layer. Developing the exposed resist layer results in a resist patterned etch mask suitable for further processing during the fabrication of nano sized semiconductor devices. Using the nanocrystal shadowmask, a 50 nm size polymethyl methacrylate (PPMA) nano sized etch mask can be produced for creating a similarly sized device such as a quantum wire or nano wire using reactive ion etching. Very small 15 nm size PMMA dot patterns can also be formed by the method. The PMMA resist pattern etch mask is obtained using the nanocrystal shadowmask. The etch mask is used masking the reactive ion etching or other processes step during further processing of the substrate. The resist etch mask can be used to pattern the substrates, for example, a silicon substrate etch by a subsequent anisotropic reactive ion etching process that is used to form nanowires under the etch mask. 
     Hence, the nanocrystal strand nanostructure form predetermined patterns that are effectively transferred onto the PMMA coated silicon wafer for patterning the substrate with nanometer scale resolution. The low energy electron beam exposes negative portions of the nanocrystal shadowmask on the resist layer on the substrate. After exposing the resist layer, the resist is then developed for removing exposed portions of the resist to expose a portion of the substrate for reactive ion etching. The method allows for the creation of nano sized structures in a semiconductor substrate for forming nano sized devices. The method uses low energy electron beam exposure that reduces the proximity effects. The method is suitable for low cost and high throughput fabrication of semiconductor nanometer scaled structures and devices. These and other advantages will become more apparent from the following detailed description of the preferred embodiment. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagram depicting a modified Langmuir-Blodgett lift-off process for forming nanocrystal structures, the modification being the addition of the resist layer upon which is transferred the nanostructures. 
     FIG. 2 is a diagram depicting transfer of a nanocrystal nanostructure onto an electron beam resist layer disposed on a substrate. 
     FIG. 3 is a diagram showing growth of the nanostructure during repeated transfers of nanocrystal nanostructure. 
     FIG. 4 is a diagram depicting Electron beam exposure during an electron beam lithography process step to form a patterned layer of electron beam resist. 
     FIG. 5 is a diagram depicting electron beam development during an electron beam lithography process step to form a patterned layer of electron beam resist. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     An embodiment of the invention is described with reference to the figures using reference designations as shown in the figures. Referring to all of the Figures, a Langmuir-Blodgett trough  10  is filled with water  12  and a nanocrystal solution  14  is dropped into the water  12  to form nanocrystal film  16 . Left pressure bar  18   a  and right pressure bar  18   b  are pushed together  19   a  and  19   b  to cause the film to condense into the nanocrystal film  16 . A transfer tool  20  having a substrate  22  is brought down upon the film  16  between the left and right pressure bars  18   a  and  18   b . The film  16  comprises nanocrystal particles that are oriented in a predetermined pattern, for examples, in arrays of dots and strands. In the preferred from of the method, the nanocrystal particles form parallel elongated strands. When the substrate  22  makes contact  25  with the nanocrystal film  16 , the nanocrystal film  16  adheres to the substrate  22  forming nanocrystal strands  26   a  and  26   b  and are lifted up and removed from the surface of the water  12  when the substrate  22  is withdrawn up and away from the water  12  out of the trough  10 . Prior to transferring  25  the nanocrystal particles onto the substrate  22 , a resist layer  24  is deposited on the substrate  22 . The resist layer  24  is preferably an electron beam resist layer. The nanocrystal strands  26   a  and  26   b  are transferred  25  as aligned in the predetermined shadowmask pattern of the film  16 . In the preferred form, the nanocrystal patterns are elongated wire-like strands  26   a  and  26   b  running longitudinally along the surface of electron beam resist layer  24 . The transferred tool  20  is repeated lowered  25  down into the trough  10  a plurality of times to repeatedly make contact  25  with the film  16 . Each time the tool  20  is lowered  25 , more of the nanocrystal particles adhere to the existing strands  26   a  and  26   b  causing the strands  26   a  and  26   b  to dimensionally grow in size as enlarged strands  28   a  and  28   b . The nanocrystal particles have linking thiols that are reduced by heating  27  to a temperature between 100° C. and 150° C. The Thiol reduction  27  reduces the distance between the enlarged strands  28   a  and  28   b  as a reduction of line spacing of the shadowmask. After growing the enlarged strands  28   a  and  28   b  to a desired thickness in diameter, the substrate  22  is removed from the transferred tool  20 . The enlarged strands  28   a  and  28   b  are exposed by low energy electron beam illumination  29 , for example, between at 500 V to 700 V. During illumination  29 , the enlarged strands  28   a  and  28   b  block the electron beam radiation from illuminating the resist  24  on the substrate  22  under the enlarged strands  28   a  and  28   b  functioning as an electron beam shadowmask. The electron beam illumination  29  of the electron beam resist  24  serves to break chemical bonds of the resist  24 . A developer rinse  30  is then applied to the illuminated resist  24  for removing exposed illuminated portions of the resist  24  to form a resist pattern comprising resist traces  32   a  and  32   b  that then function as an etch mask. Those skilled in the art of semiconductor processing are well adept at applying electron beam illumination  29  and developer rise  30 . Reactive ion etching  34  removes exposed substrate portions  35   a ,  35   b  and  35   c  to thereby pattern the substrate for further processing. The electron beam resist etch mask  32   a  and  32   b  is then removed using a conventional chemical cleaning  36  thereby creating a patterned substrate  22  having elevated portions that function as elongated quantum wires or nanowires  37   a  and  37   b  as exemplar but simple semiconductor devices. The patterned substrate  22  can be further processed to use these nanowires  37   a  and  37   b . For example, electrical contacts can be deposited  38  for interconnecting the nanowires  37   a  and  37   b . It should now be apparent that other conventional types of processing steps can be used to form other types of semiconductor devices using conventional processing steps utilizing nanometer sized shadowmasks and etch masks. 
     In the preferred form of the invention, the method can be used to form a small size nanostructure using silver nanocrystal particles. For example, 3 nm diameter octanethiol-capped silver nanocrystals are mixed into a 1 mg/ml hexane solution to form the nanocrystal solution  14 . The Langmuir films  16  are formed in a Nima Technology type 611 Langmuir trough  10  at 15° C. The nanocrystal solution  14  is a hexane nanoparticle solution comprising decanedithiol (HSC 10 H 20 SH) and hexane nanoparticles. Typically, only one drop which is approximately 3 μl of the silver nanocrystal solution  14  is dispersed on the surface of the water  12 . The nanocrystals in the solution  14  spontaneously self-assembled into high-aspect ratio wire-like structures. The water surface forces cause interactions among the nanocrystals on the water surface causing the nanocrystals to disperse as wire-like strands. The width of the strands of the film  16  can be controlled from 20 nm to 300 nm depending on the size of the nanocrystal particles and depending on the pressure  19   a  and  19   b  applied by the pressure bars  18   a  and  18   b . For a given set of wires, a narrow distribution of widths, for example 15-25% in width variances can be obtained. The interstrand distance, as well as the alignment of the strands  16 , can be controlled via compression  19   a  and  19   b  from the pressure bars  18   a  and  18   b  in the Langmuir trough  10 . During film transfer  26 , the nanocrystal nanostructures  26   a  and  26   b  retain the film structures and aligned with interwire separations of a few nanometers. The nanocrystal Langmuir film  16  is transferred onto the polymethyl methacrylate (PMMA) electron beam resist  24  that coats the substrate  22 . The nanocrystal shadowmask pattern defined by enlarged strands  28   a  and  28   b  is grown by repeatedly lowering  25  the substrate  22  with the resist  24  into contact with film  16  on the water  12 . This film transfer amplification  25  slightly increased the width of the enlarged strands  28   a  and  28   b  during each transfer step  25  immersion as the nanoparticles bond together. The nanocrystal particles comprise a few hundred atoms, such as silver atoms. Thiol tails, not shown, extend from the particles  28   a  and  28   b  to which additional particles are drawn and bonded during the growing process  25 . The growth can double the height of the wires  28   a  and  28   b  to about 18 nm. Hence, the method involves transferring  25  of nanocrystal nanostructures onto the PMMA electron beam resist  24 , amplifying  25  the nanostructures to a desired thickness as a shadowmask, exposing  29  the shadowmask  28   a  and  28   b  and electron beam resist  24  to low energy electron beams for the formation of the PMMA etch mask pattern  32   a  and  32   b  after developing  30 . Scanning electron microphotography shows that 50 nm wide PMMA wire patterns can be formed by a silver nanocrystal shadowmask after developing process. After forming the nanostructured resist pattern  30   a  and  30   b  as a shadowmask, the substrate  22  can be processed to form a variety of semiconductor devices. As shown, reactive ion etching  34  is used to etch exposed substrate portions  35   a ,  35   b  and  36   c . After etching away the exposed substrate portions  35   a ,  35   b  and  35   c , the resist  32   a  and  32   b  can be removed  36  using conventional chemical cleaning processes, thereby exposing nanowires  37   a  and  37   b  formed in the substrate  22 . 
     The low energy electron beam exposure  29  is used because low-energy electrons have less lateral scattering, higher interaction cross-sections, and shorter substrate penetration depths than the higher-energy electrons used in conventional electron beam lithography. To estimate the electron stopping power for silver nanocrystals, the formula for the electron range ρR given by ρR=0.0276AE 1.67 /Z 0.889  where R in μm is the stopping depth, A is a constant, E is the acceleration voltage in KV, Z the atomic number, and ρ is the density in g/cm 3 . The estimated electron stopping effective range R for silver at 700 V is 4 nm and is less than the thickness of the silver nanocrystal nanostructures. A Monte Carlo approach with more than 10 5  electron trajectories through the silver can be used to evaluate the electron stopping power of the silver nanocrystal shadowmask. This penetration depth is greater than the diameter of silver nanocrystals  26   a  and  26   b  but is smaller than the thickness of the amplified nanocrystal nanostructures  28   a  and  28   b . A JEOL 6401F field emission scanning electron microscope can be used to expose  29  the substrate  22  to a 600pA 700 V electron beam with the field size of 100 μm ×100 μm for ten minutes to expose the PMMA  24  with an electron dose of 50 μC/cm 2 . At 700 V, only very thin resist films  24  of less than 50 nm can be exposed. The silver nanocrystal strands  28   a  and  28   b  of 4 nm thin 50 nm wide form the showdown that was transferred onto the PMMA coated substrates  22  by the Langmuir-Blodgett trough lift-off process  25 . For improved pattern transfer, a thicker shadowmask is preferred and accomplished by the amplification  25  of the shadowmask thickness. Because the top of the silver nanocrystal nanostructures  28   a  and  28   b  are hydrophobic, multiple nanostructure layers of silver nanocrystal particles can be added to the previously deposited nanostructure layer by repeating the Langmuir-Blodgett trough lift-off process. The amplified silver nanocrystal structures  28   a  and  28   b  are kept in air for a few days to allow for organic ligands to evaporate during a chemical metalization process. This metalization process can be accomplished quickly through direct heating  27  to reduce the ligand thiols. The areas occupied by the organic ligands between the nanocrystals in the nanostructures  28   a  and  28   b  decreased through the metalization evaporation process  27 . As a result of the amplification  25  and metalization process  27 , 10 nm high silver nanocrystal nanostructures  28   a  and  28   b  are formed as enlarged strands with a reduction in the interwire distance. The amplified and metallized silver nanostructures  28   a  and  28   b  and the PMMA resist  24  are then exposed to a low energy electron beam  29 . Following the electron beam exposure  29 , the resist  24  is developed for one minute in a mixture of methyl isobutyl ketone and isopropanol with a ratio 1:3. Scanning electron microphotography can depict the 50 nm wide and 10 nm high PMMA etch mask pattern  32   a  and  32   b  fabricated using the silver nanocrystal shadowmask  28   a  and  28   b  after developing process  30 . As such, nanometer size etch mask  32   a  and  32   b  has been created using the nanometer size shadowmask  28   a  and  28   b  and the low energy electron beam exposures  29 . 
     The patterned resist  32   a  and  32   b  on the substrate  22  can be further processed to create semiconductor devices. For example, a subsequent anisotropic reactive ion etching process  34 , with a chlorine tetrafloride CF 4  to oxygen O 2  mixture of 22 mtorr to 18 mtorr, and the plasma sustaining power of 150.0 watt is used to etch away exposed portions  35   a ,  35   b  and  35   c  of the substrate. Next, the resist etch mask pattern  32   a  and  32   b  is removed by cleaning  36  resulting in 50 nm wide silicon nanowires  37   a  and  37   b  that are nanometer size semiconductor devices. The nanowires  37   a  and  37   b  can be interconnected by depositing contacts  38  at the ends of the nanowires  37   a  and  37   b.    
     As may now be apparent, the method enables the formation of a parallel array of 50 nm wide PMMA etch mask patterns  32   a  and  32   b  by using a low energy electron beam exposure  29  of silver nanocrystals strands  28   a  and  28   b  functioning as an electron beam shadowmask. The photoresist etch mask patterns  32   a  and  32   b  are obtained by using the nanocrystal shadowmask to transfer the shadowmask pattern  28   a  and  28   b  onto silicon substrates resulting in the formation of the nanowires  37   a  and  37   b  during the subsequent reactive ion etching process  34 . An advantage of the method in forming the quantum nanowires is the use of low energy electron beam exposure that reduces the proximity effects. The method is especially suitable for low cost and high throughput fabrication of quantum nanowires. 
     Field emission scanning electron microscopy micrographs can be used to image the transferred silver nanocrystal shadowmask pattern formed from the film  16 . The nanocrystal film  16  is transferred onto the substrate at a surface pressure  19   a  and  19   b  of 15-20 mN/m prior to microphotography provided by the pressure bars  18   a  and  18   b . In the preferred form, the method is a procedure of low energy electron beam lithography for creating a silver nanocrystal shadowmask through transfer of silver nanocrystal nanostructures  26   a  and  26   b  on the PMMA resist  24  disposed on the substrate  22 . The preferred form of the method enables the formation of metallized and amplified silver nanocrystal nanostructures  28   a  and  28   b . The wire-like nanostructure  26   a  and  26   b  have a narrow distribution of wire widths and the wires can be as long as 1-2 μm length and aligned into a regular pattern along the substrate  22 . 
     The method for fabricating nanometer size polymethyl methacrylate (PMMA) etch mask patterns using silver nanocrystal shadowmasks can be applied to many types of semiconductor processes. The method can be also applied to create more complex semiconductor devices of different materials. For example, the method can be used to produce bismuth nanowires  37   a  and  37   b  by filling porous anodic alumina with bismuth from the liquid phase resulting in single-crystal nanowire arrays having the same crystal structure and lattice parameters as a bulk material. A 40 nm thick bismuth single-crystal film, not shown, but in the position of nanowires  37   a  and  37   b  is formed on the substrate  22  by molecular beam epitaxy deposition. The substrate  22  may be an indium doped semi-insulating CdTe(111)B substrate that is one square centimeter in size. X-ray diffraction can be used to show sharp (0001) peaks that implied c-axis growth of the bismuth layer perpendicular to the substrate  22 . The bismuth nanowires  37   a  and  37   b  can be fabricated using low energy electron beam lithography using the silver nanocrystal shadowmasks and a subsequent chlorine reactive ion etching  34 . The reactive ion etching  38  is used to form the bismuth nanowires  37   a  and  37   b  under the etch mask  32   a  and  32   b . Submicron-size metal contacts  38  are deposited on the ends of the bismuth nanowires  37   a  and  37   b  through an in-situ focused ion beam metal deposition  38 . Two 100 nm wide platinum contact pads are deposited  38  on the ends of each of the bismuth nanowires  37   a  and  37   b . The temperature dependent resistance measurements on the 50 nm wide bismuth nanowires  37   a  and  37   b  show that the resistance increased with decreasing temperature, which is characteristic of semiconductors and insulators. Self-assembled high-aspect ratio silver wire structures are transferred to a 40 nm thick 1% PMMA coated molecular beam epitaxy grown bismuth layer on the CdTe substrates. Substrates with the transferred silver wires are exposed by the JEOL 6401F field emission scanning electron microscope at 700 V to provide the resist  24  with an electron dose of 50 μC/cm  2 . At 700 V, the 40 nm thick PMMA resist  24  can be exposed all the way to the surface of bismuth film. The penetration depth of electrons in silver was found to be 4 nm at 700 V by the previous Monte Carlo simulation. That penetration depth is smaller than the thickness of silver nanocrystal shadowmask  28   a  and  28   b . Following the electron beam exposure  29 , the resist  24  is developed  30  for one minute in a mixture of methyl isobutyl ketone and isopropanol in the ratio 1:3. A subsequent anisotropic reactive ion etching process  34  may be carried out by a PlasmaMaster model PME 1200 chlorine etcher. With a BCl 3  to Ar 2  mixture at 20 mTorr, and a plasma sustaining power of 200 W, reactive ion etching process transferred the silver nanowire resist etch mask pattern into the bismuth layer and CdTe substrate  22 . The shadowmask material consists of 30 nm thick silver on the 40 nm thick PMMA resist  24 . The electronic orbital configuration of bismuth implies that bismuth prefers to have two ionization states. A simple model of the bismuth etching mechanism for the case of Bi +3  ionization state is proposed: Bi→Bi +3 +3e−, 3/2 Cl  2 +3e−→3Cl 31 , Bi+3/2Cl 2 →Bi +3 +3Cl + →3BiCl 3  (Volatile). Following the formation of Bi +3  and Cl −3  ions by plasma, volatile products of BCl 3  are formed and washed away. The Argon gas in the etch gas mixture might contribute to a reduction of any undercut profile. A mixture of bismuth chloride BCl 3  and argon Ar 2  is a suitable choice of gases to be used in reactive ion etching  34  of bismuth layer producing vertical profiles and etching rates at about 100 nm/min. Through the reactive ion etching process, 50 nm wide and 40 nm high bismuth nanowires  37   a  and  37   b  can be fabricated on CdTe substrates  22 . After cleaning the resist  36 , submicron-size platinum contacts  38  are deposited on the bismuth nanowires. The contacts are prepared in situ focused ion beam epitaxy metal deposition at 25kV and 6 pA. The electrical resistance of the nanowires  37   a  and  37   b  can be measured and is typically on the order of 1-20 Ohms. The resistance of the nanowires increases with decreasing temperature that is characteristic of semiconductors and insulators. As quantum confinement is introduced into the bismuth nanowires, the external conduction subband and valence subband edges move in opposite directions to eventually form a positive energy band gap (Eg) between the lowest L-point conduction subband edge and the highest T-point valence band edge, thereby leading to a semimetal-semiconductor transition at Eg=0 as the nanowires size is decreased below the critical wire width of bismuth with the bismuth making a transition at a critical wire radius of 52 nm. In the bismuth nanowires  37   a  and  37   b , the carrier mobility is suppressed by carrier confinement along the direction of wire and by surface imperfection. As such, 50 nm wide bismuth nanowires can be fabricated by low energy electron beam lithography using silver nanocrystals as a shadowmask and a subsequent chlorine reactive ion etching process. Temperature dependent resistance measurements show that the bismuth nanowires fabricated have semiconductor properties rather than metallic properties. The method is suitable for fabricating bismuth nanowires as well as other nano size semiconductor devices. 
     The method employs the use of nanostructures for creating nano size shadowmask for forming nano size resist masks for enabling the fabrication of nano size semiconductor devices. Those skilled in the art can make enhancements, improvements, and modifications to the invention, and these enhancements, improvements, and modifications may nonetheless fall within the spirit and scope of the following claims.