Scanning Probe Lithography

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					                   Scanning Probe Lithography

                                       Junior Research Seminar
                                              Spring 2004

                                                       13 April 2004




Junior Research Seminar: Nanoscale Patterning and Systems
Teri W. Odom
             Intro to Scanning Probe Lithography
         •     High Resolution Patterning Using Scanning Probes
                – Chemical and molecular patterning (DPN)
                – Mechanical patterning
                       • Scratching
                       • Nanoindentation
                – Local heating
                – Voltage bias application
                       • Field Enhanced Oxidation (of silicon or metals)
                       • Electron exposure of resist materials
                – Manipulation of nanostructures
         •     Factors Important in Scanning Probe Lithography
                –    Resolution
                –    Alignment accuracy
                –    Reliability
                –    Throughput

Junior Research Seminar: Nanoscale Patterning and Systems
Teri W. Odom
                         Scanning Probe Lithography


                         Atomic Force Microscopy


                         Scanning Tunneling Microscopy




Junior Research Seminar: Nanoscale Patterning and Systems
Teri W. Odom
                             Atomic Force Microscopy
                            Mechanical scratching                          Chemical modification




                                                  50 nm



                          Electrostatic writing                                 Magnetic writing




                       C. Lieber (Harvard), Science 257, 375 (1992); H. Dai (Stanford), APL 11, 1508 (1998)
                       C.F. Quate (Stanford), J. of App. Phys. 70, 2725 (1991); www.nanoscience.de/group_r/mfm

Junior Research Seminar: Nanoscale Patterning and Systems
Teri W. Odom
                     Dip-Pen Nanolithography (DPN)




                                                            http://www.chem.northwestern.edu/~mkngrp/timeref.html#dpn

Junior Research Seminar: Nanoscale Patterning and Systems
Teri W. Odom
                         AFM Lithography Scratching
         •     Material is removed from the substrate leaving deep
               trenches with the characteristic shape of the plough used
         •     The advantages of nanoscratching for lithography
                – Precision of alignment
                – The absence of additional processing steps, such as etching the
                  substrate.
         •     Depending on the applied load, AFM can characterize
               micro-wear processes silicon for magnetic head sliders,
               polymers for electronic packaging and liquid crystals
               displays.




                                                            http://www.ntmdt.ru/SPM-Techniques/Lithographies/AFM_Lithography_-_Scratching_mode51.html



Junior Research Seminar: Nanoscale Patterning and Systems
Teri W. Odom
                    Nanoindentation and Scratching

                                                            1.5 nm deep at 4.4 µN




                        15, 20, 25 µN




                                                                               Diamond-like Carbon (DLC)


Junior Research Seminar: Nanoscale Patterning and Systems
Teri W. Odom
              Millipede: Data Storage in a Polymer
         •     Tips are brought into contact
               with a thin polymer film
         •     Bits are written by heating a
               resistor built into the cantilever
               to a temperature of ~ 400 C. The
               hot tip softens the polymer and
               briefly sinks into it, generating
               an indentation.
         •     For reading, the resistor is
               operated at lower temperature,
               ~300 C. When the tip drops into
               an indentation, the resistor is
               cooled by the resulting better
               heat transport, and a measurable
               change in resistance occurs.
         •     The 1,024-tip experiment
               achieved an areal density of 200
               Gb/in2
                                                            http://www.research.ibm.com/resources/news/20020611_millipede.shtml


Junior Research Seminar: Nanoscale Patterning and Systems
Teri W. Odom
                 Electric Field Enhanced Oxidation




         •     Voltage bias between a sharp probe tip and a sample
               generates an intense electric field at the tip
                – Oxidization of silicon
                – Anodization of metals
         •     The high field desorbs the hydrogen on the silicon surface
               and enables exposed silicon to oxidize in air
         •     Oxidation depends on humidity
         •     Can achieve sub-50 nm feature sizes
                                                            http://www.ntmdt.ru/SPM-Techniques/Lithographies/AFM_Oxidation_Lithography_mode37.html


Junior Research Seminar: Nanoscale Patterning and Systems
Teri W. Odom
                                  AFM Anodic Oxidation




                                                            Digital Instruments


Junior Research Seminar: Nanoscale Patterning and Systems
Teri W. Odom
                                   AFM Nanolithography




                                                            Digital Instruments


Junior Research Seminar: Nanoscale Patterning and Systems
Teri W. Odom
                 Parallel Field Enhanced Oxidation




        Typical scan area of
        commercial AFMs.

                    Oxidation of silicon with 50 probe tips. Probes are spaced by 200 µm.
                                                               http://www.stanford.edu/group/quate_group/LithoFrame.html

Junior Research Seminar: Nanoscale Patterning and Systems
Teri W. Odom
         Electron Exposure of Organic Polymers




         •     Electron Exposure of Resist
                – When a conducting tip is biased negatively with respect to a
                  sample, electrons are field-emitted from the tip
         •     Exposure Using Scanning Probes
                – Polymers have low threshold voltage, high sensitivity, sub-
                  100-nm resolution, and good dry etch resistance.
                – Resist can be easily deposited on substrates
                – The polymer surface is soft and pliable which minimizes the
                  tip wear
                                                            http://www.stanford.edu/group/quate_group/LithoFrame.html

Junior Research Seminar: Nanoscale Patterning and Systems
Teri W. Odom
                 Patterns in Resist Transferred to Si
             •     Feature sizes of patterns written determined by the
                   exposure dose
             •     Can be accurately controlled down to about 25 nm in
                   50-100 nm thick resist




                                                            http://www.stanford.edu/group/quate_group/LithoFrame.html

Junior Research Seminar: Nanoscale Patterning and Systems
Teri W. Odom
                      Non Contact AFM Lithography
         •     Silicon probe tip acts as a source of electrons
         •     The field emission current from the tip is used as the
               feedback signal to control the tip-sample spacing




 http://www.stanford.edu/group/quate_group/LithoFrame.html


Junior Research Seminar: Nanoscale Patterning and Systems
Teri W. Odom
                  AFM Manipulation of Polystyrene


                                       Tip Direction




                                                            Digital Instruments


Junior Research Seminar: Nanoscale Patterning and Systems
Teri W. Odom
                         Scanning Probe Lithography


                         Atomic Force Microscopy


                         Scanning Tunneling Microscopy




Junior Research Seminar: Nanoscale Patterning and Systems
Teri W. Odom
                     One dimensional (1D) tunneling
         •     Tunneling through a rectangular barrier




                                             Ψ I = Ae − ikz           Ψ III = Ce − ik ' z
                                          Incident wave              Transmitted wave
                                                                                    2m
                                                        Ψ II = Be − κ z      κ2 =    2   (V0 − E )
                                                      Exponential decay


         •     Elastic tunneling versus inelastic tunneling
                –    Elastic tunneling: energy of tunneling electrons conserved
                –    Inelastic tunneling: the electron loses a quantum of energy within the tunneling
                     barrier




Junior Research Seminar: Nanoscale Patterning and Systems
Teri W. Odom
                                 STM: General Overview

                                Current           Feedback          Position
                                Amplifier          Control          Control




                                                        Piezoelectric
                                                        Transducer

                                                                 ~30 nm        Tip Atoms



                                               Tip Path                              ~ 1nm



                                                            Surface Atoms
                                Bias Voltage




Junior Research Seminar: Nanoscale Patterning and Systems
Teri W. Odom
                                 Constant current mode
                                   SCAN                     •   ∆ Z(x,y): Constant integrated DOS
                                                            •   If surface atoms have identical p(E),
          SCHEMATIC VIEW




                                                                contour of atomic corrugation
                                                            •   Spatial resolution depends on
                                                                  • Status of tip
                                                                  • Electronic properties of sample
                                                                  • Applied bias voltage




                           Z
          SINGLE SCAN




                                                   x




Junior Research Seminar: Nanoscale Patterning and Systems
Teri W. Odom
                                   Constant height mode
                                 SCAN                       •   ∆ I(x,y): Variation of DOS at fixed height
                                                            •   High contrast and fast scanning
       SCHEMATIC VIEW




                                                            •   Improved performance
                                                                 • Insensitive to low frequency mechanical
                                                                      vibrations and electronic noise
                                                                 • Piezoelectric hysteresis reduced




                        I
       SINGLE SCAN




                                                 x




Junior Research Seminar: Nanoscale Patterning and Systems
Teri W. Odom
                                          STM Lithography
         •     Application of voltage pulse between tip and sample
         •     “Pushing” atoms
         •     Advantages of STM Lithography
                – Information storage devices
                – Nanometer patterning technique
                – Manipulations of big molecules and individual atoms
         •     Example of STM Lithography: local exposure of LB film




                                                            http://www.ntmdt.ru/SPM-Techniques/Lithographies/STM_Lithography_mode50.html


Junior Research Seminar: Nanoscale Patterning and Systems
Teri W. Odom
                         STM: Manipulation of Atoms




Junior Research Seminar: Nanoscale Patterning and Systems
Teri W. Odom
                          STM Manipulation of Atoms




                                                            M. Crommie (UC Berkeley), Science 262, 218 (1993)

Junior Research Seminar: Nanoscale Patterning and Systems
Teri W. Odom
         Feedback Controlled Lithography (FCL)
         •     FCL monitors the STM feedback signal and the tunneling
               current during patterning
                – Terminates the patterning process when a bond is broken
                – Controlled doses of electrons can be written over an area to
                  remove hydrogen atoms and create templates of individual
                  dangling bonds
         •     Examples of organic molecules patterned on Si:H (100)
               surfaces: norbornadiene, copper phthalocyanine and C60




                                                               Mark C. Hersam, Northwestern University



Junior Research Seminar: Nanoscale Patterning and Systems
Teri W. Odom

				
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