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AFM Nanolithography
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Atomic Force Microscopy: Gold Nanolithography Using Self-Assembled Monolayers Index 1. Introduction 2. Atomic Force Microscope (AFM) 2.1. AFM modes 3. Self-assembled monolayer (SAM) 4. Experimental procedures 4.1. Sample preparation 4.1.1. Silicon Wafer 4.1.2. Gold film 4.1.3. Self-assembled monolayer 4.2. AFM nanolithography 4.3. Final etching 5. References 1. Introduction Miniaturization of new technologies in the manufacture of microstructures with conventional methods is reaching its limits. The need to have nanostructures is now a reality, the progress in this area is determined by the ability to manipulate structures this size. The purpose of this paper is to provide a method for nanolithography patterning and explain the results obtained in a nanolithography experiment that was made. The Nanolithography was made on Self-Assembled Monolayers on gold in a silicon wafer surface. For this project the Atomic Force Microscope was used to create the nanolithography. An overview of Atomic Force Microscopy and Self-Assembled Monolayers was made for a better understanding of the procedure. 2. Atomic Force Microscope (AFM) The Atomic Force Microscope (AFM) also called Scanning Force Microscope (SFM) was conceived by Binning, Quate, and Gerber in 1986. Here, instead of using a Scanning Tunneling Microscope (STM) tip whose direction is normal to the surface of the sample, they positioned it almost parallel direction so that its sharp edge was just above the surface. The tip, acting as a cantilever, did exert a force on the sample the same way that the STM tip does, except that now the minute deflections of the cantilever with this force-sensing edge were of importance. To measure the deflection of the cantilever, they used a second STM tip that could resolve cantilever deflections as small as 10-3nm. In particular, the AFM technology was adapted to measure electrostatic and magnetostatic interactions, as well as long-range van der Waals forces (Sarid et. al, 1991). 2.1 AFM modes Many AFM modes have appeared for special purpose while the technique of AFM is becoming mature. There are 3 primary modes in AFM: Contact Mode AFM, Tapping Mode AFM and Non-contact Mode AFM. Contact mode AFM operates by scanning a tip attached to the end of a cantilever across the sample surface while monitoring the change in cantilever deflection with a split photodiode detector. A feedback loop maintains a constant deflection between the cantilever and the sample by vertically moving the scanner at each (x,y) data point to maintain a “setpoint” deflection. By maintaining a constant cantilever deflection, the force between the tip and the sample remains constant. The force is calculated from Hooke’s Law: F = -kx where F is the force, k is the spring constant and x is the cantilever deflection. Force constant usually range from 0.01 to 1.0 N/m, resulting in forces ranging from nN to µN in an ambient atmosphere. The distance the scanner moves vertically at each (x,y) data point is stored by the computer to form the topographic image of the sample surface. Contact mode AFM is the only AFM technique which can obtain “atomic resolution” images. Tapping mode AFM operates by scanning a tip to the end of an oscillating cantilever across the sample surface. The cantilever is oscillated at or near its resonance frequency which an amplitude ranging typically from 20nm to 100nm. The frequency of oscillation can be at or on either side of the resonant frequency. The tip lightly “taps” on the sample surface during scanning, contacting the surface at the bottom of its swing. The feedback loop maintains constant oscillation amplitude by maintaining a constant RMS of the oscillation signal acquired by the split photodiode detector. The vertical position of the scanner at each (x,y) data point in order to maintain a constant “setpoint” amplitude is stored by the computer to form the topographic image of the sample surface. By maintaining constant oscillation amplitude, a constant tip-sample interaction is maintained during imaging. Tapping Mode AFM has lower forces and less damage to soft samples imaged in air. In non-contact mode AFM the cantilever is oscillated at a frequency which is slightly above the cantilever’s resonance frequency typically with an amplitude of a few nanometers (<10nm), in order to obtain an AC signal from the cantilever. The tip does not contact the sample surface. The cantilever’s resonant frequency is decreased by the van der Waals forces, which extend from 1nm to 10 nm above the sample surface. The decrease in resonant frequency causes the amplitude of oscillation to decrease. The feedback loop maintains a constant oscillation amplitude or frequency by vertically moving the scanner at each (x,y) data point until a “setpoint” amplitude or frequency is reached. The distance the scanner moves vertically at each (x,y) data point is stored by the computer to form the topographic image of the sample surface. (DI Training Notebook, 1998) 3. Self-assembled monolayer (SAM) In terms of miniaturization, the present technologies are reaching their limits. In the fabrication of microstructures, conventional techniques, despite of their success, are getting behind and new technologies for the creation of microstructures are being needed. There are other tools capable of reaching lower limits in resolution, like electron-beam lithography, but there’s a big need of development of this method before it can reach the efficiency required for mass production of smaller microstructures. The option presented by the Self-Assembled Monolayers (SAMs) is very promising for micro and nanofabrication because of its easy production, favorable characteristics and multiple applications (Atkinson, 2004). SAM formation occurs when a reaction between gold and thiol supplied causes a covalent assembly of a monolayer of thiols on the metallic substrate. The presence of this monolayer confers and controls many characteristics of the resultant interface between the solid film and adjacent phase, such as its wettability, its control of adhesion, its susceptibility to chemical reaction, and the physical barriers that control electrical conduction or influence mass transport or neutral molecules to the underlying metal. The later property of the interface is particularly relevant in lithographic applications where differential dissolution of the substrate in sought by the localized formation of a SAM, the monolayer that serves as a sort of ultimate resist because of its thinness (1-2nm) and its tendency toward high definition an order (E. Delamarche et. al, 1998). To prepare the samples, gold films are created by different methods like electron beam, thermal evaporation, etc. over a glass or silica having in first place a thin film of titanium or chromium to improve adhesion (~5nm). Gold films are usually 5 - 300 nm wide; however, SAMs also can be formed around colloidal gold (Atkinson, 2004). SAMs form a quasi-crystalline structure where, for the case of n-alkanethiols on gold, the chains of these molecules get attached to the gold atoms tilted about 24 - 28° respect to the normal of the gold film plane. When this arrangement has been completed, the approximate distance between the sulfur atoms is about 0.5 nm. SAMs provide a unique combination of desirable properties of high packing density, very small thickness, resistance to many chemicals, and small molecular dimensions, that should allow for nanometer scale lithography (Lercel et. al, 1994). 4. Experimental procedures The objective of this experiment consisted on making a nanolithography using SAMs deposited on a thin film of gold. The proper procedure for this experiment is presented. For a better understanding of the proper procedure for nanolithography the following literature is recommended, J. M. Atkinson “Nanolithography on thin Au films aided by Self-Assembly Monolayers” (2004). 4.1. Sample preparation For this project I used SAMs to cover the gold substrate that was deposited on a silicon wafer. In this section the sample preparation procedure is described. A silicon wafer surface, a thin gold layer, and octadecanethiol (ODT) SAMs were used to make the sample; the literature recommended for understanding this decision is Lercel et. al, “Scanning tunneling microscopy based lithography of octadecanethiol an Au and GaAs” (1994). 4.1.1. Silicon wafer The surface that was decided to use to deposit gold for the nanolithography was a silicon wafer. The silicone wafer that was used was donated by the Marcelo Videa Ph.D. (LCQ director). Once obtained the silicon wafer this was cut in squares of approximately 1cm2 with a glass cutting pencil. To clean the silicon wafer an ultrasound bath with an ethanol solution was made for a lapse of 4 hours to remove particles in the surface. The clean silicon wafer is now ready for the gold deposition. 4.1.2 Gold film The gold film was created with a sputtering method on the silicon wafer. This technique consists of the deposition of gold by gravity. The sputter coater that we have in our lab is the SC7620 Mini Sputter Coater manufactured by Quorum Technologies. The SC7620 Mini Sputter is a compact magnetron sputter coater. Thickness of metal deposition will be determined by the operator, but will typically be in the region of 1 - 20 nm. An HV voltage is applied between the Target (cathode) and the Baseplate (anode), which is at earth potential. A pressure interlock ensures that the HV supply cannot be activated until vacuum chamber pressure is reduced to 10-1 mbar or better. Low-pressure gas (argon) is leaked into the vacuum chamber to provide a medium for ionization. An HV voltage is applied between the Target (cathode) and the Baseplate (anode), which is at earth potential. A pressure interlock ensures that the HV supply cannot be activated until vacuum chamber pressure is reduced to 10-1 mbar or better. Low-pressure gas (argon) is leaked into the vacuum chamber to provide a medium for ionization. The SC7620 uses a basic magnetron sputter head with a simple to replace disc target (gold, silver, or other metals). The head is hinged for easy operation and fitted with electrical safety interlocks. The plasma current created inside is variable by adjustment of the vacuum level using and Argon leak valve; the plasma voltage is preset. Coating time is controlled by a 180 second solid-state timer with 15 second resolution. Analog meters monitor the vacuum level and plasma current (Atkinson, 2004). For this experiment a ~30nm gold film was deposited (100nm recommended). Thirty deposition cycles of 5 seconds each were made with a 13mA plasma current. The gold layer width was approximated bye the following operation. 4.1.3 Self-assembled monolayer After the deposition of the gold film the SAM was created. A ODT solution in ethanol in a 4.5mM concentration was created. The gold film was then submerged for a lapse of 9 hours. The minimum time required for SAM formation is 3 hours (Lercel et. al, 1994) and a maximum time recommended is 24 hours (Kim et. al, 1992). The more formation time is given the more uniform the SAM will be. After this procedure the sample is know ready for the nanolithography experiment. When working with ODT security percussions have to be taken. ODT is an irritant substance. Wear appropriate gloves to prevent skin exposure; wear appropriate clothing to prevent skin exposure; always use an approved respirator. When using the ODT solution use glass bottles. Never use plastic with ODT ore other -thiols (20 April 2005 4.2. AFM nanolithography The nanolithography consists in removing the SAM with the AFM tip in contact mode. A tapping mode AFM tip (RTESP tip) was used for AFM patterning. The RTESP tip has a nominal spring constant k40N/m (10 May 2005, For calculating the actual force in nN or µN the force Calibration plot is presented in figure 8. 4.3 Final etching A final etching bath is required to remove the gold were the SAM pattern was created. For the ODT on gold, the patterns can be transferred to the underlying gold with a short KI-I2 bath (Lercel, 1994). A good etching solution can be made with 4g of KI, 2g of I2 in 10ml of H2O; this solution has to be at 70°C and etching rate is 280nm/min (28 March 2005, 5. References -C. F. Quate, “The AFM as a tool for surface imaging” Surface Science 299/300 (1994): 980-995. -D. Sarid and V. Elings, “Review of scanning force microscopy” Journal of Vacuum Science Technology B9.2 (1991): 431-437. -Digital Instruments, Scanning Probe Microscopy Training Notebook (1998). -Ortiz Lanoratory @ MIT, 5 May 2005 -Atkinson, J. M. “Nanolithography on thin Au films aided by Self-Assembly Monolayers” (2004). -M. J. Lercel, G. F. Redimbo, H. G. Craighead, C. W. Sheen and D. L. Allara, “Scanning tunneling microscopy based lithography of octadecanethiol an Au and GaAs” Appl. Phys. Lett., Vol. 65, No. 8 (1994): 974-976. -Y. T. Kim and A. J. Bard, “Immaging and Etching of Self-Assembled n-Octadecanethiol Layers on Gold with the Scanning Tunneling Microscope” Langmuir, Vol. 8, No. 4 (1992): 1096-1102. -E. Delamarche, H. Schmid, A. Beitsch, N. B. Larsen, H. Rothuizen, B. Michel and A. Biebuyck, “Transport Mechanisms of Alkanethiols during Microcontact Printing on Gold” J. Phys. Chem. B, Vol. 102 No. 18, (1998): 3324-3334. -E. Delamarche, A. C. F. Hoole, B. Michel, S. Wilkes, M. Despont, M. E. Welland and H. Biebuyck, “Making Gold Nanostructures Using Self-Assembled Monolayers and a Scanning Tunneling Microscope” J. Phys. Chem. B, Vol. 101 No. 45 (1997): 9263-9269. -BYU Cleanroom - Selective Chemical / Wet Etching Recipes of Metals and Semiconductors, 28 March 2005 -Veeco Probes Store, 10 May 2005, -Material Safety Data Sheet, 20 April 2005, See Also
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