Ion And Electron Beam Nanostructuring Techniques Biology Essay

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vlevel. Nanotechnology intends to imitate nature by taking advantage of the unique properties of Nanoscale matter to come up with more efficient ways of controlling and manipulating molecules.[1]Nanotechnology is the designing, fabricating and use of nanostructured systems.

A reduction in the spatial dimension or confinement of particles or quasi particles in a particular crystallographic direction leads to change in physical properties in the direction.[2] The classification of the nanostructure materials can be done in 3 ways: systems confined in 3D(Nano particles, Nano pores), system confined in 2D(Nano wires, tubes) & system confine in 1D (quantum well, clay platelets). Nanotechnology is the growing, shaping or assembling of systems either mechanically, chemically or biologically to form Nanoscale architectures, systems and devices.[2] The various techniques for developing nanostructure are shown in the figure below.

Figure1: Different Nano Structuring Techniques

Direct write technologies with focused electron beam (FEBs), focused ion beam (FIB) or photons (lasers) and scanning tunnelling microscope (STM) probes offer unique advantages over classical resist based process. [3] A lasers beam doesn't have a resolution of nm sized deposition which is needed for ultrahigh density data storage and fast. The limitation of other fabrication technologies made STEM and FIB most popular in silicon and semiconductor industry. Developing a nanostructure using electron beam is done by SEM (Scanning electron microscope), TEM (Transmission Electron Microscope) or FEG-TEM {STEM (Scanning Transmission Electron Microscope)}. Here STEM is also known as FEG-TEM (Field emission gun transmission electron microscope), since it uses a field emission gun like LMIS (light metal ion source). The ways of developing the nanostructures can be seen in the flow chat given below for the case of ion and electron beam:

Figure 2: Ways of generating Nanostructure.

Ion and electron beam Nano structuring techniques have been used by most of the semiconductor industry. There have been many developments done in order to develop the ion and electron beam techniques. There are two ways of generating nanostructure using ion and electron beam techniques: generation of positive structure (material deposition) & negative structure (materials removal). FEBs and FIB can deposit sophisticated 2D and 3D features. In both the cases of FIB and STEM they use a precursor gas for the deposition process; these can be seen in table below

FIGURE 3: Table 1- Common deposition gases and source temperature.

Table 2- Composition and resistivity of deposited metals

STEM:

In recent years the best known development done in microscope is STEM (Scanning Transmission Electron Microscope). STEM is a type of Scanning & Transmission microscope, which combines the advantageous of both TEM (Transmission Electron Microscope) & SEM (Scanning Electron Microscope). STEM is ideally suitable for the multi-signal approach. The first STEM was built in 1938 by the Baron Manfred von Ardenne while working in Berlin for Siemens.[4][5] At first the result shown by STEM was inferior to TEM but until 1970's, Albert & his crew at the University of Chicago developed the FEG (Field Emission Gun) & adding a high quality objective Len's to create the modern STEM. The crew & co-worker of Albert worked on the cold Field Emission Electron gun & built a STEM able to visualize single heavy atom.[6]

FIGURE 4: STEM (Scanning transmission electron microscope)

The field emission electron gun is source to generate electron beam which is used as for scanning purpose and also for the nanolithography process. The nanolithography process is an additive application of the STEM, which can be used to lithograph nanostructure, etch masking and can also be used in atomic force microscope (AFM) probe tips.[7]

Working:

The basic principle involve in the process of electron beam deposition process in STEM depends on the electron high energy to break the molecular bonds. In this process the decomposition of the material like substrate or the materials which are already deposited near the deposition spots absorb the primary electron from the beam. The substrate reemits the secondary electron by absorbing the primary electron and these secondary electrons are responsible for the decomposed of the precursor molecules. [8]

FIGURE 5: Basic principle of EBID

The focused electron beam of high energy (10-300KeV) is used in STEM as a source. The precursor material is in the gaseous form (liquid and solid material are converted into gaseous form using vaporization or sublimation). The gas is introduced into the high vacuumed chamber of STEM at a controlled rate. The beam is introduced through a small orifice since the small orifice maintains differential pressure in the microscope and deposition chamber. In the case of high temperature deposition or corrosive gas deposition a special designed chamber is used & the rate of deposition depends on the partial pressure. The electron beam induced deposition is caused by the dissociation of the molecules adsorbed to a surface by high energy electron. The electron beam deposition taking place in the STEM is low and higher cost as compare to FIB for the 3D nanostructures.

'The resolution obtained so far for the positive structure generation is 0.2 to 2nm'. [2]

In order to generate the negative nanostructure by removal of the material can be done using the STEM. In STEM the electron beam machining (EBM) releases high energy electrons which are focused electromagnetically onto the target in vacuum and hence these beam results in nanostructure by material removal. Nanometre patterning accuracy can be achieved by STEM. Metal halides are more sensitive electron beam that are used for this process. Writing pattern used in the STEM is either stand alone or in the case of fluoride thin film on a substrate for negative structure. The film can be lifted off and subsequently can be used a lithography mask.

'The resolution obtained so far for negative structure generation is far better than 10nm'.[9]

FIGURE 6: - Generation of negative structure using STEM

FIB:

FIB is known as Focused Ion Beam normally used for material science where we deal with nanostructures. FIB is use for the semiconductor analysis, deposition & cutting out / ablation of materials. FIB plays an important role for the development of large semiconductor manufacturers. Nowadays FIB technique has been produced commercially by the semiconductor industries. The first FIB was developed by Levi-Setti & Orloff in 1975 based on field Emission technology.[10][11][12] Within a few years the first LMIS (liquid Metal Ion Source) based FIB was developed by Seliger.[13]

Working:

The FIB uses a Focused beam of ion to image the sample in the chamber. FIB uses a finely Ga+ for the imaging of sample at low beam current and a high beam of current for sputtering or milling process. The uses of ion for imaging and sputtering using single equipment made the researcher & semiconductor manufacturer to rethink over it. The ion (Ga+) beam produce by the ion source hits the sample and making the sample to eject secondary ions (i+ or i-) or neutral ion (N0). The primary ion beam produced from the ion source also ejects some secondary electron (e-). These secondary electron, ion and neutral ion are collected in the form of image. Here the primary ion beam plays an important role in FIB device depending on the sample. If the sample turns to be a non- conductive, a low energy electron gun can be used to provide charge neutralization and hence the positive ion beam can be used for the imaging highly insulating sample. These positive ions can also be used as the milling process for the sample without using a conductive surface coating.

FIGURE 7: Basic FIB principle

There are two ways of developing nanostructures: positive way (deposition) or the negative way (sputtering or milling). Generation of positive nanostructures are done by deposition way, where a partial vapour pressure of organic gas molecules (containing the metal of interest) is inserted near the substrate surface with a hollow needle (0.5mm).These gas molecules are absorbed by the surface which forms a layer over it. In order to trigger the chemical reaction we use an external energy source like ion beam kinetic energy, secondary electrons that emerges from the surface. While providing the external trigger one should keep in mind that reaction product should be non-volatile and should have a lower sputter-rate than the pure substrate.

'The resolution obtained for FIB so far is 10-200nm.' [2]

FIGURE 8: Generation of positive structure using FIB

In order to grow the deposition, the reaction must be at faster rate than the sputter yield, which leads to formation of trench. Using the ion beam induced deposition we can develop 2D and 3D nanostructures like wires, lines, layers, dot arrays, stair case structure, pillars, X-Z diagonal lines.

The other way of generating a nanostructure is by sputtering or milling known as negative structures. One of the best ways to generate a negative nanostructure is ion implantation, which is widely used in the semiconductor and electrical device industry. In the ion implantation process the ion beam is passed through the magnetic field where the unwanted ions are resisted and the selected ion beam is made to fall on the surface. This beam sputters the substrate in order to make a nanostructure.

FIGURE 9: Generation of negative structure using FIB

Comparing the processes:

STEM and FIB are both used for fabrication of nanostructure, the way they differs is the source they use for the fabrication process. STEM uses electron beam and FIB uses ion beam for the fabrication process. In STEM Electron beam induced deposition (EBID) is caused by the dissociation of the molecules absorbed to the surface by high energy electron.[7] In the FIB chemical vapour deposition technique widely used for the deposition purpose. In STEM the rate of deposition is low and the cost is high as compare to FIB. Some of the advantage of using ion beam rather than electron: ions are larger than electron (hence ions have low penetration depth, since we deal with surface interaction and deposition), ions are heavier than electron which helps then to gain momentum easily, ions are positive and electrons are negative.[14] In order to grow the deposition rate we use a trigger like ion beam kinetic energy, secondary electrons emerging from the surface in the case of EBID. EBID at lower SEM voltage has higher deposition rate, but has a poorer resolution. Earlier it was assumed that STEM can develop only 1nm size structure using FEG-SEM, since the beam diameters was less than 1nm, but the new result shows that the beam diameter matters even below 1nm. The resolution obtained in EBID is much better as compare with the FIB; hence EBID shows much better result as compare with the FIB-ID.

Dual beam FIB/SEM has more advantage for generating positive and negative nano structures. In order to generate negative structure a mask is developed over the substrate and the beam drill through the thin layer of the material on the removal substrate. For the generation of the positive nanostructure the mask is subsequently used to selectively deposited material by spatially resolved blocking of beam for deposition. FIB is used for lithographic mask generation in dual beam FIB/SEM. The gas used for the generation of negative structure & positive structure in FIB/SEM is the same gas injection technology used in FIB. The dual beam process improves the resolution to 1-2 orders as compare with the FIB and STEM, but there are no device found in the consumer market till now.

Conclusion:

We have mentioned the various techniques for fabricating the nanostructure. Direct write techniques with focused ions beam, focused electron beam & laser technique offers unique advantages over classical resist based process. Laser technique doesn't provide resolution of nm size deposition. We have also discussed about the fabrication process for generating of positive and negative nanostructures using the ion and electron beam. Material removal technique refers to negative structure and material deposition technique refers to positive nanostructure. We have also discussed about the basic working principle of the STEM and FIB. We have also compared the sputtering technique between STEM and FIB fabrication process for nanostructures. EBID (electron beam induced deposition) is much better than FIB-ID (focused ion beam induced deposition).Both methods EBID and FIB-ID are available in dual beam FIB/SEM. EBID at lower SEM voltage has higher deposition rate, but poorer resolution. FIB can be used for imaging purpose for low current beam and it can be used as sputtering or milling process at high current beam. We have also discussed about some of the recent work done for the development of the STEM and FIB.

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