{"id":214,"date":"2018-12-10T11:06:22","date_gmt":"2018-12-10T11:06:22","guid":{"rendered":"http:\/\/msp11.epgpbooks.inflibnet.ac.in\/?post_type=chapter&p=214"},"modified":"2018-12-12T11:22:09","modified_gmt":"2018-12-12T11:22:09","slug":"214","status":"publish","type":"chapter","link":"https:\/\/ebooks.inflibnet.ac.in\/msp11\/chapter\/214\/","title":{"rendered":"Electron Beam Lithography – I"},"content":{"raw":"
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Learning Objectives<\/strong><\/p>\r\n

From this module students may get to know about the following<\/strong><\/p>\r\n

i. Overview of Lithography Techniques<\/em><\/p>\r\n

ii. Electron Beam Lithography<\/em><\/p>\r\n

iii. Working Principle of Electron Beam Lithography<\/em><\/p>\r\n\r\n

1. Introduction<\/strong><\/h1>\r\n

1.1. Lithography<\/strong><\/p>\r\n

Lithography comes from the Greek words lithos<\/strong> <\/em>means stone and grapho<\/em><\/strong> means write. Low dimensional structures are defined with writing patterns at micro\/nano-meter scale designed by printing text or graphics on the surface of materials. Lithography is currently one of the most feasible techniques used for patterning micro\/nanoscale features of desired designs with definite shape, size and material compositions, selectively on the material of choice for device fabrication. Several lithographic methods are developed with different resolution, reproducibility, speed, simplicity for processing, and acquisition costs, etc., by using different sources of energy. The process of utilizing light source for patterning is termed as photolithography and utilization of energetic beams as source are nomenclatured according to the type of energy source.<\/p>\r\n

During semiconductor processing, fabrication of micro\/nano-structures rearranges the additives by deposition, subtractives by etching, and surface dopants by annealing. Etching out the unwanted materials and depositing desired materials for fabrication of a complete nanosctructure or device structure is the key concept of microfabrication processing. Lithography is used for defining the wanted and unwanted regions by using a polymer chemical called resist layer over top of the surface to be patterned. The etching of unwanted material or metallization of new materials are pursued with the patterned resist on the surface, followed by resist cleaning to fabricate the desired structure. Simple solvents includes acetone, methyl ethyl ketone (CH3<\/sub>COC2<\/sub>H5<\/sub>), methyl isobutyl ketone (CH3<\/sub>COC4<\/sub>H9<\/sub>) for chemicals used for resists stripping\/lift-off process. Additional O2<\/sub> plasma cleaning is effective for removing residual resist or any organic contaminations during lithography process.<\/p>\r\n

1.2. Types of Lithography<\/strong><\/p>\r\n

In general, the ability to view or fabricate nano-sized objects depends on availability of strongly focused particle beams. Since, diffraction limits the spot size of the exposure, smaller the wavelength of the beam particle produces smaller spots. Here in the tabular are few beam energies correlating to particle wavelength (\u00c5) at various energies that decide the spot size are tabulated in Table 1.<\/p>\r\n

\"\"\r\nTable 1. Particle wavelength (\u00c5) at various energies<\/em><\/strong><\/p>\r\n

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1.3 Basic components and working principle of lithography<\/strong><\/em><\/p>\r\n \r\n

1) Energy Source- Modifies the resist dissolution rate.<\/em><\/p>\r\n

2) Mask \u2013 Allows patterning by shining direct energy to the resist.<\/em><\/p>\r\n

3) Exposure Systems\/Aligner - Aligns the mask to patterns to be written on wafer.<\/em><\/p>\r\n

4) Resist - Transfers the image from mask to wafer.<\/em><\/p>\r\n

Schematic of the photolithography technique is demonstrated in Figure-1. Generally, photolithography utilizes a light source that illuminates over an area of larger dimension compared to the dimension of the features to be designed. This requires a photo mask of same design with the feature sizes to restrict light to pass through and fall on to the photoresist on the substrate. The photo mask is the master pattern of the designs that transfer to wafers. The region, where light falls, could change the chemical decomposition of the polymer used as photoresist and could be dissolved out using another solvent during the lift-off process.<\/p>\r\n

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Figure 1. Components of Lithography<\/em><\/strong><\/p>\r\n \r\n

1.3.1. Steps for Photolithography<\/strong><\/em><\/p>\r\n\r\n