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Photolithography and its procedure
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- 2. LITHOGRAPHY IN ART •Lithography comes from the Greek word, lithos, means "stone“ and graphein, means "to write”. • Lithography is a method for printing using a stone (lithographic limestone) with a completely smooth surface. • This method was invented in 1796 by German author and actor Alois Senefelder as a cheap method of publishing artwork on paper or to print text. Lithography works because of the mutual repulsion of oil and water. • The image is drawn on the surface of the print plate with a fat or oil-based medium such as a wax crayon, which may be pigmented to make the drawing visible. 2
- 3. LITHOGRAPHY IN IC FABRICATION •Lithography (or patterning) refers to the series of steps that establish the shapes, dimensions, and location of the various components of the integrated circuit (IC). • The current progress in IC design, with the decreased dimensions (miniaturization) of the chip and increased density of transistors, is possible only if smaller areas on the wafer surface can be patterned. • This is primarily the function of lithography. • Thus, the success of modern IC design is due largely to lithography. 3
- 4. CONTD.. • Create apattern with the dimensions established by the circuit design. • Place the pattern correctly with respect to the crystal orientation and other existing patterns. • After the pattern is created, either the defined part of the wafer surface is removed (trench creation) or left behind (island creation) or new material is deposited. 4
- 5. CONTD.. • Lithography isalso used to expose certain parts of the wafer surface for doping (either with a hard mark for thermal diffusion or with a soft mask for ion implantation). • The correct placement of the circuit pattern involves alignment or registration of various masks. • An IC wafer fabrication process can require forty or more patterning steps. • Alignment of these individual steps is critical to form a working IC. 5
- 6. PHOTOLITHOGRAPHY • Photolithography isthe process of transferring geometric shapes on a mask to the surface of a silicon wafer. • Photolithography, also called optical lithography or UV lithography, is a process used in microfabrication to pattern parts on a thin film or the bulk of a substrate (also called a wafer) • Photolithography is a patterning process in which a photosensitive polymer is selectively exposed to light through a mask, leaving a latent image in the polymer that can then be selectively dissolved to provide patterned access to an underlying substrate. 6
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- 8. PHOTOLITHOGRAPHY STEPS: 1 WaferCleaning: In the first step, the wafers are chemically cleaned to remove organic, ionic, and metallic impurities. 2 Barrier Layer Formation: After cleaning, silicon dioxide, which serves as a barrier layer, is deposited on the surface of the wafer. 8
- 9. 3.Photoresist Application: • Photoresistis applied to the surface of the wafer by high-speed centrifugal spinning. • This technique, known as "Spin Coating," produces a thin uniform layer of photoresist on the wafer surface. • In this process a liquid solution of photoresist is give out from the wafer by rapid spin and produce uniform thin layer (0.5µm to 2.5µm). • Spin coating/ spinner typically runs at 1200rpm to 4800rpm for 30sec to 60sec 9
- 10. COMPONENTS OF PHOTORESIST •Polymer - this is a light sensitive polymer whose structure changes on exposure to light. The desired property is usually change in solubility in a specific solvent. • Solvent - The solvent is used to thin the resist so that is can be applied on the wafer by a spin on process. The solvent is usually removed by heating to around 100 ◦C, called soft bake process. • Sensitizers - these are used to control the chemical reaction during exposure. • Additives - various chemicals that are added to achieve specific process results, like dyes. 10
- 11. Positive Photoresist: • Positivephotoresists is exposed to UV light, the underlying material is to be removed. • In these resists, exposure to the UV light changes the chemical structure of the resist so that it becomes more soluble in the developer. • The exposed resist is then washed away by the developer solution. In other words, "whatever shows, goes”. 11
- 12. CONTD.. • Positive resistsdirectly transfer the pattern from the mask onto the wafer. • This is because the mask protects the portion of the resist below it from exposure to UV radiation. • The rest of the resist, that is exposed, becomes more soluble and can be easily removed. 12
- 13. Negative Photoresist: • Negativephotoresists behave in just the opposite manner. Exposure to the UV light causes the negative resist to become polymerized, and more difficult to dissolve. • Therefore, the negative resist remains on the surface wherever it is exposed, and the developer solution removes only the unexposed portions. 13
- 14. CONTD.. • Negative resists,transfer the negative of the mask pattern to the wafer. • This is similar to the negative process in film photography. • For negative resists, the portion that is protected by the mask pattern is more soluble, since it is not exposed to UV radiation, while the radiation hardens the rest of the resist 14
- 15. 4.Prebaking • Prebaking isthe step during which almost all of the solvents are removed from the photoresist. • The photoresist become photosensitive after prebaking. • Photoresist is prebake at 90Co to 100Co for 5min to 30min. 15
- 16. 5.Mask Alignment andExposure • A mask or "photo mask" is a square glass plate with a patterned combination of metal film on one side pattern transferred onto the wafer surface. • The mask contains the hard copy of the pattern that has to be transferred to the different wafers during lithography. For a given integrated circuit, there are multiple masks, which have to be aligned for proper device fabrication. • Masks have alignment markers included with the pattern, which can be used for this purpose 16
- 17. MASK MAKING • Thealignment markers are usually much smaller than the typical dimensions of the pattern. • The mask material is made of borosilicate glass or quartz with a sputter deposited chrome layer on top. The chrome layer is 100 nm thick. • There is also a photoresist layer deposited on top of the chrome. • A laser writer is used to ‘write’ the pattern on the mask. Different laser wavelengths (365, 248 or 193 nm) and lenses are used to write the pattern on the mask. • The choice of the wavelength depends on the smallest dimension on the pattern. 17
- 18. • The laserwriting process is sequential (line by line) and can take hours depending on the complexity of the pattern. • The mask pattern took approximately 7 hours to write, using a 365 nm laser wavelength. After the pattern is written, a suitable developer is used to remove the unexposed photoresist. • After that, the exposed chrome layer is removed (using an acid bath etch) and then the remaining photoresist is removed to leave behind the chrome desired pattern on glass. • There are also cleaning and drying steps to remove any excess solvent and keep the mask free of dust particles 18
- 19. METHODS OF MASKALIGNMENT There are three primary exposure methods: • contact • proximity • projection. Contact Printing: • In contact printing wafer is brought into physical contact with photo mask. Because of the contact between the resist and mask, very high resolution is possible. • The problem with contact printing is that fragments trapped between the resist and the mask, can damage the mask and cause defects in the pattern. 19
- 20. Projection Printing • Projectionprinting avoids mask damage entirely. • An image of the patterns on the mask is projected onto the wafer, which is many centimeters away. • To achieve high resolution, only a small portion of the mask is imaged it has about 1-micron 20
- 21. Proximity Printing • Theproximity exposure method is similar to contact printing except that a small gap, 10 to 25 microns wide is maintained between the wafer and the mask. • This gap minimizes (but may not eliminate) mask damage. • Approximately 2 to 4 micron resolution is • possible with proximity printing 21
- 22. 6.Development • Development isa process in which exposed/non- exposed area is dissolved by developer. • Most commonly used developer is tetra methyl ammonium hydroxide is used in concentrations of 0.2 - 0.26. • Developer is important in controlling the development uniformity. 22
- 23. • Therefore twomethods are mainly used i.e. spin development and spray development. • During spin development wafers are spun and developer is poured onto the rotating wafer. • In spray development, the developer is sprayed rather than poured, on the wafer by using a nozzle that produces a fine spray over the wafer. 23
- 24. 7.Hard-Baking The hard bakeis used to harden the final resist image at the temperature (120°C - 150°C), so that it will hold out the harsh environments of etching. 24
- 25. 8.Etching • Etching isperformed either using wet chemicals such as acids, or more commonly in a dry etching (by exposing the material to a bombardment of ions) . • The photoresist will “resists” the etching and protects the material covered by the resist. • When the etching is complete, the resist is stripped leaving the desired pattern. 25
- 26. DRY AND WETETCHING The etching process that involves using liquid chemicals or etchants to take off the substrate material is called wet etching. In the plasma etching process, also known as dry etching, plasmas or etchant gases are used to remove the substrate material. 26
- 27. 9.Stripping • After theimaged wafer has been etched the remaining photoresist must be removed. • There are two classes of stripping techniques; wet stripping and dry stripping. • A simple example of stripper is acetone. • Acetone tends to leave residues on the wafer. • Most commercial organic strippers are phenol-based. 27
- 28. ADVANTAGES Photolithography can etcha pattern into an integrated circuit with a single beam of ultraviolet light and does not require any additional materials. highly efficient Controls the exact size and shape of the entire substrate. 28
- 29. DISADVANTAGES Very expensive Photolithography requires extremelyclean conditions that are void of all contaminants, liquids, and environmental hazards. 29
- 30. APPLICATIONS OF PHOTOLITHOGRAPHY • ICdesigning process • Fabrication of microcircuits • Semiconductor industry • Sensors • Microprocessor 30
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