Photolithography

Why Photolithography Low cost IC manufacturing Due to large number of lithographic steps needed in IC manufacturing, lithography typically accounts for about 30% of cost of manufacturing. Photolithography cost minimize result in low cost IC Ultra miniaturized electronic device manufacturing With photolithography ultra miniaturized IC manufacturing is possible resulting in miniature electronic device

Basic Process of Lithography Substrate Base layer consists of either glass, silica etc. Depositing Film A thin layer of SiO2 coated at the top of substrate Pattern the Film Imaging the pattern from the template to the film Etching Removing the unnecessary part of the film and keeping the required pattern. Can be performed using different chemicals( acids)

Photolithography

Procedure of Photolithography Cleaning(Surface Preparation) Chemicals like H2O2, acetone are used to clean the surface of substrate to clear from dust particle or any contaminant. A layer of silicon is placed on the wafer or substrate to act as masking film Photoresist Viscous solvent have binding and adhesive property Photoactive compound that activates in presence of light Activate at the wavelength of the light that we plan to use Coated with the thickness of 0.5 to 2.5 micrometer at the top of SiO2 layer using spin coating which runs typically 1200 to 4800 rpm for 30 to 60 sec. Types Positive photoresist Area exposed to light becomes soluble in chemical while developing Negative photoresist Area exposed to light get polymerize and remains the same and area that is not exposed becomes soluble Commonly used negative photoresist is SU8

Contd.. Soft Bake Align and Exposure After coating, the resulting resist film will contain between 20 – 40% by weight solvent. At room temperature, an unbaked photoresist film will lose solvent by evaporation, thus changing the properties of the film with time. Film thickness is reduced Adhesion is improved The film becomes less tacky and thus less susceptible to particulate contamination Dry off excessive photoresist solvent so that it becomes stable at room temperature. Exposed at a temperature of 90 degrees to 100 degrees for 30 sec. Align and Exposure The basic principle behind the operation of a photoresist is the change in solubility of the resist in a developer upon exposure to light  To imprint the correct pattern in the wafer mask alignment is required. If the alignment of mask is not accurate, the printed pattern will not be correct.

Contd.. Developing Hard Bake Etching Stripping Chemical used to remove the unwanted photoresist. These are mostly aqueous base chemical as developer Generally used tetra-methyl ammonium hydroxide (TMAH) as developer Hard Bake The hard bake is used to harden the final resist image so that it will withstand the harsh environments of implantation or etching. The high temperatures used is 120°C - 150°C. If the temperature used is too high, the resist will flow causing degradation of the image.  Etching This removes the unwanted SiO2 layer Normally acids are used for this purpose Stripping Stripping out unwanted photoresist material.

Capacity Enhancement The ability to project clear image(Projection Printing) of small feature in wafer is dependent on wavelength and ability of capture lens to capture the diffraction light. Smaller feature uses smaller wavelength of light As per light energy required, various wavelength with various light intensity can be used UV 365 nm – 436 nm Deep UV(DUV) 157 nm – 250 nm Extreme UV(EUV) 11 nm – 14 nm X-ray <10 nm

Resolution The achievable, repeatable minimum feature size Determined by the wavelength of the light and the numerical aperture of the system. The resolution can be expressed as

Summary Photolithography uses three basic process steps to transfer a pattern from a mask to a wafer: coat, develop, expose. The pattern is transferred into the wafer’s surface layer during a subsequent process. Capacity enhancement is dependent on wavelength and numerical aperture.