PhotolithographyPhotolithography is a process used in semiconductor device fabrication to transfer a pattern to the surface of a wafer or substrate. The transfer of this pattern will allow for the definition of features to be etched in underlying film or to provide a mask for ion implantation. In a complex integrated circuit a wafer will go through the photolithographic area on the order of 20 to 30 times.
A wafer is introduced onto an automated "wafertrack" system. This track consists of handling robots, bake/cool plates, and coat/develop units. The robots are used to transfer wafers from one module to another. The wafer is initially heated to a temperature sufficient to drive off any moisture than may be incorporated to the film surface. Hexa-methyl-disilizane (HMDS) is applied in either liquid or vapor form in order to promote better adhesion of the photo-sensitive polymeric material called photoresist. Photoresist is dispensed in a liquid form onto the wafer as it undergoes rotation. The speed and acceleration of this rotation are important parameters in determining the resulting thickness of the applied photoresist. The photoresist coated wafer is then transferred to a hot plate where a "soft bake" is applied, the purpose of this bake is to drive off excess solvent before introduction into the exposure system.
The desired pattern is then projected onto the wafer in either a machine called a stepper or scanner. The stepper/scanner functions similarly to a slide projector. Light from a mercury arc lamp or excimer laser is focussed through a complex system of lenses onto a "mask" (also called a reticle), containing the desired image. The light passes through the mask and is then focused to produce the desired image on the wafer through a reduction lens system. The reduction of the system can vary depending on design but is typcially on the order of 4X-5X in magnitude.
When the image is projected onto the wafer, the photoresist material undergoes some wavelength specific radiation-sensitive chemical reactions, which cause the regions exposed to light to be either more or less acidic. If the exposed regions become more acidic, the material is called a positive photoresist, while if it becomes less susceptible it is a negative photoresist. The resist is then "developed" by exposing it to a alkalai solution such as sodium hydroxide (NaOH), which removes either the exposed (positive photoresist) or the unexposed (negative photoresist) photoresist. This process takes place after transfer from the exposure system back to the wafertrack. A post-exposure bake is performed before develop typically to help reduce standing wave phenomena caused by the destructive and constructive interference patterns of the incident light. The develop chemistry is delivered in a similar fashion to how the photoresist was applied. The resulting wafer is then "hardbaked" on a bake plate at high temperature in order to solidify the remaining photoresist to serve as a protecting layer against future, ion implantation, wet chemical etch or a plasma etch.
The ability to project a clear image of a very small feature onto the wafer is limitted by the wavelength of the light that is used and the ability of the reduction lens system to capture enough diffraction orders off of the illuminated mask. Current state-of-the-art photolithography tools use DUV(Deep Ultraviolet light with wavelengths of 248 and 193 nm, which allow minimum feature sizes on the order of 130-90 nm. Future tools are under development which will use 157 nm wavelength DUV in a manner similar to current exposure systems. Additionally Extreme Ultraviolet (EUV) radiation systems are currently under development which will use 13nm wavelengths, approaching the regime of x-rays and should allow feature sizes below 45 nm.
The image for the mask is originated from a computerized data file. This data file is converted to a series of polygons and written onto a silicon-dioxide (Quartz) square substrate covered with a layer of chrome using a photolithographic process. A beam of electrons is used to expose the pattern defined in the data file and travels over the surface of the substrate in either a vector or raster scan manner. Where the photoresist on the mask is exposed the chrome can be etched away leaving a clear path for the light in the stepper/scanner systems to travel through.