Photolithography Overview for MEMS

welcome to the photo lithography overview for Microsystems presented by the Southwest center for Microsystems education this presentation will outline the photo lithography steps used to fabricate micro sized devices more detailed information about these steps can be found in the photolithography learning module available on the SEM II website Microsystems are MEMS fabrication uses several layers of various thin films to build a variety of devices layers typically consist of silicon dioxide silicon nitride poly silicon and metals the graphic illustrates the layers of a micro sized linkage assembly each layer is a different component of the device and requires a different pattern some layers are used to spacers as you can kind of see spaces between the linkages and then removed at the end of the process however even these layers require a pattern to open up spaces for subsequent layers so how do we get these patterns on each layer photo lithography photolithography is the fabrication process that defines and transfers a pattern for each layer of a device photolithography is used at any point in the process where a pattern needs to be defined which in the case of Microsystems is pretty much each layer and each layer requires a unique pattern the basic concept of photolithography is to transfer the pattern of a mask or reticle into a layer by first transferring the pattern into a photosensitive layer called photoresist to resist once the pattern is within the resist then it is transferred to the underlying layer or layers using an etched process after the prot after the pattern transfer the resist is usually stripped or removed the three primary steps of photolithography are coat expose and develop encode the surface of the wafer is coated with federalists sensitive material as I mentioned called photoresist in expose the photoresist is expose using a light source such as ultraviolet or UV or x-ray after expose comes develop and develop selects portions of the photoresist are dissolved with a chemical developer leaving the mask pattern in the resist so now we're going to break down each of these steps into their secondary processes in order to achieve a proper coating of resist with specific resist thickness and a uniform thickness across the wafer the wafer surface must first be conditioned surface conditioning prepares the wafer to accept the photoresist by first ensuring a clean surface that's free of particles of moisture surface conditioning also coats the wafer with a primer this primer is normally a chemical called HMDs or hexa methyl dyes Alassane starting at the beginning of the surface conditioning process the wafer is first clean then baked to remove any remaining moisture on the wafer surface once clean and dry HMDs is applied to create a hydrophobic surface the surface that has a fear of water making the surface hydrophobic makes it more adhesive for the application of the photoresist the wafer is then cooled to room temperature after the surface is conditioned the wafer surface is coated with photoresist this is a critical step for many reasons the thickness of the photoresist in conjunction with exposure affects the critical dimensions are line widths of the pattern therefore the resist be a specific thickness and the thickness must be uniform across the entire wafer in addition the resist must be thick enough and durable enough to withstand the next process steps the most common method of coating a wafer with resist is spin coating in the spin coat process the wafer is placed on a Chuck and a vacuum is applied to hold the wafer on the Chuck when the Chuck is spinning resist is applied either before the Chuck starts spinning which is called a static dispense R once this Chuck start to spend which is called a dynamic defense once the resist is deposited on the surface the Chuck accelerated accelerates to a specific spin speed called the casting speed the faster the spin speed the thinner the resist once at the casting speed the chuck continues to spend to throw off any remaining solvents in the resist let's talk about this photoresist for a moment photoresist is a mixture of organic compounds held together in a solvent solution there are two basic types of photoresist one is negative resist the other is positive resist the primary difference is how they respond to the light source this graphic illustrates the difference with positive resists the the regions of the resist exposed to the UV light become more soluble when developed the soluble resist dissolves and the unexposed resist remains on the wafer the result is a positive resist pattern on the wafer with negative resists the regions of the resist exposed to u UV become insoluble and hardened when developed the hardened resist remains on the wafer and the non exposed resist dissolves the result is a negative resist pattern on the wafer in micro systems fabrication the majority of resists used is positive resist after the resist is applied to the desired thickness a soft bake is used to remove the residual solvents of the photoresist after soft bake the wafer is cooled to room temperature before transferred to the expose equipment expose consists of two steps align and expose align is one of the most critical steps in the entire Microsystems fabrication process due to the microscopic size of these devices a misalignment of one micrometers or micron or even smaller can destroy the device and all the devices on the wafer each layer must be aligned properly and within specifications to all previous layers take a look at the microscopic hinge that you see in the scanning electron microscope image notice the one micrometer scale at the bottom in the bottom right using the scale we can estimate that the width of the space between the hinged component component and its enclosure the loop is approximately a half of a micron or 500 nanometers wide just think about the outcome of the process if the mask for the loop was aligned by as little or missile I'd buy as little as 500 nanometers we would have a wafer full of rejected devices after a lines expose exposed uses a pattern of mask or reticle which is a quarter glass plate with the desired pattern for a particular layer the pattern has been fabricated onto the mask surface using a film such as Chrome this picture shows a mask used that is used to expose an entire wafer notice notice that there's a repeating pattern throughout the mask and that each of the patterns each of these patterns is a die or the pattern for one component or a set of components if you look closely at each die you can see that there are nine devices within each die or denying device components within each die this quartz piece in the bottom right hand corner that contains only four die is called a reticle some photo lithography equipment uses reticles instead of masks when a mask is used the wafer and the light source are held in one position and the light is turned on for the required dose when a reticle is used the wafer is stepped under the radical a small portion of the wafer is exposed with each step during expose the resist is exposed using a UV or ultraviolet light source the UV travels through the openings in the mass to the resist while the areas of the mask with chrome block the UV a chemical reaction occurs between the resist and the light energy as we discussed before the type of resist whether it's positive or negative determines the results of the chemical action whether it hardens or makes it soluble once the resist has been exposed the wafer moves to develop and develop portions of the photoresist are dissolved by a chemical developer with positive resists the exposed resist dissolves while the unexposed resist remains on the wafer with negative resists the unexposed resist dissolves while the exposed resist remains the develop process leaves a visible pattern within the resist as you can see in that top picture develop is a wet process wafers are either immersed in the developer or the developer is sprayed or puddled on this graphic illustrates two types of develop immersion and spray-on develop to stop the chemical reaction of the developer with the photoresist the wafers are rinsed with deionized water then spin dried a post developed hard-baked hardens the photoresist for the next process step the temperature of the hard-baked is higher than that of the soft bake however too high of a temperature and a hard bake could cause the photoresist to reflow and that would destroy the pattern therefore it is important to keep the temperature and the time of the hard bake within specifications after the hard bake the wafer is cooled to room temperature then inspected there are a couple of inspect steps in foot during photolithography one is after the coat process to check for resist thickness and uniformity and the other is the final photolithography inspection in the final inspection we look for three critical parameters alignment line widths and defects with alignment the pattern must be positioned accurately to the previous layer in order for the inspection to pass line width the critical dimension is measured to ensure that the patterned images are in focus and have the correct size width and height the wafer is also inspected for defects things that could affect subsequent processes and eventually the operation of the devices in summary photolithography consists of three primary process steps that transfer a pattern from a mask to a wafer these steps are coat develop and expose in addition to these three primary steps photolithography also includes a surface conditioning prior to code and a soft fake after coat alignment before expose in a hard baked after develop in subsequent steps the resist pattern can be transferred to the underlying layer the resist layer can be used as a hard mask for an etch process or it could be used as a sacrificial layer for more detailed information on each step of the photolithography process be sure to download the photolithography learning module from the SEMA website under educational materials thank you for viewing this photo lithography presentation produced by the Southwest Center for micro systems education

Channel: Support Center for Microsystems Education

Views: 158,486

Keywords: MEMS, microtechnology, photolithography, photoresist, lithography

Id: 1bxf9QRVesQ

Length: 12min 3sec (723 seconds)

Published: Mon Aug 06 2012