Pattern Transfer Reading Chapter 7 and 8 Fabrication Engineering at the Micro and Nanoscale OutLine Photolithography Photolithography Experiment Photolithography Process Photolithography process Dehydration Bake Apply Adhesion Promoter Apply Resist Soft bake Exposure with Mask Post Exposure Bake Develop Hardbake Optional Processing Photolithography Process Dehydration Bake In order to obtain a smooth uniform coverage of the photoresist with good adhesion of the resist to the wafer for positive resists the wafer must be pretreated before resist application Firstly A dehydration bake Done at 150 200C in either vacuum or dry nitrogen as long as 30 minutes This step is intended to drive off most of the adsorbed water on the surface of the wafer Dehydration bakes can be done at much higher temperatures to further remove all of the adsorbed water but these high temperature bakes are much less common Photolithography Process Apply Adhesion Promoter Secondly Apply Adhesion Promoter Immediately following the bake the wafer is often primed with hexamethyldisilazane HMDS which acts as an adhesion promoter HMDS forms a hydrophobic surface improving resist wetting and adhesion The surfactant HMDS is ensure a more uniform coating between the photoresist and the wafer It helps ensure that patterned features do not lift off the wafer during rinse steps Photolithography Process Apply Resist Spin coating photoresist after it is primed The wafer is first mounted on a vacuum chuck When a wafer is placed on its surface the vacuum draws the wafer into intimate contact with the chuck A predetermined amount of resist is dispensed on the surface of the wafer Torque is applied to the chuck to rapidly accelerate it at a controlled rate up to a maximum rotational speed usually 2000 6000 rpm The acceleration stage is crucial to obtaining good uniformity since the solvents begin evaporating from the resist as soon as it is dispensed The wafer is spun at this speed for a fixed period of time then decelerated in a controlled manner to a stop Photolithography Process Apply Resist Resist can pile up at the wafer edge making contact lithography non uniform You can spin on the resist and pre expose dissolve this beaded up area to remove it before proceeding with pattern transfer Photolithography Process Apply Resist Resist thickness and thickness uniformity are critical parameters in developing a good photolithography process Thickness is not a strong function of the dispense amount Typically less than 1 of the dispensed resist remains on the wafer after spinning The rest flies off during the spin The thickness of the resist is primarily determined by its viscosity and the spin speed Higher viscosities and slower spin speeds will produce thicker layers of photoresist The resist thickness is found to vary with spin speed as A typical process might be a 30 sec spin at 5000 rpm to produce a resist thickness of about 0 5 m Photolithography Process Apply Resist Specially Stages of Resist Coating Spinning Artifacts Photolithography Process Soft bake After spinning the wafers must undergo a softbake or prebake The function of this step is to drive off most of the solvent in the resist and to establish the exposure characteristics The dissolution rate in the developer will be highly dependent on the solvent concentration in the final photoresist Generally shorter times or lower temperature softbakes lead to an increased dissolution rate in the developer and so to a higher sensitivity but at the cost of lower contrast High temperature softbakes can begin to drive the photochemistry of the PAC leading to resist dissolution of unexposed regions in the developer Photolithography Process Soft bake After spinning the wafers must undergo a softbake or prebake In practice the softbake cycle is empirically determined by optimizing the contrast while retaining an acceptable photosensitivity Typical softbake temperatures are 90 100 C Times range from 30 sec on a hotplate to 30 min in an oven The solvent concentration remaining after softbake is usually about 5 of the original concentration Photolithography Process Exposure with Mask After softbake the wafer is exposed Mask to Wafer Alignment Photolithography Process Develop After exposure the wafer must be developed Positive resists use nonalkaline developers such as tetramethyl ammonium hydroxide TMAH During the develop cycle Care must be taken to constantly maintain a pH of at least 12 5 in the developer in order to maintain a consistent process Develop process is very temperature sensitive It is important to control the develop temperature in order to maintain accurate control of linewidths During the develop process the developer solution penetrates the surface of the The depth of the gel called the penetration depth is negligibly small in positive exposed resist creating a gel photoresistor For many negative tone resists the swelling of the penetration region can lead to a distortion of resist features Photolithography Process Develop The develop process can affect the resist contrast and therefore the resist profiles When a 60 sec single spray puddle application of the developer was used the contrast was 3 14 However if two spray dispenses are done each of which is 30 sec long the contrast increases to 4 12 Photolithography Process Develop HMDS Surfactants may be added to the developer solution HMDS Surfactants reduce the surface tension and improves the ability of the developer to wet the surface of the wafer HMDS Surfactants serve as a dissolution inhibitor blocking the developer from the unexposed regions Ultrasonic waves may be used in conjunction with the surfactant to optimize the contrast Applying and Developing Photoresist Hardbake Hard bark High temperature bakes after develop this process is used to cross link the resist and so harden it against further energetic processes such as ion implantation and plasma etching Photolithography Process Hardbake Applying and Developing Photoresist Hardbake Applying and Developing Photoresist Hardbake Hardbake with Temperature 1 0 m line and space and large feature resist profiles for 60 sec hotplate bakes of varying temperatures At sufficiently high temperatures the resist profile begins to reflow producing a shallow profile during a subsequent etch the angle can be reproduced in the underlying film Basics of Photolithography for Processing Etch back Lift off Photoresists Photoresists Used for Pattern transfer into oxides metals
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