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SJSU EE 225A - Photolithography

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2/9/08Course PhotolithographyD.W. Parent2/9/08Course Photolithography I• Without photolithography there would be no integrated circuits because every process done to a wafer to fabricate diodes and transistors (implantation, oxidation, diffusion, and etching), would be done to the whole surface of the wafer. We would be limited to diodes and MOS capacitors the size of the wafer (Yes we could cut the wafer up into parts, but this is quite limited.)2/9/08Course Photolithography• Our Process: Singe Spin on photoresist Pre-Bake Expose Develop Post-Bake Etch Remove photoresist2/9/08Course Singe• What is it? Heat wafers to about 800 oC for five minutes in.• Why do we do it? SiO2and Si attract water and absorb it. Photoresist repels water, thus if the SiO2or Si have absorbed water, the photoresist will not stick. We can also ash particles on the wafers.2/9/08Course Spin on photoresist• What is it? Photoresist is a photosensitive film that can be selectively patterned, and can protect the underlying structures from your etch process. It is a polymer that contains a interlocking mechanism, photosensitive chemicals, and solvents.• Why do we spin it on? Spinning it on is the quickest way to uniformly coat the wafer with photoresist. It also drys out the solvent from the PR.2/9/08Course Spin on photoresist• Problems? Most of the PR is spun off the wafer. PR is about $900 per gallon.2/9/08Course Pre-Bake• What is it? Bake the wafers for 90 oC for 30 minutes.• Why do we do it? Drive out solventConvert liquid to solid Relieve stress during spin on step• It needs to be carefully optimized with exposure time. Too much softbake and the film will not be very sensitive to the developer. Too little and the film will be too sensitive.2/9/08Course Expose• What is it? Selectively expose the PR coated wafers with UV light with a predetermined amount of energy.• Why do we use UV light? We can resolve features down to about λ/2.2/9/08Course Develop• What is it? Mechanically agitate the exposed wafers in a developer solution.• Why do we do it? This removes the PR that we do not want. We have a selective pattern on the sample that will protect features from etching.2/9/08Course Post-Bake• What is it? We post-bake at 120oC for 20 minutes• Why do we do it? This drives out all the elements that would allow the PR to be attacked by the etch.2/9/08Course Etch or Process• Once we have completed these step we have to remove oxide so that we can diffuse n or p, implant or grow a different quality oxide on the wafer.• Note: We are not able to diffuse or oxidize until we have removed the patterning PR.2/9/08Course Survey of 90nm transistor issues• Poly Flare Changes Resistance and Capacitance makes hard to Model in standard Design KitDrawn DataDrawn DataPoly FlarePoly Flare2/9/08Course Survey of 90nm transistor issues• Line edge roughness Many transistor parameters depend on L. L varies for the same transistor!LW2/9/08Course Survey of 90nm transistor issues• Variance in channel length Can vary from run to run, die to die, and transistor to transistor! Off current VT2/9/08Course Survey of 90nm transistor issues• RC Delay Resistors Resistors impede current flow (R, Ω). Ohm’s Law Everything has resistance! except super conductors. Resistance increases the time it takes for data to get form one point to another, and causes heat dissipation (P=IV=I2R) as well as a Voltage drop (V=IR) The big issue for the future is that L is remains the same while W is shrunk each successive generation. (Researchers try to keep Hthe same or higher or use copper (low ρ). g An electric circuit element used to store charge temporarily, consisting in general of two metallic plates separated and insulated from each other by a dielectric (C, Farads).V=IRHWLR××=ρLWH2/9/08Course Survey of 90nm transistor issues• RC Delay Capacitor An electric circuit element used to store charge temporarily, consisting in general of two metallic plates separated and insulated from each other by a dielectric (C, Farads). Delay is how long it takes information to travel from one end of a wire to another. This can be found to be about .69RC.Metal LayersLWHInside filled with aDielectricHWLCr××=εε02/9/08Course Survey of 90nm transistor issues• RC Scaling IssuesAB A BA BAll dimensions are scaled, C betweenA and B reduced by scale factor.R is increased by the scale factor squared.The height of the metal lines is increased to decrease the resistance.This increases C!Data lines can now “talk” to each other more easily (cross-talk) and actually change the data.2/9/08Course 18Survey of 90nm transistor issues• Contact misalignment impacts the timing on timing.Resistance is directly proportionalto the distance from the contact tothe gate from Contact to GateMisalighnment causes a one sideof the transistor to have moreresistance than the other.2/9/08Course 19Sample Process to Etch Aluminumsilicon dioxidealuminumphotoresistThis is the lithography process Spin on the photoresistPhoto-resist is a light-sensitive polymer.2/9/08Course Expose the photoresistsiliconsilicon dioxidealuminumphotoresistglass maskUltraviolet lightSample Process to Etch Aluminum2/9/08Course Develop the photoresistThis is the lithography processsiliconsilicon dioxidealuminumphotoresistSample Process to Etch Aluminum2/9/08Course silicon dioxidealuminumphotoresistsiliconEtch the aluminum in acidSample Process to Etch Aluminum2/9/08Course siliconsilicon dioxidealuminumRemove the photoresist in a basic solutionSample Process to Etch Aluminum2/9/08Course Basics• Pattern registration How well can a previous mask step be fit to to following mask step.DiffusionContactWindow2/9/08Course Photolithography MetricsResolutionSmallest feature that can be resolved.Ultimately, it is not how thin a line of PR we can pattern, but also how thin we can carry out our processDiffusion under masksUndercutting of masksPattern registrationThroughputN+Original DimensionDiffsion undermaskOriginal Dimension under cuttingof mask2/9/08Course Photolithography• Nature of light Light can behave as if it were a particle and as if it were a wave. Depending on the type of photolithography exposure system will dictate how you have to treat light in your model.wavelength of lightIndex of refraction (1.0 in a


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SJSU EE 225A - Photolithography

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