EE C245 Discussion 3 Ilya Gurin 9/22/2008 I: Examples 1. Lithography Going from mask to features: how do we determine the polarity of the features? What if we want a feature with the opposite polarity? (remove all PR except in the L-shaped area)? Use a clear-field mask (dark feature on a clear background). Negative PR is also possible, but more difficult. 2. Doping: limited-source diffusion Say we have a bare silicon wafer with a certain dose Q of boron predeposited on the surface. We diffuse it with a diffusion constant for time . (How would we find the diffusion constant?) How deep does it go? As an approximate measure, find the characteristic diffusion length: But what’s the actual junction depth? This is a limited-source diffusion (Gaussian profile). First, we need the surface concentration . We would find it by setting the integral of the the Gaussian distribution equal to the dose. Let’s say we’ve found that . Now, we know that: Try some numbers: Sidenote: when calculating the sheet resistance, need to keep track of the electrical activity limit 3. Etching: anisotropic etch Perfectly anisotropic etch. SiO2: 1 μm/min; Si: 0.5 μm/min Initial profile: Final profile: Mask (dark-field) Expose photoresist Develop photoresist (positive) SiO2 2 μm Silicon wafer4. Etching: isotropic release etch Perfectly isotropic etch (HF). SiO2: 1 μm/min; Si3N4: 1 nm/min Initial profile: Final profile: II: Tradeoffs in fabrication 1. Lithography Benefits Technology choice Benefits Depth of focus Exposure system Contact Proximity Projection Resolution Quality of alignment Die-to-die uniformity Avoids mask contamination Depth of focus Behavior in developer Resolution Easier to work with Photoresist Thin (< 2 μm) Thick (~ 10 μm possible) Topography coverage Uniform exposure over topography Ability to etch thick layers 2. Etching Benefits Technology choice Benefits Selectivity Etch rate Throughput Etch type Wet Dry Uniformity Anisotropy possible Repeatability Allows lower selectivities to underlying layer and PR Avoids PR “burning” Overetch Short Long Removal of stringers Mitigates nonuniformity Process simplicity Masking layer Photoresist only PR + “hardmask” Allows lower selectivity to photoresist Vertical sidewalls Silicon wafer SiO2 Si3N4 500 nm each 100
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