Keith Warner, Andy Loomis April 7, 2000 150mm Wafer Transfer Keith Warner, Andy Loomis April 7, 2000
Transfer Process Goals Adhesive-based wafer-to-wafer bonding 2 – 5 µm bondline thickness, thermal limits on post-transfer processing, +/- 2 µm alignment accuracy with existing tools Complete transfer of NU processes, also useful for other device types Oxide fusion bonding (Adopt existing process from CCD work) < 1 µm bond layer, post-transfer processing ~400°C (Tungsten CVD plugs, “hot” Al), +/- 1 µm alignment with tool to be purchased Determine technology limits on this program Sub-micron via placement with oxide bonding Build/specify tools using stepper technology techniques
Wafer Bonding - Voids Voids are formed after the SOI handle wafer is removed Particulate - Cleaning, clean area, inspection, filtration Pinholes and oxide quality - Starting material Adhesion – Cleaning Bubbles – Vacuum outgassing of epoxy
Wafer Bonding - Voids 12/16/99 3/2/00 (After wafer thinning decorative etch) 12/16/99 3/2/00 Void – free wafer transfer achieved
Wafer Bonding - Alignment Alignment method – depends on type of tool Early work used Research Devices flip-chip bonder Limitations of optical system restricted accuracy to +/- 5 µm
Wafer Bonding - Alignment Karl Süss mask aligner with infrared optics Better than +/- 2 µm accuracy, repeatable
Wafer Bonding - Alignment (Same wafer without voids) Accurate alignment achieved
Wafer Bonding – Bondline Uniformity (2-5 µm required across wafer) Wafer flatness – Double side polished ultra-flat wafers Adhesive curing cycle – Experiments started (Research Devices tool is not designed for wafer-to-wafer bonding; available pressure is inadequate) Spacers – Latex microspheres Particulate – Cleaning, inspection, filtration Still working on optimization
Wafer Bonding Apparatus and Operation
Status Void-free transfer successful Alignment accuracy adequate for devices currently available (“North2”) Method identified to meet bondline requirements – experiments are underway Full-time effort has started