1. Keith Warner, Andy Loomis April 7, 2000
150mm Wafer Transfer
Keith Warner, Andy Loomis
April 7, 2000

2. 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

3. 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

4. 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

5. 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

6. Wafer Bonding - Alignment
Karl Süss mask aligner with infrared optics
Better than +/- 2 µm accuracy, repeatable

7. Wafer Bonding - Alignment
(Same wafer
without voids)
Accurate alignment achieved

8. 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

9. Wafer Bonding Apparatus and Operation

10. 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