520.495/580.495 Microfabrication Laboratory

Laboratory Assignment #7

Objectives:

I) To scribe the wafer and harvest the fabricated DNA microarray chips.

II) To mount the chips on printed circuit boards  for testing and characterization.

Preliminaries:

1. When scribing the wafers be extra careful and gentle.

2. Use tweezers to handle the dice; dispose propertly the extra silicon pieces in the "sharps" container.

I. Prelab Work:

  1. Visit the wafer map to pick a sensor pattern that you like to package and write down the number that corresponds to your DNA micro-array. 

II. Lab Work:  

The final step in the lab process is the packaging of the dice. Our package is simple; a  printed circuit board  with copper wire patterns etched on it. The wafers will be scribed by hand by dragging a diamond tipped tool across the wafer to score the surface. Subsequently, the individual DNA microchips will be separated from the wafer, and mounted on the package using epoxy. The wire bonder will be used to make connections from the bonding pads on the die to the leads of the package.

A.  Scribing  the wafer

  1. Place your wafer on a hard but flat surface on a clean filter paper.
  2. Using the  tool with a diamond tip, scribe lines at the locations indicated in the wafer map. Scribe each line twice or three times, being very careful not to break the wafer.

B.  Dicing the wafer and harvesting the dice.

  1. Place your wafer face down on a clean filter paper, on a piece of rubber mat, noting the orientation and location of the scribe lines. 
  2. Position a pencil over the scribe line and press down gently. Rock the pencil back and forth while increasing the downward pressure until the wafer breaks. 
  3. Repeat this until the individual dice are harvested.
  4. Inspect the individual chips and try to pick the best one or two for packaging

C.  Packaging (please see schematic diagram here)

  1. Mix up the epoxy following the instructions provided by the Teaching Assistant
  2. Place some epoxy at the edge of the printed circuit board and carefully place the die on the epoxy making sure that the square bonding pads on die and the copper bonding surfaces on the printed circuit board are on the same side! The mask that we used to make the printed circuit boards is shown here.
  3. Cure the packaged dies in the Blue-M oven for 15 minutes at 40°C.

D. Wire bonding

  1. Use the wire bonder to place bonding wires from the die to the printed circuit board. Do two bonding wires per pad for each of the two pads.
  2. Put a drop of epoxy on each of the bonding pads on the die and on the printed circuit board for extra protection (please see diagram here.)
  3. Cure in the Blue-M oven for 15 minutes at 40°C.

III. Postlab Work

  1. Draw a schematic for your sensor  that you packaged in the lab, marking its dimensions. Use the equipment in the lab to to make additional measurements if necessary.