Boron etch stop process

1

• Boron etch stop process
• (Michigan Probes)
• Theory EDP(ethylene diamine pyrocatechol)
  etch rate for P-Si is much slower than Si.
• Step 1) Microstructure formation, including
  deep and shallow boron diffusions
• 2) Wet etching in EDP release the
  probe

• Good
• Can build shank with rounded cross-section and a
  rounded sharpened tip,
• Cheap

• Bad
• EDP is toxic, and can cause cancer
• Process is hard to control
• Hard to make the process compatible with the 2
  direction DRIE Bellows structure fabrication

2

• SOI (Silicon on Insulator) Wafer technology
  (Stanford, Berkeley, Sweden)
• Theory Plasma etching rate for SiO2 is much
  slower than Si.
• Step 1) SOI wafers with different silicon layer
  thickness are commercial available
• 2) Front side DRIE to define the probes
• 3) Back side DRIE to release the probes

• Bad
• Expensive
• Probe shank are in the same thickness

• Good
• Process is easy to control
• process is compatible with the 2 direction DRIE
  Bellows structure fabrication

3
16-site Neural Probe Design
Trace width 5?m Site size 10 ?m 10 ?m
4
Process Flow

• SOI wafer
• Coat parylene as an insulating layer, deposit and
  pattern (lift off) Ti/Au as the electrode traces
  and the bonding pads
• Coat and pattern (plasma etch) parylene to open
  the electrode sites and the bonding pads
• Deposit and pattern (lift off) Ti/Pt as the
  electrode sites
• Deposit parylene as a protective layer and
  pattern it
• Font side silicon etch
• Back side silicon etch
• Oxide etch and resist strip

5
Connection
Standard Bus Interface
PCB
Bonding Pads for Gold Wire Bonding
Silicon Chip
6
Parylene Flexible Cable
7
Power Interface
Power Management
RAM
MUX
ADC
AMP
Bus Interface
RPROM
Wireless Interface
IC
IC
IC
IC
8
Probe Array