Step by step:Improving reproducibility

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Hands-on design training for MultiMEMS
D. Lapadatu and P. Ohlckers

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| 5.4 Step by step:Improving reproducibility

A model for electrochemical passivation is presented in 'A model for the etch-stop location on reverse-bias pn junctions’ by Lapadatu et. Al. Sensors and Actuators A 66 (1998) 259-267 which describe how the potential of the p-regions effect the passivation in a four electrode setup.
Considering the passivation to terminate when the depletion layer between the alkali etchant and the p-type silicon meets the metallurgical depletion region, the potential of the p-type silicon plays an important role. If it is left floating as in the three electrode setup (a discussion on the electrode configurations can be found in Kloeck 1989), the potential of the p- region might build up, effectively resulting in a premature passivation compare to the four electrode setup used by MultiMEMS.
We will therefore add structures to our circular membrane that will help control the potential of the p-region under the circular membrane. You will find a dedicated section on this in the design handbook under 'Extended p-contacts'.
Extended p-contacts
To improve the control on the potential of the p-region under the circular membrane, buried conductors will be connected to the region. This can be done as illustrated above.
Step by step

Open cell 'Bridge_Branch' | The cell 'Bridge Branch' contains one quarter of the leads for the Wheatstone bridge. In addition , it contains the outline for an extended p- contact. This has been done because we need to not only connect to the p- region under the membrane, but also to cross the leads of the Wheatstone bridge. |
Open cell 'Extended P contact' |

Add a substrate contact in both ends

- Add to '6x6_SUBS_CONTACT' to the cell.
- One centred at the left bend
- One centred over the cross

Flatten cell and delete the NOWEL at the outer end

- Flatten cell can be done in L-edit via the menu 'Cell- >Flatten'
- Flatten cell can be done in LayoutEditor via the menu 'Cell- >Cell flat'

Add a NOWEL rectangle to the inner end | Best done by using a wire that follows the help structure |
Connect the NOWEL to the BUCON at the contact | Best done by using a wire that follows the help structure |

Connect the two contacts with MCOND | The metal is on top of the silicon and if you look at the 'Bridge branch' cell, you will find that the Wheatstone bridge has a region with only BUCON, enabling crossing the metal over the conductors of the Wheatstone bridge. |

Your 'Bridge branch' cell should now look like this |

Subassignment finished. Save the file

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Last updated: June. 2009 24.06.2009