Assignment:Fluid flow through

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

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| 4. Assignment:Fluid flow through

The goal of this assignment is to learn about some of the key features of the process:

- Anisotopic etching of silicon
- pn-junction etchstop
- Frontside deep reacitve etching

To do this we will design a hole through the silicon wafer for which the liquid can pass through the wafer. This is useful because the fluid ports of the sensor are both on the backside and in order to use the pressure sensor to be designed in Assignment : pressure sensor as a differential pressure sensor rather than an absolute, we need to transport the fluid of interest on both sides of the sensor.

A tow view and corresponding cross section of the flow sensor illustrating how the fluid flowing through the sensor. The flow enters at the bottom side. Flows through a hole to a cavity in the front side glass and over the topside of the pressure sensor (blue arrow region) before entering the Venturi channel(red arrow region). Before exiting the sensor, the liquid flows through a recess etched in the bottom glass, under the membrane of the pressure sensor(note it is the same membrane as the fluid flows over), before exiting the sensor.

Before starting
The through hole is fabricated using several process steps.

NOWEL and 'Deep N- wells'

With no masking, a wafer processed in the MultiMEMS process will have three layers, which from the bottom and up are:

- Bulk silicon. P-type
- A n-well of about 20 micron thickness
- An epitaxial grown n- layer of about 3 microns thickness

The mask NOWEL is a masking mask, meaning wherever you define it, the deep n- well will not be present. This is implied by the name NOWEL suggesting there is NO N- WELl where you draw the mask. This is illustrated in the cross section featured through the region of the whole before the backside etching has been done.
Notice that over the arrow marked NOWEL, indicating the extent of the a NOWEL mask feature, there is no deep n-well underneath.

Anisotropic etching

Crystal silicon has a structure that governs the etching speed in different directions when etched with alkali etchants such as KOH, TMAH and EDP. In the illustration to the right, this is illustrated on the same cross section as above. The Backside ETCH (BETCH) mask defines an opening in the oxide on the back of the wafer which is used as a masking material. When the (100) wafers used in the process are etched, the resulting cavity is governed by the crystal planes of the silicon, giving slanted sidewalls of 54.7 degrees angle. In the setup files provided, there are some projection masks included, which of one of them is illustrated here. Recognized by the square clamps, [BETCH] is a helping geometry you must add yourself that will help you see what dimensions the etching will result in.
In addition to the anisotropic etching in TMAH, MultiMEMS utilize electrochemical passivation to passivate the n- regions. This means that the etching will terminate against whatever pn- junction it first meet. In the illustration to the right, this is illustrated as if the etch has terminated against the thin epitaxial n- region.

Deep reactive ion etching through the surface | The only thing that is missing to complete the through hole now is etching a hole through the thin region. This can be done using deep reactive ion etching, a process etching trenches with near vertical sidewalls. In the MultiMEMS process, the nominal depth of the etch is 10 micron, more than enough to remove the thin membrane left after the anisotropic etching(but not enough to etch through the thick n- well !). |

Assignment outline

- Define a region without n-well
- Define mask opening for backside etching with BETCH
- Add help features using [BETCH]
- Add a rectangle on RECTH to etch through the thin membrane

The through hole has to be larger than 400 X 750 micron, and we will use the bulk etch calculator, which found under support files, MultiMEMS bulk etch calculator , to calculate what dimensions is needed for NOWEL and BETCH

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Last updated: June. 2009 24.06.2009