Assignment: Glass etching

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

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| 7. Assignment: Glass etching

The goal of this assignment is to learn about isotropic etching of glass and how to use the help cells provided in by MultiMEMS.
Why do we need to etch the glass?
The MultiMEMS process provides a triple stack, meaning a stack of glass,silicon and glass. The three structural layers provide the means of making cavities with a reference pressure, ports and channels. For our sensor containing a pressure sensor and a Venturi channel, the glass will provide the upper boundary for the Venturi channel as well as the channels to transport the flow from the outside of the sensor, over the pressure sensor membrane and to the entry of the Venturi channel. At the other end of the Ventruri channel, the flow is brought to the bottom glass, where channels are etched to allow the flow to pass under the bottom side of the pressure sensor membrane.

The path of the flow through the sensor. Note that the cross section is along the arrow, and that it is the same membrane the flow passes over and under, thus providing a differential pressure sensor rather than an absolute.

Isotropic etching of glass
Glass is not really a solid, but rather an liquid with very high viscocity. One could also think of it as an amorphous solid. By other means are there no repeating atomic structure in glass. It means that when chemically etched, the etch rate will be, in principle, equal in all directions. In MultiMEMS the process is tuned to give trenches of about 310 microns depth. Because of the isotropic etching, the mask will be equally underetched This is documented in the design handbook and illustrated below.
Because both the top and bottom glass is etched from both sides, it is possible to fabricate holes in both glasses. For this exercise we will use three of the four glass etch layers available:

- Bottom glass front side etch( BOGEF)
- Bottom glass backside etch(BOGEB)
- Top glass backside etch(TOGEB)

The naming convention relates to as if we see the sensor from the top side, that is the side of the bonding pads.

Two sensor dies seen from the top(left) and bottom(right)

Help cells
To ease the design of the glass etch masks, MultiMEMS provides a range of help cells. The intention is that you can position these at t he corners of you bottom or top glass mask features to help visualize where what the glass etch will actually produce.

MPW_ASSIST_CORNER_BOTG_308 | MPW_ASSIST_CORNER_TOPG_308

The layers featured in the help cells are projection layers equivalent to [BETCH]. The use of the different help layers is illustrated in Step by step: Bottom hole .

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Last updated: June. 2009 24.06.2009