MEMS is layout plus a recipe

You are trying to make a tiny comb of silicon that can move, the way a microphone diaphragm moves, not a picture of a comb. The picture at the top is that comb after the factory has done the part the drawing cannot show: fingers suspended over a pit, free to deflect. The top view you drew looked like any other chip — rectangles, holes, named sheets. The object in the photograph moves only because a recipe removed a film from underneath. Double the plate width and keep the same release holes, and the center can stay stuck after a timed undercut.

That family of moving, sensing, tiny machines is MEMS, micro-electro-mechanical systems. A MEMS layout without a process is a coloring book. Two labs can build different objects from the same drawing because the sandwich differed.

The recipe is half the letter

A process recipe is a run sheet: grow or deposit this thickness, pattern this layer, etch with this chemistry, strip this film. One color in the layout might mean “this is the structural beam.” Another might mean “this is the sacrificial film that will vanish.” Another might mean “cut a hole so the etch can crawl under the beam.” The hole is how the undercut sees daylight. Give each film of that recipe a thickness and a place in the order, and an editor can extrude the layout into a three-dimensional stack that shows the films in height.

The step that frees the moving part is release. The sacrificial material leaves, and the comb or membrane can move. If the recipe never deposits that sacrificial film, the fingers in the photograph would be a ridge stuck to the floor. The top view would still show a nice comb. Stiction — the comb snapping down and staying down — is not a spelling error in the geometry. It is physics the recipe failed to plan for.

The second picture is the scale the top view hides. A whole die sits on a fingertip. The comb you saw in the first picture is a small region of that die. Release holes, anchors, and the pit have to be drawn at that real size, not at the size the screen feels when you are zoomed in.

What the drawing still owes

You still draw in top view. Each color is one instruction, and the instruction only works if the recipe agrees. A hinge 2 µm wide that was supposed to be free, and is instead glued down, is a broken sensor. Enclosure of a release hole over the sacrificial film is the difference between a beam in air and a beam on the substrate.

Send the layout and the recipe name together. “The comb cell” is not an order. “This structural layer, this sacrificial layer, this release-hole layer, this process revision” is an order. The photograph at the top is what you are allowed to expect when those sentences match. It is not what the editor stored.

Open the cell and turn the structural layer off. The comb should disappear. The pads should not. If the pads vanish with the fingers, they were drawn on the film the release etch removes, and a wire on the die in the second photograph will have nothing solid to land on. Turn the structural layer back on only after you have named the layer that stayed. That name has to be a film the run sheet does not sacrifice. A seal ring, if the part must live in air or in vacuum, has to survive the same test.

Then put the released die under a microscope and focus on one anchor in the first photograph’s comb. Shift the focus to the fingertip of that same finger. If the tip goes soft while the anchor stays sharp, the finger is off the floor. If both stay sharp and you cannot find a gap, the finger is still stuck, which is what a timed undercut does when the plate grew and the holes did not. The file check and the focus check are one visit: the layer that must remain, and the finger that must leave. Pin the process revision next to the cell name before you call the photograph a success. A comb drawn for one sacrificial thickness is a different part on another thickness, even when every top-view edge matches. If the fingertip and the anchor are sharp in the same focus, count the release holes across the middle of that plate and compare the count with the pitch on the process sheet. A plate whose holes stop at the ends, with an empty middle, is the wide plate that a timed undercut will leave stuck. Mark that middle on a print of the structural layer and add holes before the next release.

A hinge that is free, and a plate that is not

Take the hinge the run sheet calls 2 µm wide and find it on the structural layer, then on the photograph of the comb. The hinge has to be on the film the recipe keeps. The film under it has to be the one the recipe removes. Turn the sacrificial layer on and confirm it runs under the hinge and under the plate, and that a release hole actually punches through the structural film into that sacrificial film. A hole drawn beside the plate, missing the sacrificial sheet, does not feed the undercut. The top view still looks like a comb.

Then compare two plates that share the same holes. Double the plate width and leave the hole pitch alone. The timed etch that cleared the narrow plate stops in the middle of the wide one. That stuck center is the failure in the first photograph’s cousin: fingers that were supposed to hang over a pit and instead sit on leftover film. The die on the fingertip in the second picture is the scale. The comb is a small region of that die, so the 2 µm hinge and the hole pitch have to be drawn at the size the recipe names, not at the size the zoom makes comfortable. Send the cell name and the process revision in one note. A comb whose edges match and whose sacrificial thickness does not is a different part.