Looking at a cross-section

You are trying to understand a stack of films by looking down on it, the way you would guess a cake from the frosting. The screen is a map. The object has height. A cut through the stack — as if you sliced the cake — is a cross-section. Layout is the top of that sandwich. The picture at the top is the side, the way an electron microscope sees it: substrate, thin metals, insulator, a connection between floors. Nothing in that grayscale image is a color you picked in the editor. A 0.5 µm metal drawn 4 µm wide looks like a fat brick on screen. The brick is wide, not tall.

The top view lies about height

Take a via that is supposed to connect a lower metal to an upper metal. In top view the opening looks like a small square on a larger rectangle. In the side view you need the lower metal present, the insulator present, a hole through the insulator, and the upper metal filling that hole. If the hole is drawn where the lower metal is missing, the top view can still look like a sandwich because another color peeks through. The side view shows an opening to nowhere.

People who build tiny moving machines (MEMS) learn this early. A beam that looks free in top view is glued in side view if the sacrificial film was never under it. Motion, etch, and leftover films happen in the dimension the map does not store.

The second picture is the same idea in an optical microscope, in color. Metal layers read as stripes. A via is a column. You still cannot tell which stripe is which process step unless you bring the recipe. You can tell that the films are stacked, and that a connection is a vertical object, not a square sticker.

How to draw the cut before you trust a picture

Draw a line on the top view through the interesting place: through a via, through a MEMS gap, through a lid over a liquid channel. Then list the films in order, with thicknesses. Label which layout color is which film. This is the cheapest simulation available. Confusing width with thickness is how people invent impossible stacks.

Some colors never become film. They are notes: pins, blockages, keep-outs. A cross-section that extrudes those notes into matter is a cartoon of the process. Leave them out of the side view.

Board people already think this way. Their sandwich list is a stackup, thicknesses in millimeters and micrometers. Chip people hide the same list in a technology file. The pictures on this page show what that list looks like after someone polishes a real edge. They are not a substitute for the list. They are the check that the list was about matter.

Use the grayscale picture as a reading exercise. Find the substrate, the lowest continuous region. Find a metal that runs sideways. Find the vertical plug that joins two metals. If you cannot point to the insulator between those metals, you do not yet know whether the plug is a via or a smear from the polish. Real cross-sections have scratches and contrast that a textbook brick does not. That roughness is a reason to read slowly, not a reason to prefer a cartoon.

Then do the same with the color picture, and refuse to let the brightest stripe win. Display color in an editor is a choice. Color in an optical micrograph is also a choice of illumination. Process order is the factory’s choice. Write the order down beside the cut line you drew on the layout. When the side view and the top view disagree, the side view of a real edge wins, and the layout gets a layer map correction — not a prettier extrusion.

Print the foundry cross-section that matches the grayscale picture, and lay it beside the layout. Point at the vertical plug. Turn every layer off except the via layer. A shape should sit in the top view at that same place. If the micrograph shows a plug and the via layer is empty there, the contact was drawn on a metal layer or on a note, and the circuit is open even though a rectangle is still on the screen. Move the shape onto the via layer, then turn the metals back on and confirm the plug still lands on both of them.

A note you leave extruded will not appear in the next micrograph. The plug will. Name that plug on the grayscale picture before the brightest stripe in the color picture is allowed to choose a layer for you.

Width is not height

Take the metal the opening called a 0.5 µm film drawn 4 µm wide. On the screen that shape is a fat brick. In the grayscale picture the same metal is a thin stripe: wide across the page, short in the stack. Four divided by one half is eight. The brick is eight times wider than it is tall. A person who reads the on-screen height as the film thickness has invented a metal as tall as it is wide, and no recipe grew that.

Draw the cut line through that via on the top view. List the films in the order the grayscale picture shows: substrate, lower metal, insulator, the plug, upper metal. Write 0.5 µm next to the metal that is the thin stripe, and 4 µm next to its run along the page, so the two numbers cannot trade places. Then do the color picture second. The brightest stripe is illumination, not the top metal, until the list says so. If the plug in the side view has no via shape under the cut line, move the shape onto the via layer and look again. The side view of the real edge wins.