Etch Rate: Chip Nanoengraving

In a chip factory (fab), the lithography process is like a top designer, drawing beautiful circuit blueprints on silicon wafers. But what really transforms this planar blueprint into a three-dimensional micro-nano structure is the etch process. The core indicator to measure the speed of the etching process is the etch rate.

What is the etch rate?

In simple terms, etch rate is the thickness of the material that the etching process removes per unit of time. Its standard unit is Å/s or nanometers/second (1 nanometer = 10 Å). For example, if the etch rate of silicon is 5 Å/sec, then 1 second of etching can remove 0.5 nanometers of silicon material.

It can be compared to the productivity of a sculptor:

Material = Board to be carved

Etching process = Engraver and his tools

Etching rate = How deep the engraver can chisel off wood per minute.

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Why is etch rate so critical?

Etch rates are never always better, they require the perfect balance between speed, precision, and control, and their importance is reflected in three aspects:

1. Precision control (accuracy) - "No more, no less, just right"

Modern chips have complex and delicate structures, and some film layers may be only a few dozen atomic layers thick. The goal of etching is to precisely remove a specific thickness of material, all the way to the next layer without excessive damage.

Too fast rate: It is difficult to accurately control the endpoint, and it is easy to over-etch, damaging the substructure and causing a short circuit or performance failure.

Too slow rate: Although the control is precise, it can seriously affect production efficiency.

One of the core challenges of the etching process is achieving a highly uniform and stable etch rate.

2. Productivity (Economy) – "Time is Money"

In a fab, an etching machine is worth millions to tens of millions of dollars. A wafer goes through dozens of etching steps.

The etch rate directly determines how long each wafer stays in the etcher.

Higher rates mean shorter wafer processing times, higher equipment throughput, and lower production costs. Therefore, on the premise of ensuring accuracy and yield, improving the etch rate is the unremitting pursuit of wafer fabs.

3. Process selection ratio (selectivity) - "Only peel the apple, do not damage the pulp"

This is a more critical concept derived from etch rate. In practice, we rarely etch only one material. For example, using photoresist as a mask to etch the underlying silica layer, we hope to:

High-speed etching silica.

Low-rate etching of the photoresist and the silicon substrate underneath.

The selection ratio is the ratio of the etch rates of the two materials:
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A high selection ratio means that the etching process is highly selective in removing the target material with little to no damage to the mask and stop layer. This is the basis for implementing complex 3D structures such as FinFET fin structures and 3D NAND storage holes.

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