Etching process: end-point detection technology

In nanoscale engraving in chip manufacturing, the core challenge of the etching process is not only "how to engrave" but also "when to stop". Stopping the etching precisely is known as "end-point detection". This technology is like a pair of "eagle eyes" for real-time monitoring of etchers, and is the cornerstone of ensuring the high-yield operation of the trillion-dollar semiconductor industry. Its failure can lead to the scrapping of entire batches, and its accuracy directly determines transistor performance and chip reliability.

What is the endpoint? Why is it so critical?

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The end of etching refers to the ideal moment when the etching process completely removes the target material layer and stops at the interface of the underlying material. Its importance stems from two fundamental reasons: (1) If not stopped in time, the etching will continue to attack the underlying material that should be retained, causing irreparable damage. For example, if the silica dielectric layer is etched too through, the underlying silicon substrate or transistor gate will be damaged, resulting in device failure. (2) If stopped prematurely, the residual target material will obstruct subsequent electrical connections, resulting in an open circuit or high resistance.

In advanced processes, the thickness of the film is only a few tens of nanometers, and there are small fluctuations in the etch rate of different wafers and different regions of the same wafer. As a result, relying on fixed-time "blind engraving" is no longer feasible and must rely on real-time, dynamic end-point detection to ensure that each etch is accurately in place.

Core principles

The core idea of endpoint detection is that during the etching process, a physical or chemical signal that changes with the change of material is monitored in real time, and when the signal changes characteristically, it is determined that the end point has been reached. The main methods are as follows:
1. Optical emission spectroscopy: the most mainstream and classic "spectral eye". This is the most widely used endpoint detection technique, and its principle is like the "fingerprint spectrum" of analyzing etching reactions. The principle is that the plasma in the etching cavity contains various active groups (atoms, ions, free radicals). When the target material is etched, its atoms or reaction products (such as Si, Cl, F, etc.) are excited and emit a characteristic spectrum of specific wavelengths. Through the quartz window on the cavity, the optical sensor collects the plasma luminescence signal and imports it into the spectrometer for analysis. The system continuously monitors the intensity of the characteristic spectral lines representing the target material. For example, when etching silicon nitride, the CN group or SiN spectral line is monitored. As the target layer is gradually engraved, the intensity of the spectral line decreases sharply; At the same time, the intensity of the spectral lines representing the underlying material will increase suddenly. The intersection or inflection point of two curves is defined as the end point of the etching.

2. Laser interferometric endpoint detection: "thickness radar" for thin films. This method is particularly suitable for etching transparent or translucent media films. The principle is to irradiate a specific wavelength of laser light onto the wafer surface. The laser is reflected on the upper and lower surfaces of the film, i.e., at the interface with the underlying material. These two beams of reflected light interfere due to the difference in optical path. The interference signal becomes thinner as the film is etched, and the optical path difference changes continuously, resulting in periodic changes in the interference intensity of the reflected light. Monitor the number of periods or final waveform mutations in the interference signal. When the film is etched to zero thickness, the interference disappears and the signal undergoes a characteristic jump that indicates the arrival of the endpoint. This is accurate to a single interference cycle, corresponding to thickness changes at the nanoscale.