End point detection, plasma etching, laser light polarization, interferometer, optical spectrometer.
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End point detection for plasma etching

In IC manufacturing, it is necessary to structure layers. In order to do that, layers have to be removed partially. This is done nowadays by dry etching or plasma etching.

un-etched	incomplete etching	complete etching	over-etching

First a resist is deposited on top of the layer, then the resist is illuminated and developed. A mask is formed. The mask is protecting some areas, while the unmasked regions will be etched, using plasma. The goal of the etching is to remove this one layer completely in the unmasked areas, but it should not etch the next layer below. In order to achieve this goad you have several possibilities:

1. Use chemicals which etch the layer that should be removed selectively, but not the layer below. In most cases, it is only possible to find chemicals which etch one layer better than another one.
2. Stop etching after a certain time. The necessary etch time is calculated from the etch rate, that has been evaluated by test runs before. Unfortunately this methode is not very precise.
3. Stop etching after removal of a certain thickness of material. This methode requires a thickness measurement system in the plasma etcher. A very common measurement system for thickness measurement is interferometry.
4. The polarization and interference of reflected light can be used to recognize the change of the optical properties of the surface, when the top layer is removed completely and the light is suddenly reflected from a different material surface.
5. Different chemical materials emit different wavelengths when they are brought into a plasma and vary the color of the plasma. When the emission spectrum of a plasma is measured by an optical spectrometer, then it is possible to recognize, when the top layer has been etched through completely, because the composition of the plasma changes slightly.

The last three methods are called "end point detection". All three methods can be implemented in Trion plasma etchers.

Interferometer

An interferometer is measuring the difference of two or more light paths by overlapping the residual light from both pathes, generating interferance fringes. A monochromatic light source is used and reflected from the surface of the sample. It is overlapped with a reference light beam. Small changes in the range of the light source wavelength can be recognized.

The graph shows the overlapping of two wavelength (green and blue) as well as the residual interferogram (red).
Trion offers its own interferometer for their plasma etchers. This endpoint detection system utilizes a 670nm laser with a spot size that can be focused down to 1-2 mm in diameter. The reflected beam from the wafer is received by a light sensitive photocell circuit which converts the intensity of the light into an analog voltage signal. This signal is then sent to the I/O in the process computer. As the thickness of the material changes during the etch process, the photocell circuit detects the destructive interference and displays this change in voltage versus time on the computer display. Knowing the characteristics on the film, the software can be configured to stop at a certain endpoint, either a certain total etch thickness, or complete removal of a material layer. This methode can be used also for end point detection during PECVD deposition and stop the deposition at a particular film thickness.
This assembly can also be used on ICP system with slight modifications in the positioning of the laser and detector.

Laser endpoint detection

This methode is a combination of interferometry and reflectometry in combination with powerful software to evaluate the measured signal.

The reflected light is a combination of signals from each layer within the sample and special interference fringes are formed for each sample and can be displayed on a monitor. The measurement spot can be positioned either on etching material solely or partially on the mask surface. For end point detection the interference fringe pattern can be simulated for various layers and then compared during etching with the measured signal. The method is very effective and can be used for monitor etching and end point detection of samples with two or more layers. It can be also used to monitor the etch back of dielectric on metal for a number of applications including failure analysis.
The system is installed in front of a window in the plasma etch chamber with incidence view of the wafer being etched. The upper electrode has a hole to allow the measurement. The system consists of an optical head, containing a 670nm laser source, detector and CCD camera, and a rack mounting electronics module. The optical head mounts on a XY and tilt adjustment so that the user can target the laser spot at a particular etch site. The spot size is around 50µm.

Optical spectrometer

The color of a plasma is determind by the species in the gas. Beside the moleculs and their fractions of the etching gas, also atoms from the etched layer get into the plasma and influence the color and the optical spectrum of the plasma. In some cases, when the etched material changes, the shift of the color is so strong that you can see it with your eyes trough the window of the plasma chamber. In order to measure the light emmission from the plasma, it is possible to use an optical spectrometer. Trion uses a small spectrometer, which reads light intensities of the plasma in the 250nm to 800nm range. A single-strand fiber optic cable attached to the side of the plasma chamber is used to transmit the light to the spectrometer, in which a fixed grating disperses the light across the linear CCD array detector. The system used by Trion is fast, can capture and store a full spectrum into memory every millisecond, and has a high sensitivity and resolution to 0.35 nm (FWHM). The software can display the intensity spectrum as the etch proceeds or it can show a specific wavelength versus time. The software can be set up to trigger an endpoint based on the change of intensity of a specific line.

Crystec Technology Trading GmbH will be pleased to further discuss details with you.
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