Oxford Plasma Technology Plasmalab 80+ Reactive Ion Etch (RIE)

Operating Instructions (cont.)

This manual is © Oxford Instruments and Oxford Plasma Technology.
Issue 5: August 99
Converted to HTML by webmaster Anders Liljeborg

5.11 Endpoint detection

The endpoint detection system provides the facility to determine the endpoint of an etch process by either optical emission or laser interferometric techniques.

5.11.1 Optical Emission Spectroscopy endpoint strategy

Choose light emission from a gas component in the plasma, which is present when etching. The light intensity falls when the etch is completed. It is also possible to use a gas component which is more abundant after etching has completed, in which case the light intensity will rise at endpoint. A good endpoint will usually need at least a 10% change in light intensity at endpoint for reliable detection. The light intensity graph v time is typically an "S", curving from one nearly constant level to another. The level at the midpoint of the "S" should be used for endpoint threshold, with enough overetch time to complete the full transition.

Endpoint detection is detecting a threshold. For example, if the light output during etching is 5 volts, and becomes 3 volts when etching is completed, then a falling threshold of 4 volts can be used to detect the end of etching. All the other software features are used to make the detection of this event reliable even if the endpoint detection view port becomes dirty, or there is a noisy plasma or electronics.

During endpoint detection, the software carries out the following sequence:

1. NORMALISATION. This means "scaling the signal" or "varying the gain". At the end of the normalisation time the software stops using a 0 – 10 Vdc scale, and uses a 0 – 100% scale. It takes the current value of the signal and increases or decreases it to set it to the normalisation level. This can take a small signal and amplify it to a useful level. In production use, it can also take a signal which varies slightly from process to process and start it off at the same level each time. At the end of the normalisation time, the signal level on the graph should suddenly jump to the normalisation level.
2. CLOSED TIME. To limit the possibility of an early false endpoint, the software disregards all endpoint events ( signal crossing the threshold level ) during this time.
3. ACTIVE. After the closed time, the software is active and ready to look for a threshold-crossing event. If no event occurs, the process step should continue until the end of the normal process time.
4. CONFIRMATION. When a threshold-crossing event occurs, a timer is started. The signal must stay low (for a falling endpoint) or stay high (for a rising endpoint) for the whole of the Endpoint capture time to be counted as a valid endpoint.
5. ENDPOINT. This is marked on the graph when the threshold-crossing event occurs during the "Active" state and continues throughout the capture time.
6. OVERETCH. Endpoint detection by threshold crossing usually finds the time when about half the wafer has been completely etched. It is normally necessary to continue etching for a short time to etch the wafer completely. Overetch time sets a fixed time beyond the threshold-crossing endpoint during which the plasma stays on. Overetch level is an alternative way of setting the overetch time: it takes the time from start of process until the detected endpoint, and gives an extra percentage of this time. For example, a 20% overetch of a process which took 5 minutes to produce an endpoint event would continue for 1 more minute.

5.11.2 Laser interferometer endpoint strategy

If etching a transparent film completely, with an underlying layer which does not etch (e.g. silicon dioxide on metal), position the laser spot on an area which is being etched.

The interferometer output will oscillate while the film is etching, then become constant at some unknown value when the etch is complete. The time derivative of this signal will also oscillate and go constant, but the steady level will be close to zero. Use a "less than" threshold on the derivative to detect the complete etching of a film down to a non-etching underlayer.

1. Threshold value should be low enough that an unchanging reflectance gives a derivative value less than the threshold. This could be found beforehand using a plain wafer and no etching. It may need to be adjusted after etching a test wafer if the derivative gain is altered. If the threshold is set too low, then the noise level will make sure that the derivative signal never stays below threshold, so there will not be an endpoint. If it is set too high, then endpoint will happen early.
2. The condition "time derivative < threshold value" will be true many times while etching a thick film. While etching is still progressing, the derivative value will climb again from a low value, but it will stay low at the real end of etching. The endpoint capture time must be set longer than the dwell time at an oscillation point, to avoid a premature endpoint. When the threshold condition has remained true for the entire capture time, then an endpoint is marked.
3. The derivative gain is a scaling number to bring the time derivative signal on to the display graph. Note that a slow etch will need a much higher gain to get the same oscillation amplitude.

5.11.3 PC 2000 endpoint control panel settings

The endpoint control panel allows you to set up the endpoint parameters for either an automatic or manual process run. The panel is displayed on the Chamber 1 process page, accessed by selecting the Process menu, then the Chamber 1 option.

Note that the panel's appearance depends on the type of endpoint detection system fitted.

The facilities provided by the panel are as follows:

5.11.3.1 Etch depth panel

This panel is displayed as a result of selecting the Etch Mode button on the endpoint control panel and gives you access to the "etch depth" laser interferometry tools.

A software algorithm calculates an etch rate and an etch depth based on the number of turning points detected, the elapsed time, the laser wavelength and the refractive index of the layer to be etched. It can only make a calculation after two turning points have been detected, which requires an etch in the range:

or 85 – 170 nm for 670 nm laser wavelength and a silicon nitride film. Etch depths cannot be calculated for smaller etches than this.

5.11.4 Typical values for endpoint software

1. Threshold value should be low enough that an unchanging reflectance gives a derivative value less than the threshold. This could be found beforehand using a plain wafer and no etching. It may need to be adjusted after etching a test wafer if the derivative gain is altered. If the threshold is set too low, then the noise level will make sure that the derivative signal never stays below threshold, so there will not be an endpoint. If it is set too high, then endpoint will happen early.
2. The condition "time derivative < threshold value" will be true many times while etching a thick film. While etching is still progressing, the derivative value will climb again from a low value, but it will stay low at the real end of etching. The endpoint capture time must be set longer than the dwell time at an oscillation point, to avoid a premature endpoint. When the threshold condition has remained true for the entire capture time, then an endpoint is marked.
3. The derivative gain is a scaling number to bring the time derivative signal on to the display graph. Note that a slow etch will need a much higher gain to get the same oscillation amplitude.

This is the end of the Operating Instructions.