7.18 7.18

9.209.20

8.608.60

6.40

6.20

6.40

6.806.80

6.20

5.005.00

Infineon Technologies DresdenAdvanced Data Processing

Combining full IEP spectrum with an interferometric signal for depth etch endpoint detection

Frank Hoffmann, Infineon Technologies Dresden GmbH & Co. OHG Knut Voigtländer, Advanced Data Processing GmbH

Motivation• In absence of a stop layer, etch process and

equipment control is typically done by depth monitoring using an interferometric endpoint system.

• The state-of-the-art method of a single UV/VIS wavelength approach has to balance between appropriate signal-to-noise ratio and interference frequency.

State-of-the-Art Approach

Fig. 1: Process Scheme, shallow recess etch with no stop layer.

8th European Advanced Equipment Control / Advanced Process Control (AEC/APC) Conference Dresden - Germany, April 18 - 20, 2007

Method Description• This approach uses full spectra

time signals to form the interferometric reference trace by combining the individual wavelengths.

Fig. 2: Set of reference measurements containing the full spectra; the time behavior even of the shortest available wavelength has long signal period and is not suited for exact endpoint control.

• A PCA (principle component analysis) based method is used to decompose the full spectra into its linear independent components. A nonlinear optimization is used for a suited superposition of these components providing the final combined interferometric time signal.

Fig. 3: With PCA decomposition the independent signals can be discovered. A Modeling procedure is used to try to superimpose these signals to fit a given fast oscillating target signal.

• Model quality depending on target signal period and phase:- blue – very good modeling possible with

large signal period.- green – sufficient modeling with fast oscillating target- red –too fast oscillating target – no good modeling possible

modeling notpossible for 5starget period

very good modeling for 20s target period

for 10s target modeling is just possible

Fig. 4

• The resulting inter-ferometric refe-rence signal has shorter periods (more frequent Min/Max features), less noise and more clear inter-ference information.

Results • The full OES and IEP spectrum is collected by a standard EyeD endpoint system from Applied Materials, Inc. (spectral range 200 … 800 nm, time resolution 0.1 sec).

• The incoming IEP spectra is multiplied by the weighting vector during etch.

• The final interferometric time-based signal is generated by summing up all weighted spectral intensities within a relevant spectral range.

Fig. 5: Weighting vector for online calculation of the interferometric reference signal.

Fig. 6: Interferometric reference signal obtained by single wavelengths and by the PCA method. Final IEP trace with pattern (t1 … t4) detection.

blue: Interferometric reference signal provided by the pca method (smoothing over 5 data points)

red/green:Interferometric reference signal obtained by a typical single wavelength (280/229 nm, smoothing over 5 data points)

• The final interferometric time signal was used for in situ depth calculation using standard methods for endpoint algorithms.

• Using this method for endpoint detection a more reliable depth control for these shallow recesses is achieved in high volume production.

Fig. 7: Depth distribution (productive data) with single wavelength (left) and pca algorithm (right).

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