Submillimeter absorption spectroscopy in semiconductor manufacturing plasmas and comparison to theoretical models Yaser H. Helal, Christopher F. Neese, Frank C. De Lucia Department of Physics The Ohio State University Ankur Agarwal, Barry Craver, Paul R. Ewing, Phillip J. Stout, Michael D. Armacost Applied Materials, Inc. Sunnyvale, CA June 23, 2015

Semiconductor Chip

Overview A submillimeter absorption spectrometer was designed for Applied Materials as a potential diagnostic tool. Comparisons will be made between measurements and theoretical model for densities of plasma products. An evaluation for the model as well as the application of absorption spectroscopy.

Spectroscopy and industrial plasmas Submillimeter spectroscopy of plasmas is a well established technique Semiconductor processing plasmas are similar to plasmas developed to study radicals and ions Pressure of many semiconductor plasma processes ( mtorr) is especially advantageous Many species have been studied and in literature, some are cataloged in astrophysical databases (JPL, Cologne, …)

Density and Temperature Measured Absorption: JPL Intensities in catalog are reported for T 0 = 300 K Temperature Dependence: Plot of Intensity Ratio vs. Lower State Energy: It has been shown* that in the ground electronic state for a molecule, there is equilibrium in rotational-translational temperature. JPL Intensity, corrected for temperature Density n f = 1 if linear molecule n f = 3/2 if not InterceptSlope *Shimada et al., J. Vac. Sci. Technol. A 24, 1878 (2006)

To Chamber Spectrometer and transceiver were designed for use on Applied Materials’ plasma reactors. Evaluation of submillimeter absorption spectroscopy as a diagnostic tool in the field of semiconductor processing FM Heterodyne Spectrometer

Temperature Measurement at OSU Fitting for these lines with a 2 nd derivative Gaussian and computing the ratios of observed intensities, a Boltzmann plot was made and temperature was measured. Snippet

Temperature Measurement at OSU Temperature is observed to increase with plasma power, an expected trend.

Ar, CF 4, CHF 3 Plasma Model Plasma models include, as a function of location, EM fields, electron trajectories, fluid kinetics, and reaction rates and products Extremely complex and difficult science which relies on many parameters which are not always known with high accuracy Inductively Coupled Plasma (ICP) was modeled and measured for various conditions o Pressure: 20 – 50 mtorr o CF 4 Flow: 100 – 350 sccm* o CHF 3 /(Fluorocarbon) Flow Ratio: 0.3 – 0.7 *sccm = Standard Cubic Centimeter per Minute

Ar, CF 4, CHF 3 Plasma Model Power distribution in chamber Peak power deposition near top of chamber, below coils Ion density and gas temperature distribution correlated with power Submillimeter beam probes location where stable products survive downstream Electron and ion temperature range: 3 – 5 x 10 4 K Gas temperature: ~360 K Coils Wafer Nozzles Pump Chamber Axis Submillimeter Beam Path Azimuthal slice of chamber (cylindrical symmetry)

Model: Reactions and Products *Agarwal et al., J. Vac. Sci. Technol. A 30, (2012) Ar, CF 4, CHF 3 ~ 70 reactions included in model* Expected Products: Ar, Ar*, Ar +, e -, CF 4, CF 3, CF 2, CF, F, C, F 2, CF 3 +, CF 2 +, CF +, F -, CHF 3, CHF 2, HF, H 2, H, CHF 2 +, H 2 +, H + Positive identification was made for CF 2, CF and CHF 3.

Model Predictions CHF 3, CF, CF 2 were compared with models (52 mtorr plasma shown) CHF 3 is a feed gas; peak density near center of chamber. CF and CF 2 are produced in plasma. Peak density is away from center, towards coils, substrate and pump. CF 2 is more stable than CF, greater survival to chamber wall and pump. CHF 3 CFCF 2 [CHF 3 ] max = 4.5 x cm -3 [CF] max = 2.6 x cm -3 [CF 2 ] max = 5.7 x cm -3 Coils Wafer Nozzles Pump Coils Wafer Nozzles Pump Coils Wafer Nozzles Pump

Transition frequencies were chosen for high intensities, and small differences in lower state energies caused large uncertainties in determining temperature via Boltzmann plot. Pressure broadening was assumed to be zero, although it contributes as much as 10% of the width. While linewidth measurement from fits for FM lineshapes has significant variance, an approximation of temperature was made from measurements. Plasma model prediction for temperature: 360 K Approximate temperature measurement from spectra: 450 K Spectroscopic Temperature Linewidth (FWHM) determined for unmodulated line using parameters from 2 nd derivative Gaussian fit

For all cases, density measurements are a factor of ~10 below model prediction. It appears that more dissociation than is predicted occurs. Trends for plasma pressure and flows will be compared despite the absolute difference. Model and Measurement

Increase in pressure results in increased density for all three species studied. For CHF 3, the resulting slope of the prediction and measurement match. Pressure: CHF 3

For CF and CF 2, measurements show that the increase is not as much as predicted. Pressure: CF, CF 2

CHF 3 expected to decrease with increase of CF 4. Instead, it was measured to slightly increase. Increase in CF 4 flow results in decrease of CF. This is due to a decrease in electron density, since CF is a product of electron collisions with CF 4. Measurement shows that CF decreases and stabilizes sooner than the model predicts. CF 4 Flow: CHF 3, CF

CF 2 density is inversely related with CF 4 and CHF 3 flow. Electron density is inversely related to CF 4 as well, indicating electrons are not the cause. A product of the dissociation of CF 4 may be the dominant reactant with CF 2. CF 2 CF 4 Decreasing CF 4 Increasing It was found that an important reaction for CHF 3 was initially missing from the model

Summary Comparisons were made between measurements and model for densities of plasma products. Temperature is an important quantity to determine model validity. More work needs to be done to obtain accurate rotational temperature. Although models and measurements differ in absolute numbers, an evaluation of the model was done based on trends. An in situ measurement of density and temperature helps to improve plasma models, with a potential for impact on how we understand the reaction mechanisms in plasma.

Acknowledgements This work is supported by Semiconductor Research Corporation (SRC) through Texas Analog Center of Excellence at the University of Texas at Dallas (Task ID: )