Passivation of Copper During Chemical Mechanical Planarization

Transcription

1 1 Passivation of Copper During Chemical Mechanical Planarization SFR Workshop & Review November 14, 22 Amnuaysak, Chianpairot and Fiona M. Doyle Berkeley, CA 23 GOAL: to characterize the composition of passive films on copper by ex-situ spectroscopic and microscopic methods by 4/3/23.

2 2 Why do we care about passivation? Pad Asperities Abrasive Particles Semiconductor Substrate Motivation Layer of Liquid between Asperities and Passive Film Passive Film of Copper Layer of Copper Passivation desirable for planarization. Planarization occurs Passive film at the protruded region is preferentially removed Recessed region remains intact. Removal rate of the protruded region is greater than that of the recessed region. Correlation of passivation with solution chemistry and knowledge of nature of passive films is essential for predictive modeling of CMP. Kaufman s Model for Chemical Mechanical Planarization

3 3 E mv vs. SHE ph 4 ph 9 ph The Problem Removal Rate, nm/min Dissolution Rate Polish Rate H 2 O 2, wt% Removal rate of copper at ph 4 i, A/m 2 Polarization curve of copper without polishing Passivation occurs At ph 12 by electrochemical polarization Lower ph values in presence of hydrogen peroxide Removal Rate, nm/min Dissolution Rate Polish Rate H 2 O 2, wt% Removal rate of copper at ph 9

4 4 The Problem (Continued) E, V vs. SHE Cu 2+ CuL + Cu CuL 2 CuO Cu 2 O ph CuO2 2- Potential-pH diagram suggests that no solid oxidized phase forms at ph lower than 11 Nature of passive film induced by hydrogen peroxide needs to be investigated Investigation of passive films should identify conditions for effective CMP and allow CMP process to be modeled for known chemistries.

5 5 Our Approach Characterization of the degree of passivation. Verify the concentration range of H 2 O 2 that gives passivation occurs. Determine removal rate of copper by weight loss measurement. Determine the amount of Cu(II) dissolved in solution by flame atomic absorption spectrometry. Difference between these measurements is correlated with uptake of other oxygen species in films. Besides composition of H 2 O 2, time of exposure is an important parameter because the dissolution rate may be non-linear if passive films are forming. Monitoring the activity of Cu(II) in solution gives insight into non-linearity of dissolution rate of copper.

6 6 Our Approach (Continued) Characterization by X-ray photoelectron spectroscopy (XPS) and cross-sectional transmission electron microscopy (TEM) Copper samples showing various degrees of passivation will be selected for surface analysis by XPS and crosssectional TEM. Analysis will be done through the thickness of the film because layers of different oxide films form on copper under certain conditions. Beside analyzing films that are formed on copper in solutions containing H 2 O 2, analysis will be done on copper samples that were held electrochemically at potentials corresponding to various H 2 O 2 concentrations. The results will be compared.

7 7 Experimental Setup and Method Stir Bar Stirrer Plastic Lid PTFE String Copper Coupon Setup for Copper Dissolution Experiment Dissolution Rate Experiments Cleaned, weighed, copper coupons (23 x 23 x 1 mm, %) suspended in stirred solutions After testing, coupons dried and weighed Copper removal rate determined by weight loss Analyze Cu(II) concentration in solution after the test by flame atomic absorption spectrometry

8 8 Dissolution Rate of Copper and Concentration of Cu(II) in ph-9 Aqueous Glycine Solution Dissolution Rate of Copper in ph-9 Aqueous Glycine Solution (Weight Loss) Expected Concentration of Cu(II) in ph-9 Aqueous Glycine Solution (Calculated from Weight Loss) Removal Rate (nm/min) Nominal Concentration of H2O2 (wt%) Concentration of Cu(II) (mg/l) Nominal Concentration of H2O2 (wt%) Concentration of Cu(II) (mg/l) Measured Concentration of Cu(II) in ph-9 Aqueous Glycine Solution (AA Measurement) Nominal Concentration of H2O2 (wt%) -Tests done with old hydrogen peroxide solution. -Spontaneous decomposition appears to have lowered actual H 2 O 2 concentration. -New tests are to be done with fresh H 2 O 2.

9 9 Difference in Measured and Expected Concentrations of Cu(II) Difference in Cu(II) Concentrations (mg/l) Difference in Measured and Expected Concentrations of Cu(II) in ph-9 Aqueous Glycine Solution Concentration of H2O2 (wt. %) Green films were observed on copper after exposure in aqueous glycine solution at all concentrations of H 2 O 2 except at wt.% of H 2 O 2. Therefore the difference in measured and expected concentrations of Cu(II) accounts for this presence of oxide film. These oxide films must play a crucial role in dissolution rate of copper. The nature of these oxide films must be different at different concentrations as can be seen from the variation of dissolution rate with concentrations of H 2 O 2.

10 1 Expected Information from XPS and TEM Analysis and comparison of films on copper that were formed in solution containing H 2 O 2 and those that were formed in solution at potentials corresponding to various H 2 O 2 concentrations should reveal the chemical compositions of films that passivate the surface of copper. Knowledge of the compositions of the passive films, and correlation with the solution chemistry should enable us to construct a predictive model for CMP.

11 11 Future Work Monitor the variation of Cu(II) activity in aqueous glycine solution containing H 2 O 2 as a function of exposure time. This should give insight into non-linearity of copper dissolution rate. Characterize and compare films formed on copper by ex-situ microscopic and spectroscopic techniques under the following conditions Concentrations of H 2 O 2 representative of various degree of passivation. Electrochemical potentials corresponding to the selected H 2 O 2 concentrations. The nature of the films and knowledge of solution chemistries will provide information on the interaction between hydrogen peroxide and copper; this will allow CMP processes to be modeled.

12 12 23 Goals Characterize the composition of passive films on copper by ex-situ spectroscopic and microscopic methods by 4/3/23. Correlate the composition of passive films with solution chemistry and integrate that knowledge to CMP model by 9/3/23.