Issue No. 251〔Technical Papers〕
Application of In-situ Observation Technologies in CMP
Process for Upgrading the Process Integrity
Author
Shohei SHIMA*
Satomi HAMADA**
Chikako TAKATOH*
Yutaka WADA**
Akira FUKUNAGA**
- *
Technologies, R&D Division Advanced Analysis Department
- **
Precision Machinery Company
1. Introduction
2. In-situ (in-liquid) observation techniques useful for CMP process development
2.1 Optical evaluation technique 1: ellipsometry

Fig. 1 (a) Measurement section of the ellipsometry device and
(b) a cell containing a Cu sample
Cu samples were soaked in pure water solutions (with a concentration of 10 mmol/L) of two different inhibitors, BTA and m-BTA (methyl BTA), for three minutes to allow a protective film to form on the surface of each Cu sample. After theses samples were soaked in the pure water solution contained in the cell for a long time, changes in the Cu surface conditions were observed, i.e., the durability of the protective films was tested. Figure 2 shows the results of the durability test, indicating the temporal changes in the conditions of the protective films of the inhibitors formed on the Cu surfaces.

Fig. 2 Temporal changes in the thicknesses of the protective films on the
surfaces of the Cu substrates soaked in pure water, formed by two different
inhibitors: BTA (benzotriazole) and methyl BTA
Using an estimated optical-structure model of the Cu surface, ellipsometry estimates the actual surface structure by fitting the spectral characteristics estimated from the structure model with the measurement data. It is assumed that on the Cu substrate, a Cu oxidized film forms, on which a Cu-BTA complex layer develops into a protective film. The thicknesses of the protective layers shown in Fig. 2 are also derived from this optical model. Then, we focused attention on the changes in the thicknesses of the Cu oxidized films located at the interfaces between Cu and BTA and between Cu and m-BTA.
The in-liquid ellipsometry measurements shown in Figs. 2 through 4 require several minutes per measurement because the wavelength is changed during measurement. A technique is also available for evaluating changes in a short time. Specifically, it evaluates optical characteristics without changing the wavelength. Since this technique makes measurements without changing the wavelength, the evaluation duration is less than one second, enabling evaluation of short-time surface reactions.

Fig. 3 Temporal changes in the thicknesses of the Cu oxidized films located at the interfaces between Cu and the BTA protective layer and between Cu and the m-BTA layer

Fig. 4 Temporal changes in the refractive indices of the Cu-BTA and Cu-mBTA complex protective layers
Immediately after the Cu film is soaked in pure water, the measured Δ values start to decrease. The decreases in Δ value mean that an oxidized film is growing on the Cu film surface; we clearly observed that the Cu surface immediately started to oxidize from the oxygen dissolved in the pure water. At a certain point in time, a high-concentration BTA solution with 0.1 mol/L of BTA was dropped. When this solution completely mixed with the pure water contained in the cell, the BTA solution showed the same concentration used in the previous evaluations, i.e., a concentration of 10 mmol/L. Immediately after the BTA solution was dropped, the Δ value decreased abruptly and then gradually. An analysis of the growth of the BTA layer on the Cu surface during this process has been reported through joint research3) with University of Yamanashi based on EOI (Ebara Open Innovation); specifically, the research provided a detailed analysis stating that, first, a BTA molecular layer is absorbed, which is followed by the growth of a Cu oxidized film at the interface between Cu and BTA. The results shown in Fig. 5 probably indicate that the abrupt decrease in Δ value reflects the BTA adsorption and the subsequent slow decreases reflect the growth of a Cu oxidized film at the interface.
We subsequently evaluated the etching behavior of the BTA complex layer using the same technique. Figure 6 shows the results of the evaluation. Specifically, we evaluated the etching behavior of the Cu-BTA layer by soaking in pure water a Cu film with a Cu-BTA layer developed on it, and then dropping a solution of TMAH (tetramethylammonium hydroxide) onto it. At first, the Δ value stayed constant because the film was soaked in pure water. Then, we dropped a TMAH solution with a high concentration of 5% to evaluate changes in Δ value. Right after the TMAH solution was dropped, the Δ value started to increase, meaning that the BTA complex layer on the surface was etched. The Δ value peaked and then started to decrease gradually. The TMAH was diluted and resulted in a 0.5% concentration, which was still strongly alkaline; it is thought that a protective film of, for example, Cu(OH)x grew on the Cu surface once cleaned, which caused the changes

Fig. 5 Measured growth of Cu surface oxidization in pure water
based on a method for measuring Δ (phase difference) with the wavelength kept constant (633 nm) and measured growth of the BTA layer with the BTA solution dropped

Fig. 6 Etching behavior of the Cu-BTA layer caused by a TMAH solution based on a technique that measures Δ (phase difference) with the wavelength kept constant
2.2 Optical evaluation technique 2: fluorescent microscopy
It is reported that as a technique for observing how abrasive grains behave during the polishing process, an attempt was made to make measurements using total reflection microscopy based on evanescent light4).
We used red and green fluorescent silica particles that had a large diameter of 1.0 μm and a diameter of 50 nm, which was close to those of abrasive particles. In a preparatory experiment, green fluorescent silica particles were used and, as a result, both of the IC-1000 pad and the PAV roll brush for cleaning glowed green, demonstrating that abrasive particles could not be separately detected. On the other hand, when the red fluorescent silica particles were used, both the pad and brush only glowed weakly, revealing that only the red fluorescent silica particles selectively glowed, allowing the behavior of the abrasive particles to be observed.
This observation technique is characterized by its ability to allow for observation not only of silica particles at the interfaces between the cover glass and brush and between the glass and pad but also, by changing the focus, of particles inside the brush and pad. Figure 8 shows the observation results. Specifically, it shows the light emissions from the red fluorescent silica particles with a 1.0 μm diameter at the interfa between the PVA roll brush and cover glass and inside the brush (at points 20 μm and 40 μm from the surface). The brush is porous, with holes connected with each other. It was observed that on the surface, silica particles are not uniformly distributed and were making Brownian movement without adhering to the brush surface. This clearly indicates that the silica particles pushed out from the flat section of the PVA surface entered the inside of the roll even to a depth of 40 μm.
Figure 9 shows the distribution of the fluorescent silica particles in the vertical direction, obtained from much data about two-dimensional distributions of silica particles at different depths as shown in Fig. 8. In this way, the technique also allows for the observation of the distribution in the depth direction inside the PVA roll brush. The distribution shown in Fig. 9, that of particles of 1.0 μm diameter and the distribution of fluorescent silica particles of 50 nm diameter, which are almost the same as the diameters of abrasive particles, was also observed through a similar experiment; the use of this technique allows for the observation of the behavior of the actual abrasive particles. Although there is some concern that the fluorescent silica particles have physical properties different from those of abrasive particles, we think that fluorescent silica particles are almost the same as normal silica particles in adhesiveness, suggesting that fluorescent silica particles can be safely likened to abrasive particles.

Fig. 7 Observation of the behavior of fluorescent silica particles
likened to abrasive particles based on a laser confocal microscope

Fig. 8 Light emitted from fluorescent silica particles on the
surface and inside the PVA roll

Fig. 9 Light emissions indicating the depth-direction distribution of fluorescent silica
particles on a cross section of the PVA roll brush
2.3 Probe technique: Kelvin probe
For Kelvin-probe evaluation, we conducted an experiment using a tungsten probe with a diameter of 20 μm, based on a scanning electrochemical microscope system from Princeton Applied Research.
First, we measured the changes in potential when the oxidized film on the Cu surface was etched. Figure 11 shows the changes in potential on the surface of Cu film on which an oxidized film had naturally grown after the Cu film was soaked in pure water and then citric acid was dripped on it. Citric acid is a selective etching solution for Cu oxidized films. The results indicate that the dropped citric acid etched the Cu oxidized film on the surface, which accordingly caused rapid decreases in potential. Subsequently, the potential decreased at a lower rate and minimized, which then gradually increased. The experiment mentioned earlier5) demonstrated that the fact that the potential is low indicates an electrochemically active state. It is thought that the naturally oxidized film on Cu was etched to make the clean Cu surface exposed, resulting in decreases in potential. The subsequent gradual decreases probably resulted from complexation between Cu and citric acid because citric acid is an organic acid that forms a Cu complex. The increases in potential that followed were probably caused by reoxidation, which must be considered in detail in light of the results obtained with ellipsometry and other evaluation techniques.

Fig. 10 Kelvin probe sample stage for measuring the surface
potential of the sample in a wet atmosphere

Fig. 11 Changes in work function when a Cu film having a
natural Cu oxidized film is etched by citric acid
We then also made similar measurements of the formation of a BTA protective film. Figure 12 shows the changes in Volta potential during the Cu-BTA complex layer formation caused by a BTA solution dropped into the pure water solution in which Cu was soaked. The first few drops of the BTA solution abruptly decreased the potential, which immediately started to increase, but stayed at low levels without returning to the original level. After that, we dropped the BTA solution again. The potential also decreased but did not exhibit the same level of recovery as in the first dropping. This phenomenon is in line with the abrupt increase in film thickness followed by the subsequent slow increases observed through the ellipsometry-based evaluation (Fig. 5). The first abrupt decrease in potential and the subsequent increase represent the adsorption of the BTA and the formation of Cu-BTA complex protective film, respectively. The second dropping of BTA also caused a decrease in potential. This fact can be explained if it is assumed that, although BTA is adsorbed, the Cu oxidized film does not grow large because a complex layer already exists. In the first place, we assumed that the BTA protective layer would act as a barrier layer and the Cu potential would increase from the potential in the original state where no BTA layer exists; however, the experiment results revealed that the potential decreases. We measured the potential values in air for the regions with and without BTA layers to find out that the potential in the region with a BTA layer formed was higher than that in the surrounding regions, causing a barrier layer. It is thought, however, that the BTA layer does not provide a complete barrier layer (as demonstrated by the results of the ellipsometry experiment in Fig. 4) and consequently the reaction on the Cu surface continues, resulting in low surface potential values.

Fig. 12 Changes in Cu surface potential caused by dropping a BTA solution into the Cu film in pure water
This Kelvin probe technique, which uses a probe with a large diameter of a few tens of μm, does not provide a sufficient spatial resolution for evaluation of LSI wiring. Similar techniques for evaluating surface potential include the Kelvin force microscopy (KFM), a method based on an atomic force microscope (AFM) having an nm-level resolution, which cannot be used for in-liquid measurement evaluation. As a technique that enables in-liquid potential measurement, open-loop electric potential microscopy (OL-EPM)7) has been developed. We are currently trying to apply the in-liquid potential evaluation to the CMP process as the EOI.
3. Conclusion
References
Recommended articles
Inquiry about Ebara Engineering Review