J.P. Wiff, Y. Takatsuru and K. Matsumaru
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1833-122X) Volume 6 November 2010
Results and Discussions
Typical chemical mechanical polishing (CMP) pad dresser consists of diamond
grains bound to a metallic substrate normally by a nickel electroplating layer.
The main problem with traditional CMP pad dressers is the drop out of diamond
caused by corrosion in the nickel plating layer. This work proposes to induce
a chemical bonding between diamond and a titanium substrate by utilization of
pulsed electric current sintering (PECS). One hundred diamond grains on titanium
discs were PECSed at different sintering temperature and time. The number of
retained grains after sintering increase with increase of sintering temperature
or period. XRD shows TiC, TiB, Ti2N and TiN on the titanium surface. It is considered
that TiC layer between diamond and titanium substrate is a key material for
bonding of diamond grains. TiC layer of 3 µm was formed by diffusion of
carbon in Ti.
Chemical Mechanical Polishing (CMP), Pulsed Electric Current Sintering, Pad
Dresser, Diamond, Ttitanium
In semiconductor industries, silicon and sapphire surface flatness plays an important role for accurate high performance in a wide spectrum of electronic devices. Chemical mechanical polishing (CMP) is one of the most common methods for planarizing the top surface of semiconductor wafers as it provides a very high surface quality at low cost [1-4].
During the fabrication of a CMP pad dresser, many diamonds are attached to a metallic surface usually by a nickel electroplating layer . However, during dressing some diamonds can drop out from the pad dresser and generate damages on the substrate surface in form of scratches.
The attachment of diamonds to the metallic surface is affected directly by the chemical environment used during polishing and dressing, producing corrosion around the diamond and increasing the drop out. TiC, TiN and TiB have been extensively studied as effective barriers against extreme pH conditions for different applications [5-7]. For this reason, it is expected that their use in a CMP pad dresser could avoid the diamond drop out and hereby improve the dressing performance.
Pulsed electric current sintering (PECS), which is also called commercially spark plasma sintering (SPS) or plasma activation sintering (PAS), is used as a rapid sintering method (some minutes) to densify various materials . PECS method provides an effective tool for generating a formation of carbide layer between diamond and titanium substrate. Bonding of diamond and titanium substrate by a chemical reaction is expected to have a stronger anchoring effect . Therefore, CMP pad dresser was fabricated by PECS. The attachment of diamond grains after sintering was investigated to obtain a minimum requirement of TiC layer for a stronger anchoring effect.
One hundred diamond grains around 100 μm in size were mechanically incrusted on titanium discs of 20 mm in diameter and 5 mm in thickness. Afterwards, the titanium discs and diamonds were PECSed under vacuum (1 Pa) at different sintering temperatures and times.
When using PECS a powder sample is pressed in an electroconductive die and a pulsed electric direct current is applied during all process to both sample and die. The internally generated heat at high rate (hundreds of Kelvin per minute) enhances the densification of small particles and reduces the sintering time to a few minutes.
A titanium disc was introduced into a graphite die and it was surrounded by a thick layer of boron nitride (BN) powder. Two graphite layers were used as a protection shield between sample and furnace as shown Fig. 1.
Figure 1. Sample set-up in PECS.
The attachment of diamond grains after sintering was calculated using the PECS performance factor (PP) defined as the percentage of diamond grains effectively attached onto the titanium substrate after sintering by PECS:
After sintering and dressing, all samples were characterized by: scanning electron microscopy (SEM, Keyence VE-7800) for calculating PP. Furthermore, X-ray diffraction (XRD, Shimadzu LabX XRD-600) was utilized for identification of the crystallographic phases.
Results and Discussions
There are two parameters for controlling the PECS process: 1) sintering time and 2) sintering temperature. The PP factor was evaluated in samples sintered for 5 min. under 10 MPa and at different sintering temperatures as shown in Fig. 2.
Figure 2. Diamond grain retention rate after sintering (PP) as function of temperatures, 10 MPa for 5 minutes.
Fig. 2 shows that at low sintering temperatures, the PP factor increases quickly as the sintering temperature raises. A maximum value for PP is achieved at 1300°C, but at high temperature, a high graphitization of diamonds is expected as well as a strong reduction of tool life of CMP dresser . Thus, in an ideal case, the sintering temperature should decreases below 1000°C, but simultaneously achieve a high PP value.
Fig. 3 shows the influence of sintering time on the PP value under 10MPa for different sintering temperatures. PP increases with increase of sintering period and is higher than that of a high sintering temperature.
Figure 3. Diamond grain retention rate after sintering (PP) under 10 MPa for different bonding temperatures as function of temperatures.
Figs. 2 and 3 suggest that at sintering temperatures over 1000°C the PP value should increases. However, the graphitization of diamond should also reduce the tool life for this CMP pad dresser. Therefore, the PECS conditions for evaluating the dressing performance of this CMP dresser were established as following: 1000°C as sintering temperature, 10 MPa as applied sintering pressure, and 120 min. as sintering time under vacuum (1 Pa).
After sintering, different crystalline phases on the titanium surface, namely: TiC, TiB, Ti2N and TiN compounds were detected (Fig. 4). Hereinafter all compounds will be indicated as “based” due to small observed changes in the lattice parameters, suggesting that some inclusion could be inside of their crystal structures. After sintering, metallic titanium diffractions were not detected evidencing the formation of a thick layer containing boron, nitrogen and carbon on the titanium surface.
Figure 4. XRD pattern of a sintered sample at 1000°C, 10 MPa for 30 minutes. No metallic titanium was detected, proving the existence of a thick layer consisting of TiC, TiB and TiN based layers on the titanium.
Fig.5 shows the influence of TiC layer thickness on the PP value. Thickness of TiC layer was estimated by carbon diffusion in TiC layer :
Figure 5. Diamond grain retention rate after sintering (PP) as a functions of thickness of TiC layer which estimated by carbon diffusion in TiC layer.
where χ, D t T R are thickness of TiC layer, diffusion coefficient, period of sintering, sintering temperature and gas constant, respectively. Fig.5 indicates that diamond grain retention increase with increase of thickness of TiC layer. 3 μm of TiC thickness is necessary for a retention of almost diamond grains around 100 μm in size. It is concluded that sintering conditions can be estimated to fabricate a CMP pad dresser with strong bonding on a titanium disks.
CMP pad was fabricated by a pulsed electric current sintering method for chemical reaction between diamond and titanium substrate to achieve a stronger anchoring effect.
TiC layer was formed between diamond and titanium during sintering. TiC layer is bonding material of diamond grains. Diamond grain retention increases with increase of thickness of Tic layer. Three μm of TiC layer is required for retention of almost diamond grains around 100 μm in size.
The authors wish to express their gratitude to the Japanese government for partially supporting this work through the 21st Century Center of Excellence (COE) Program of the Ministry of Education, Culture, Sports, Science.
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National Institute of Advanced Industrial Science and Technology (AIST)
2266-98 Anagahora, Shimoshidami, Moriyama-ku, Nagoya 463-8560, Japan
Y. Takatsuru and K. Matsumaru
Nagaoka University of Technology
Nagaoka, Niigata 940-2188, Japan
This paper was also published in print form in "Advances in Technology
of Materials and Materials Processing", 10 (2008) 63-66.