プラナリゼーションCMP委員会

Characterization of the aluminum and PMMA CMP (chemical
mechanical polishing) for transparent plastic display
Nam-Goo Cha, Young-Sam Yu, Young-Jae Kang, Chang-Hwa Park and Jin-Goo Park †
Division of Materials and Chemical Engineering, Hanyang University, Ansan,
426-791, Korea
†
[email protected]
A Transparent display seems to become popular in these days. This concept is simple,
really cool looking, and has many applications. The future transparent display demands
lightweight, low cost and high power efficiency. Flexible and transparent flat panel display
will be generating a technology based on plastics. For the high performance of plastic
displays, new materials and fabrication technology should be developed. Conventionally,
the metal line is fabricated by lift-off method that introduces the surface swelling and
roughness. Additionally, solvent based lift-off method easily attacks the plastic surface
during process. In this paper, we suggested that HECMP (hot embossing and chemical
mechanical polishing) process as a new metal line fabrication method on plastics. The
effects of polishing parameters of Al and PMMA (polymethylmethacrylate) were
investigated. The CMP process experiments were conducted using an IC1400 pad and
home-made slurry which was composed of Al2O3 abrasive in acidic solution. Patterned
transparent PMMA wafer was fabricated by hot embossing process. In order to make metal
lines, a 200 nm thick aluminum film was deposited on the patterned PMMA wafer by an
evaporation method. CMP condition was tested as functions of pressure, acid concentration,
and slurry content. Low pressure and low slurry content showed good result. Optimized
CMP process showed almost same removal rates of Al and PMMA respectively. The top of
deposited aluminum was removed by optimized CMP condition. Finally, the inlaid metal
lines which had a various line width from 10 to 50μm were fabricated in PMMA.
Keywords : Hot embossing, Al CMP, PMMA, Transparent display.
INTRODUCTION
Plastics have replaced conventional materials in many industries because of their low cost, lighter
weight, unique electrical and mechanical properties. These properties are desirable for the flat panel
industry where develops a flexible, lightweight and power efficient display. The transparent plastic
display has recently demonstrated using polymeric OLEDs (organic light emitting devices).
Furthermore, the transparent display has reached the point where commercial production is
commencing. To achieve a high speed operation in a large area transparent display, it is necessary to
have high conductive lines. A conventional transparent electric wire made by ITO (indium tin oxide)
has problems such as a high sheet resistance and poor flexibility. Organic conductive polymers still
have a high sheet resistance [1-5]. One of issues of making metal lines on a plastic substrate is that a
transparent plastic can not be patterned by a conventional photolithography method due to its optical
limitation. Also organic chemicals may attack the plastic surface in a conventional lithography process.
Hot embossing lithography (HEL) is recently developed for a high-resolution and low-cost pattern
generation method. [6-8]. PMMA (poly methyl metacrylate) is well known material for optical devices.
PMMA has a low cost, good transmittance and climatic resistance, and harmless to people. [9, 10].
CMP (chemical mechanical polishing) has been used as an important process in semiconductor chip
fabrication. CMP is also being applied in fabrication processes for micro electromechanical systems
(MEMS). As semiconductor chips are highly integrated, more precise planarization of layers on the
chips is required. Aluminum thin films have been used as metal lines of the semiconductor chip since
they have low electrical resistance and an easy manufacturing process [11, 12]. But the developed Al
CMP process is optimized between Al and oxide surface which are relatively harder materials than
plastics so that it can’t directly apply for the Al and plastic mixed surface. Conventional Al CMP
process shows higher removal rate of PMMA and poor selectivity.
In this paper, we suggested that HECMP (hot embossing and chemical mechanical polishing)
process as a new metal line fabrication method (Fig. 1) and developed CMP process which has same
removal rates of Al and PMMA.
Fig. 1. A schematic diagram of the HECMP (hot embossing and chemical mechanical polishing)
method to fabricate metal lines on a plastic substrate.
EXPERIMENTAL MATERIALS AND PROCEDURE
A 4-inch Si(100) wafers, which had a 400 nm thermal oxide layer, was used to make a mold for hot
embossing. Semiconductor grade chemicals were used in the experiments. A thermal oxide wafer was
cleaned by piranha solution (H2SO4:H2O2 = 4:1) for 10 minutes and D.I. water rinsed. A layer of
positive photoresist (AZ1512, Clariant Corporation, USA) was spin-coated at 3000 rpm for 30 second
using a spin coater (CEE4000, CEE, USA). The coated photoresist had a 1 μm thickness and was
baked on a hot plate at 100 oC for 1 minute. The surface was exposed by a UV Aligner (EVG, EVG620,
Austria) through a photomask and developed. The photomask was generated using a CAD software
(AutoCAD, Autodesk, USA) and was printed on a plastic film by a photoplotter (LPP, Pentax, Japan).
The photomask had 10 μm line and space zones and 50 μm single lines. The patterned photoresist
wafer was dry-etched in a dry etcher (AOE, STS, England) with C4F8. After oxide etching, the
photoresist was removed in acetone. Silicon wafer with patterned oxide was dry-etched in the dry
etcher with SF6 and C4F8. Finally silicon mold with 6 μm height was fabricated. A height of the
patterned silicon mold was measured by a non-contact profiler (Microsurf, Nanofocus, Germany).
To insure a clean demolding process, an antistiction layer was coated on the silicon mold. Variable
angle spectroscopic ellipsometry (VASE, J.A. Woollam, USA) with rotating analyzer was used to
measure the thickness. The dynamic measurement was carried out at an incident angle of 70o and 75o.
The wavelength of lights was 400 nm to 800 nm and a Cauchy model was applied for the optical
modeling. A 1 mm thick PMMA (polymethylmethacrylate) (Sewha Polytech, Korea) wafer was
prepared for hot embossing replica. Hot embossing was achieved at 120 oC, 12.6 bar for 10 minutes
using an EVG520HE (EVG, Austria). The 200 nm thickness of Al film was deposited on the imprinted
PMMA wafer after hot embossing by a homemade thermal evaporator. A sheet resistance of the
evaporated Al was measured by a 4-point probe (CMT-SR2000N, Advanced Instrument Technology,
Korea). A pattern width of the inlaid metal lines on PMMA wafer was measured by an optical
microscope (L150, Nikon, Japan)
CMP process to make Al metal lines on PMMA wafer was achieved by a Poli500 (G&P Tech,
Korea). An IC1400 (Rohm&Haas, USA) pad and a γ-alumina (99.99%, 13nm primary particle size,
Degussa, German) was used as a polishing pad and slurry, respectively. Polishing conditions was done
at a flow rate of 150 ml/min, head speed of 50 rpm, platen speeds of 50 rpm. The base of the slurry
was a mixture of H3PO4+H2O2+citric acid+Al2O3 in de-ionized water. Various amounts of H3PO4 and
Al2O3 were added respectively for different experimental demands.
RESULTS AND DISCUSSIONS
Fig. 2 shows a photograph and 3-D profile of the fabricated silicon mold by photolithography
method. All patterns were well fabricated over 4 inch silicon wafer with the average pattern height of 6
μm.
Fig. 2. (a) Photograph of the fabricated Si mold and (b) 3-D image of a patterned silicon mold.
Dimension of pattern was 10 μm width, 10 μm space and 6 μm height.
An antistiction layer of around 50 nm thick was coated on the silicon mold before the hot
embossing. After hot embossing process, the patterned PMMA wafer showed the same pattern
dimensions as that of the silicon mold. After, Al was deposited on the patterned PMMA wafer by
evaporation method. The sheet resistance of the deposited Al was about 150 mΩ/□.
To get a 1:1 selectivity of Al and PMMA, CMP process conducted Al wafer and PMMA wafer
individually. Fig. 3(a) presents the CMP removal rate of Al and PMMA as a function of H3PO4
concentration. The higher concentration of H3PO4, the higher removal rate of Al. But removal rate of
PMMA just slightly changed. Fig. 3(b) indicates that increasing the concentration of Al2O3 over 5 wt%
did not enhance the CMP removal rate. In case of PMMA, removal rate was higher than Al. Fig. 3(c)
illustrates the relation between the CMP removal rate and pressure. As pressure was decreased, the
removal rate of Al and PMMA was decreased. When pressure fell at 2 psi pressure, a 1:1 selectivity
was obtained. Fig. 3(d) shows pH was linearly decreased with the increase of H3PO4 concentration.
We guessed that this range where the balance between the chemical effect and mechanical abrasion.
Fig. 3. Removal rate of Al and PMMA as a function of (a) H3PO4 concentration, (b) Al2O3 slurry
contents and (c) pressure. (d) A pH changes as a function of H3PO4 concentration.
The optimized CMP process was performed to make inlaid metal lines on PMMA wafer with Al
with a slurry of 0.5M citric acid, 2wt% Al2O3, 48.75vol% H3PO4 at 2 psi. Fig. 4 shows the fabricated
inlaid Al lines on a 4 inch PMMA wafer. All of long and dense metal patterns were well fabricated
over the whole PMMA wafer scale. Optical microscope images shows the fabricated 50 μm long metal
lines (Fig. 4(b)) and 10 μm line width and space dense patterns (Fig. 4(c)).
Fig. 4. (a) Photograph of the transparent PMMA wafer which had inlaid metal lines after CMP process
and microscopy images of (b) 50 μm line and (c) 10 μm line width and space.
3. Conclusions
Hot embossing combined with CMP was proposed to fabricate metal lines on transparent plastic
substrates. Hot embossing was performed on a 1 mm thick PMMA wafer. In order to make metal lines,
a 200 nm thick aluminum film was deposited on the patterned PMMA wafer by evaporation and the
top of deposited aluminum was removed by CMP process. Hot embossing process could make patterns
on a transparent plastic wafer which was difficult to fabricate by the conventional optical lithography
technique.
Al CMP condition was tested as a function of slurry content, acid concentration and pressure.
PMMA showed relatively high removal rate than Al and poor selectivity with Al at high pressure and
high slurry concentration. In optimized condition, removal rate of PMMA and Al showed almost same
values. Damascene CMP process made inlaid metal line patterns on a plastic without using any
organic chemicals which was able to attack the plastic surface. Using proper slurry and polishing
conditions, Al metal patterns with 10 μm line and space were successfully fabricated on a transparent
plastic surface.
Acknowledgement
This research was supported by a grant (code#:05K1401-00215) from the Center for Nanoscale
Mechatronics and Manufacturing (CNMM) under the 21st Century Frontier R&D Programs of the
Korean Ministry of Science and Technology.
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