4. Results
4.3. Adhesion
It is known that adhesion of copper to dielectric substrates, including
SiO2 and glass, is poor. Poor adhesion to glass substrate will be difficult to
employ copper in advanced TFT-LCDs, especially for bottom gate
staggered a-Si:H TFT-LCDs. It is therefore necessary to improve the
adhesion of copper to glass substrate. Scratch tests were carried out to
evaluate the adhesion between the Cu(Mg) alloy films and the SiO2/Si
substrates. The results showed that Cu film adhesion was enhanced
significantly as the Mg content increased, as seen in Fig. 22. This was
found to be due to the reaction between Mg in Cu(Mg) films and SiO2.
To investigate the effects of annealing on adhesion to glass, an annealing
process of as-deposited Cu(2.3at%Mg)/glass and Cu(4.5at%Mg)/glass
multilayers at a fixed temperature of 350℃ in 10mTorr of O2 pressure was
also carried out. Fig. 23 shows that adhesion of Cu(Mg) alloy films was
significantly improved by annealing process. To further improve the
adhesion to SiO2, the SiO2/Si substrate was treated by O2 plasma in
100mTorr at 300℃ for 10min. O2 plasma was created as a function of RF
power of 150W, 200W, 250W, and 300W at a fixed bias power of 30W.
Cu(4.5at%Mg) films deposited on the O2 plasma treated SiO2 were
annealed in vacuum at 500℃ for 30min. It was found that improved
adhesion strength was obtained, as shown in Fig. 24. Fig. 25 shows the
acoustic emissions of Cu(4.5at%Mg) films deposited on O2 plasma treated
SiO2/Si. After annealing, the peak of Cu(4.5at%Mg) film deposited on
SiO2/Si substrate without O2 plasma treatment appeared around 10N. On the
other hand, as RF power of plasma increased adhesion strength of Cu(Mg)
alloy films was generally improved. It can be clearly seen that O2 plasma
treatment provides the excellent adhesion strength of Cu(Mg) alloy films to
SiO2/Si substrate.
Fig. 26 shows AES profiles of Cu(4.5at%Mg) films annealed in vacuum at
500℃ without and with O2 plasma treatment. Upon annealing, the amount
of Mg moving to the interface between Cu(Mg) alloy film and O2 plasma
treated SiO2/Si substrate increased compared to that to the interface without
O2 plasma treatment. The increase in the oxygen amount at the interface
between Cu(Mg) film and SiO2/Si substrate by due to the treatment of O2
plasma caused the strong reaction with alloying element in Cu(Mg) alloy
film upon annealing, leading to excellent adhesion to SiO2/Si substrate.
To investigate the effects of O2 plasma on the adhesion of Cu(Mg) alloy
films to glass, glass substrate was also treated by O2 plasma in 100mTorr at
300℃ for 10min. O2 plasma was created as a function of RF power of
150W, 200W, 250W, and 300W at a fixed bias power of 30W.
Cu(4.5at%Mg) films deposited on the O2 plasma treated glass were
annealed in vacuum at 350℃ for 30min. Fig. 27(a) shows that adhesion
strength of as-deposited Cu(4.5at%Mg)/glass is poor. However, as RF
power of O2 plasma increased up to 200W, adhesion strength of annealed
Cu(4.5at%Mg) films to glass was significantly enhanced. From these
results, it was concluded that O2 plasma treatment of glass substrate
followed by annealing process provides significant improvement of
adhesion strength.
Fig. 22. Scratched images of (a)Cu(4.5at%Mg)/SiO2/Si,
(b)Cu(1.0at%Mg)/SiO2/Si, and (c)Cu/SiO2/Si multilayer.
(a)
(c)
(b)
Fig. 23. Scratched image of (a)as-deposited Cu(2.3at%Mg)/glass,
(b)as-deposited Cu(4.5at%Mg)/glass, (c)annealed Cu(2.3at%Mg)/glass,
and (d)annealed Cu(4.5at%Mg)/glass.
(a)
(d)
(c)
(b)
Fig. 24. Scratched images of Cu(4.5at%Mg) alloy films deposited on
(a)SiO2, (b)150W O2 plasma treated SiO2, and (c) 250W O2
plasma treated SiO2.
(a)
(c) (b)
20N
Fig. 25. Acoustic emissions of Cu(4.5at%Mg) films deposited on O2 plasma
treated SiO2/Si.
0 5 10 15 20
0 10 20 30 40
ref 150W 200W 250W 300W
Acoustic emission
Load (N)
Fig. 26. AES depth profiles of Cu(4.5at%Mg) films annealed in O2 pressure
of 10mTorr at 500℃ (a)without and (b)with O2 plasma treatment.
0 5 10 15 20 25
Fig. 27. Scratched images of Cu(4.5at%Mg) alloy films deposited on
(a)glass, (b)150W O2 plasma treated glass, and (c) 250W O2 plasma
treated glass.
(a)
(c) (b)
40N
4.2. Device characteristics
Fig. 28(a) shows the transfer characteristics of a-Si:H TFT using copper
metals as a gate electrode. The subthreshold slope and on/off current ratio
obtained from the transfer curve at drain voltage VD = 5V are 0.9V/dec and
∼106, respectively. The off-state leakage current is ∼10-13 A at VD = 5V and
gate voltage VG = -5V. The fabricated a-Si:H TFT exhibited a threshold
voltage of 11V. But its performance was not reliable. We have fabricated a
pure Cu gate an a-Si:H TFT with a silicon nitride gate insulator. The silicon
dioxide buffer layer was introduced between glass substrate and pure Cu
layer to improve the adhesion to the substrate. However, the Cu layer was
peeled off during wet etching process. Moreover, the silicon nitride leakage
current was very high because Cu diffused into silicon nitride.
Another try was to use BCB(Benzocyclobuten) as a diffusion barrier
between silicon nitride and Cu gate. The thin film Cu gate was planarized
with BCB. Cu metal was deposited on glass substrate by sputtering and
then was patterned for gates. The BCB layer was coated by a spin-coater.
Curing of the organic dielectric (BCB: Benzocyclobuten) was carried out in
a convection oven with nitrogen ambient at 350℃ for 1hour, resulting in
the BCB layer thickness 700nm. The BCB layer was etched by CF4/O2
plasma in order to remain the predetermined thickness. But BCB layer
could not be removed clearly. It appears to be due to the reaction between
BCB and Cu during BCB curing at 350℃.
Figs. 28(b), 28(c), and 28(d) shows the transfer characteristics of a
Cu(Mg)-gate TFT. Using a source/drain voltage (VD) of 5V and gate
voltage (VG) of -5V, off-state leakage current was in the order of 10-13A and
on/off current ratios was in 107. The threshold voltage VT, defined as the
linear intercept of the ID(VG) curve with the VG axis, was found to be 6.8V ~
7V. The subthreshold slope S = (d(log ID)/dVG)-1, was 0.91V/dec ~
0.93V/dec. Fig. 29 shows the output characteristics of the a-Si:H TFT
employed various types of gate electrode. As the content of Mg in the gate
electrode increases drain current also increases. The field-effect mobility was 0.37 ∼ 0.56cm2/Vs. The field-effect mobility of the TFT was obtained
from the formula:
Id1/2=(WCiµfe/2L)1/2(Vg-Vt),
where Id, µfe, Vg, Vt and Ci are drain current, field-effect mobility, gate
voltage, threshold voltage and the capacitance per unit area of SiNx,
respectively. The dielectric constants of MgO and SiNX were 3.2 and 6,
respectively, which were obtained from the experiments. However, there
have not been found any significant difference in device characteristics
between various types of electrodes.
Fig. 28. Transfer characteristics of the fabricated a-Si:H TFT’s using
(a)pure Cu, (b)Cu(1.0at%Mg), (c)Cu(2.3at%Mg), and
(d)Cu(4.5at%Mg) alloy as a gate electrode.
GATE VOLTAGE (V)
Fig. 29. Output characteristics of the fabricated a-Si:H TFT’s using (a)pure
Cu, (b)Cu(1.0at%Mg), (c)Cu(2.3at%Mg), and (d)Cu(4.5at%Mg)
alloy as a gate electrode.
DRAIN VOLTAGE
5. Conclusion
We have demonstrated a self-aligned process for the Cu-gate of a-Si
TFTs in AMLCD’s, which yields a gate line resistivity ranging from 8 µΩ-cm to 2.6µΩ-µΩ-cm, well below that of refractory metal. Upon annealing of
as-deposited Cu(Mg)/SiO2/Si multilayer samples in various oxygen ambient at
400℃ for 30min, self-aligned MgO layer of 150Å in thickness has been
formed to be applied for the passivation layer. During the PECVD process,
the self-aligned MgO provided effective passivation against the surface
reaction. O2 plasma treatment was found to be very effective to enhance the
adhesion of Cu(Mg) to glass or SiO2.
Consequently, the self- passivated Cu(Mg) films as a gate electrode can
solve the problems related to the routine poor adhesion, oxidation of Cu
surface, and surface reactivity during PECVD process in advanced high
resolution and large area TFT-LCDs.
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국 문 요 약
박막 트랜지스터 액정 표시장치의 게이트 금속에 적용 가능한
Cu(Mg) alloy 금속의 적합성을 조사하였다. 1000Å, 2000Å, 3000Å의
다양한 두께의 Cu(Mg) alloy 시편을 다양한 산소분위기에서 30분간
열처리하였다. O2 분위기 열처리는 급격한 입계성장과 Mg의 확산
을 유도하였다. Cu를 게이트 금속으로 적용할 때, Cu 와 SiH4, NH3,
CF4 등의 반응가스간에 상호확산이 일어난다. SiH4, NH3 가스는
SiNx 의 증착시에 PECVD법으로 증착되며 CF4 가스는 SiNx, a-Si
등의 etch 시에 사용된다. 이와 같은 상호확산은 게이트금속의 비
저항 증가를 일으킨다. 그러나 열처리를 통하여 표면에 MgO 막을
형성시킨 시편의 경우 반응가스와의 표면반응을 억제하는
passivation 막의 역할을 하는 것을 알 수 있었다. glass 기판과 SiO2
기판에 대한 접착력은 Mg 과 Al 의 첨가에 의해서 향상되었다.
또한, O2 plasma 처리는 glass 나 SiO2 기판에 대한 접착력을 향상시
키는데 매우 효과적임을 알 수 있었다. TFT 소자의 전기적 특성은
0.52cm2/Vs 의 전계이동도를 나타냈고, 0.91V/dec 의 slope 특성을
보였다.
결론적으로, Cu(Mg) alloy 는 열처리를 통한 MgO 막 형성을 통
해서 낮은 비저항을 유지하면서도 우수한 접착력과 산화방지능력
및 상호확산을 방지하는 passivation 막을 얻을 수 있음을 알 수
있었다. 따라서 Cu(Mg) alloy는 공정을 단순화시키면서 우수한 성
능을 가지는 대면적, 고화질 TFT/LCD의 게이트 금속으로서 적용
가능할 것으로 판단된다.
감사의 글
2년여의 대학원생활동안 아낌없는 가르침과 보살핌, 그리고 때로
는 채찍질로 작으나마 이 한편의 결실이 있도록 도와주신 이재갑
교수님께 진심으로 감사드립니다. 언제나 진심어린 충고를 아끼지
않으셨던 김지영 교수님께도 깊은 감사를 드립니다. 긴 시간은 아
니었지만 저에게 때로는 교수님처럼 때로는 형처럼 대해주신 이
원희 교수님께도 감사를 드립니다. 지난 대학원 생활동안 많은 가
르침을 주신 조남돈 교수님, 이진형 교수님, 박화수 교수님, 지충
수 교수님, 권훈 교수님, 김용석 교수님, 이재봉 교수님, 정우광
교수님, 남원종 교수님, 이건배 교수님께도 감사를 드립니다.
대학교와 대학원에서의 생활에서 언제나 자기일처럼 신경써주신
충서누나와 대학원생활동안 조언과 충고를 아끼지 않았던 기주형,
재명이형께도 감사드립니다.
현재의 우리 대학원 실험실이 있기까지 고생한 선배님들, 성호형,
면학이형, 원화형, 재호형, 지용이형, 상헌이형, 성원씨, 상기형, 흥
렬이형, 형석이형, 우성이형, 일환이, 정환 모두에게 고마움을 표
합니다. 카리스마를 갖기위해서 끊임없이 애쓰고 있는 우리실험실
의 현재 최고의 우두머리, 희정씨에게도 고마움을 표합니다. 2년동
안 생사고락을 같이 한 선배이자 동기, 영원한 동반자였던 인재형
에게도 깊은 감사를 표합니다. 나와 면담을 시작하면서부터 지금
까지 꼬박 만 1년동안 내 부사수이자 동생처럼 따라준 봉주, 실험
실의 실질적인 맏형역할을 하고있는 느림보, 개그맨 종호형, 삐지
기 잘하고 술잘먹고 성격더러운 그러나 시키면 잘하는 서범석, 우
리실험실의 막내인 내 손녀 성진, 그리고 어디갔다 이제왔는지 때
늦은 만학도 두식이에게도 고마움을 표하며 앞으로도 좋은 결실
있기를 빕니다. 또한 전자재료실험실의 태호형, 준모씨, 형섭이,
창배씨, 주환이에게도 그동안의 많은 도움에 감사드립니다. 그동
안의 대학원생활동안 많은 도움을 주신 경희대에 김은옥씨, 소자
제작을 도와주신 우인근씨, 그 많은 시편을 싫은 소리없이 AES
분석해 주신 이 세회씨, XRD 분석을 해준 서울대 조종회 선생님,
그리고 삼성전자 정창오 박사님께도 깊은 감사를 표합니다.
내 대학생활의 많은 부분을 함께한 유니팩친구들, 태호, 재윤, 우
호, 그리고 19기 동기와 선후배님들께도 감사드리고, 92학번 동기
들께도 고마움을 표합니다. 내 인생의 영원한 우정을 함께할 친구
들인 폭약제조 및 검사로 국방의 임무를 충실히 수행중인 태민이,
우리나라 벤처업계를 이끌어갈 벤처사장 중일이, 지질토목업계의
거목 명수, 대우건설의 마지막 지성이자 장래의 연극배우 형석이
모두에게 그 동안의 끊임없는 지지와 성원을 보내준것에 감사드
모두에게 그 동안의 끊임없는 지지와 성원을 보내준것에 감사드