Investigation of Wet Etching with Surfactant Applied on the

Tamkang Journal of Science and Engineering, Vol. 10, No. 2, pp. 155-158 (2007)
155
Investigation of Wet Etching with Surfactant Applied
on the Releasing Process of Devices Fabricated by
Surface Micro Machining
Chien-Chung Tsai*, Y-Cheng Huang, Chia-Jung Lee and Chia-Ching Chang Chien
Department of Optoelectronic System Engineering, Minghsing University of Science and Technology,
Hsinchu, Tauwan 304, R.O.C.
Abstract
Investigation of surfactant applied on wet etching for releasing the micro optical devices with
Micro Array Thermal Actuator, MATA, fabricated by Surface Micromachining common process is
proposed in this paper. BOE (buffered oxide etch), concentrated HF (49%), concentrated HF (49%)
with 10% surfactant are adopted as the etchants for releasing the micro optical devices in experiment,
respectively. Low Temperature Oxide, LTO, is a sacrificial layer material and the etching phenomena
will be demonstrated by the three different etchants during the releasing process. The experimental
results exhibit Poly-Si is seriously damaged and geometrical dimensions of the micro structure are
reduction by BOE (6:1) compared with the original design. On the other hand, the geometrical features
of the micro devices are consistency with the original design and successfully released by concentrated
HF (49%) etching. However, there are some residual LTO on the surface of Poly 1 layer during the
period of concentrated HF (49%) etching process. Nevertheless, the residual LTO on Poly 1 layer
surface is significantly improved by the concentrated HF (49%) with 10% surfactant. Finally, the
micro optical devices fabricated by SMart common process are successfully released by the
concentrated HF (49%) with 10% surfactant and the surfaces of devices are relatively smooth.
Key Words: Micro-optics, Surface Micromachining, Wet Etching, Sacrificial Layer
1. Introduction
In high technology age, all kinds of products are expected to be of high quality, high capability, and be miniaturized. Micro Electro-Mechanical System, MEMS,
has played an important role in industry since the 1980s
[1]. This field includes opto-electrical engineering system, communication engineering, mechanical engineering, electrical engineering, electronics, material science,
control engineering, chemical engineering, and semiconductor technology, etc. MEMS devices have the characteristics of miniaturization, high precision and high reliability [2]. Micro structure of MEMS could be integrated
with a signal circuit on a chip [3]. And that would be
*Corresponding author. E-mail: [email protected]
mass production by semiconductor process. There are a
lot of advantages on MEMS technology so it becomes
the start and highly competitive technology in industrial
sector of each country. MEMS technology has gradually
matured. Many products with high precision and high
quality are manufactured by micro-fabrication common
process, recently [4].
Releasing the micro structure is the key step during
the post process of surface micro machining. Especially,
the performance of device shall be still functioned after
the releasing process in common process, such as CIC
CMOS-MEMS and SMart. Hence the releasing process
dominates whether the devices fabricated by common
process are success or not. Etching technology is usually
adopted for the releasing process. There are two divisions for etching technology. One is dry etching, the
156
Chien-Chung Tsai et al.
other is wet etching. However, dry etching is anisotropic
and high precision. Nevertheless, there are advantages
on the simple process, low cost equipment and mass production for wet etching [5]. Wet etching with surfactant
applied on the releasing process of devices fabricated by
surface micro machining is investigated in the work.
2. Fabrication Process and
Experimental Process
The SMart (Surface Micromachining for applications
and research technology platform) supplies a multi project wafer (MPW) technology platform. It is a low-cost
and a rapid prototyping surface micromachining process.
The process platform could serve as a commercial program for the industrial, governmental and academic communities. The SMart process is a general surface micromachining process involved three layers polysilicon. The
related material and its composition in the process are
demonstrated in Table 1. Eight masks adopted to achieve
the process are defined as well. For the purpose of executing process more simply and effectively, two structure
layers Poly 1 and Poly 2 are equal in thickness; more-
over, two sacrificial layers LTO 1 and LTO 2 are also the
same as in thickness [6]. The device fabricated by SMart
process is composite of micro mirror, MATA and elevating structure, as shown in Figure 1 [7,8].
The detailed experimental procedure is elaborated
on each step as shown in Table 2. Then the phenomena
are observed and recorded by the Optical Microscope
(OM) with Charge Couple Detector, CCD. BOE (buffered oxide etch), concentrated HF (49%), concentrated
HF (49%) with 10% surfactant are the etchants for the
study.
Figure 1. Mask configuration of the device.
Table 1. Layer composition overview of SMart
Level
Mask #
THOX
LS SiN
Poly 0
SiN 0
N/A
N/A
#1 Poly 0
#2 SiN 0
#3 Dimple (7500 Å)
#4 Anchor 1
#5 Poly 1
#6 Anchor 2
#7 POLY 2
#8 Metal
LTO 1
Poly 1
LTO 2
Poly 2
Metal (Cr/Ni/Au)
Thickness (Å)
GDS number
Dark/Clear
5000
5000
5000
5000
N/A
N/A
1
2
3
4
5
6
7
8
N/A
N/A
Dark
Clear
Clear
Clear
Dark
Clear
Dark
Clear (lift-off)
17500
20000
17500
20000
100/1000/5000
Table 2. Procedure of Etching
Step
Acetone
IPA
DI-Water
Etchant
DI-Water
IPA
Heating Plate
Observation
Time
Temperature
Remark
05 min
05 min
10 min
te
10 min
10 min
05 min
-
room
room
room
room
70 °C
room
70 °C
room
Remove Photoresist (PR)
IsoPropyl Alcohol cleans the residual Acetone and PR
Clean
Remove LTO, te is the etching time and dependent on the etchant
Clean
Diminish Sticking
Dry
OM with CCD
Investigation of Wet Etching with Surfactant Applied on the Releasing Process of Devices Fabricated by Surface Micro Machining 157
3. Results and Discussion
3.1 The Effect of BOE
The effect of BOE etchant with the concentration 6:1
on the releasing process is investigated. Figure 2 reveals
there is a lot of the residual LTO under MATA which is
Poly 1 layer after 40 minutes etching. And the geometry
features of MATA are reduction compared with the original design, as shown in Figure 3. The micro structure of
the device is seriously damaged due to the long etching
time for removing LTO thoroughly.
3.2 The Effect of Concentrated HF (49%)
The effect of HF etchant with the concentration 49%
on the releasing process is demonstrated. There are some
residual LTO on the surface of Poly 1 layer in black area,
as shown in Figure 4(a). On the other hand, the surface
roughness is affected by the residual oxide, as shown in
Figure 4(b). Hydrofluoric dissolves the SiO2 by forming
the water-soluble H2SiF6 with H2 bubbles [9]. And Polysilicon is hydrophobic. The H2 bubbles will partly pre-
vent the HF from dissolving the LTO which is near the
surface of Poly 1 layer. That is why there is some residual
LTO 2 on the Poly 1 layer; the surface is black and not
uniform due to the H2 bubbles during the period of concentrated HF (49%) etching process, as shown in Figure
5. Fortunately, the sacrificial layer oxide is thoroughly
removed after 5 minutes by concentrated HF (49%) etching. The estimated etching time is 5 min 8 sec, as shown
in Table 3. Because the room temperature is higher than
the referenced temperature 20 °C, 5 minutes are adopted
as the etching time in the experiment. And the releasing of
micro structure is successful except the surface condition.
3.3 The Effect of Concentrated HF (49%) with
10% Surfactant
The effect of concentrated HF (49%) with 10% surfactant on the releasing process is implemented to improve surface condition of the releasing results. The
etching time is also 5 minutes; the releasing of micro
structure is success, too. The significant improvement of
concentrated HF (49%) with 10% surfactant etchant for
the releasing process is nearly perfect surface condition
on Poly 1 layer, as shown in Figure 6.
Figure 2. Residual LTO under Poly 1.
Figure 5. Mechanism sketch of the residual LTO.
Table 3. The estimated etching time for HF [9]
Etchant Recipe
HF49%:
1:10
water
HF
49%
Etching
rate
Etching LTO
thickness
Etching
time
30 nm/min
9.25 mm
1.8 mm/min
9.25 mm
5 hr 8 min
18 s
5 min 8 s
Figure 3. Size reduction after etching by BOE.
Figure 4. OM photos of device by 49%HF etching.
Figure 6. OM photos of device by concentrated HF (49%)
with 10% surfactant etching.
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Chien-Chung Tsai et al.
The results demonstrate the residual LTO and surface condition shall be diminished and improved by the
surfactant additive in concentrated HF (49%). And that
reveals the surfactant can effectively reduce H2 bubbles
which block the oxide and HF etchant. Since the LTO is
dissolved by concentrated HF (49%) without residue, the
surface of micro mirror is almost not affected by concentrated HF (49%) with 10% surfactant compared with by
concentrated HF (49%) only, as shown in Figure 6(b).
4. Conclusion
Investigation of surfactant applied on wet etching for
releasing the micro optical devices with Micro Array
Thermal Actuator, MATA, fabricated by Surface Micromachining common process is proposed in this work.
BOE (6:1), concentrated HF (49%) and concentrated HF
(49%) with 10% surfactant are adopted as the etchants
for releasing the micro optical devices in experiment,
respectively. The experimental results exhibit Poly-Si
is seriously damaged and geometrical dimensions of the
micro structure are reduction by BOE (6:1) compared
with the original design. On the other hand, the geometrical features of the micro devices are consistency with the
original design and successfully released by concentrated HF (49%) etching. However, the surface of Poly 1
layer becomes black due to the residue of LTO during the
period of concentrated HF (49%) etching process. Nevertheless, the residual LTO on the surface of Poly 1
layer is significantly improved by the concentrated HF
(49%) with 10% surfactant. Finally, the micro optical devices fabricated by SMart common process are successfully released by the concentrated HF (49%) with 10%
surfactant and the surfaces of devices are relatively
smooth. Besides, the etching time of the concentrated HF
(49%) with 10% surfactant is shorter than that of BOE
for releasing the micro optical devices during the post
process.
the authors for their assistance. The authors are deeply
grateful to the president of tMt, Chien-Yung Ma Ph. D.
and the manager of tMt, Jhy-Ping Wu Ph. D, for their
kind and liberated support in this work. This work is sponsored by the funding of National Science Council of Taiwan, NSC 93-2212-E-159-002 and MUST 95-OES-02.
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Acknowledgement
National Science Council of Taiwan and Touch Micro-system Technology Cooperation are appreciated by
Manuscript Received: Feb. 23, 2007
Accepted: Mar. 17, 2007