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542/0441
Microsystem Technologies 9 (2004) 1–10 Ó Springer-Verlag 2004
DOI 10.1007/s00542-002-0441-7
Bulk micromachining fabrication platform using the integration of DRIE and wet
anisotropic etching
Huai-Yuan Chu, Weileun Fang
1
Abstract This study presents a bulk micromachining undercut effect has been extensively employed to fabri-
fabrication platform on the (100) single crystal silicon cate free suspended thin film structures [3, 4]. Presently,
substrate. The fabrication platform has employed the the high aspect ratio (HARM) devices become more
concept of vertical corner compensation structure and important in MEMS applications. For instance, the single
protecting structure to integrate the wet anisotropic crystal silicon wafer has also been employed as the
etching and DRIE processes. Based on the characteristics material for MEMS devices to increase their inertia and
of wet anisotropic etching and DRIE, various MEMS stiffness [5, 6]. The wet anisotropic etching and DRIE
components are demonstrated using the bulk microma- (deep reactive ion etching, by BOSCH process) are the
chining platform. For instance, the free suspended thin two most common techniques for deep silicon etching.
film structures and inclined structures formed by the {111} However, the applications of wet anisotropic etching are
crystal planes are fabricated by the wet etching. On the limited to the crystal planes of silicon substrate and
other hand, the mesas and cavities with arbitrary shapes convex corner undercut effect; and the DRIE technique is
and the structures with different leve l heights (or depths) unable to fabricate free suspended structures. Hence, the
are realized by the characteristics of DRIE. Since the existing bulk silicon micromachining processes have
aforementioned structures can be fabricated and inte- limited available components to become a powerful fab-
grated using the presented fabrication platform, the rication platform.
applications of the bulk micromachining processes will The concept of using vertical corner compensation
significantly increase. structure to prevent the undercut during wet anisotropic
etching was presented in [7]. The protecting structure
Keywords DRIE, Wet anisotropic etching, Bulk was also used to protect the non {111} crystal planes
micromachining from etching. Hence, it is possible to employ the
DRIE to assist the wet anisotropic etching to increase
1 the variety of bulk micromachined MEMS devices. In
Introduction short, under the assistant of DRIE, the mesas and
Bulk silicon micromachining is regarded as one of the cavities with arbitrary shapes can be fabricated using
primary MEMS fabrication technologies. Various mi- wet anisotropic etching. Moreover, these mesas and
cromachined structures such as V-grove and cavity be- cavities can further integrate with suspended thin film
come available after etching the silicon substrate structures and the structure formed by the inclined
anisotropically [1, 2]. Moreover, the convex corner {111} crystal planes.
This study presents a bulk micromachining fabrica-
tion platform on the (100) single crystal silicon sub-
strate. The fabrication platform has employed the
concept of vertical corner compensation structure and
protecting structure in [7] to integrate the wet aniso-
Received: 2 September 2003/Accepted: 24 November 2003
tropic etching and DRIE. Based on the characteristics of
wet anisotropic etching and DRIE, various MEMS com-
H-Y. Chu, W. Fang (&) ponents are demonstrated using the bulk micromachin-
Power Mechanical Engineering Department
National Tsing Hua University ing platform. For instance, the free suspended thin film
Hsinchu 30043, Taiwan structures and inclined structures formed by the {111}
E-mail: [email protected] crystal planes are contributed by the wet etching. On the
other hand, the mesas and cavities with arbitrary shapes
This research is based on the work supported by WALSIN and the structures with different level heights (or
LIHWA Corporation and the National Science Council of Taiwan depths) are realized by the characteristics of DRIE. Since
under grant of NSC-91–2218-E-007–034. The authors would like the aforementioned structures can be fabricated and
to thank the Central Regional MEMS Research Center of National
Science Council, Semiconductor Research Center of National integrated using the presented fabrication platform, the
Chiao Tung University and National Nano Device Laboratory for applications of the bulk micromachining processes will
providing the fabrication facilities. significantly increase.
5 4 2 _ 0 4 4 1
Journal number Manuscript number B Dispatch: 26.4.2004
Author’s disk received 4
Journal: Microsystem Technologies
Used 4 Corrupted
No. of pages: 10
Mismatch Keyed
542/0441
2
Concept and Fabrication Processes
Wet anisotropic etching is the primary process for the
bulk silicon micromachining. The wet anisotropic
etching has two characteristics. First, its etching rate
depends on crystal plane orientation. In general, the
{111} planes always have the slowest etching rate, and
will form the "etching stop layer." [8, 9] Thus, the
sidewalls of MEMS structures formed by the silicon
substrate are limited to {111} crystal planes. Second,
2 the convex corner structure will be undercut during
the wet anisotropic etching. This characteristic has
been extensively exploited to fabricate free suspended
thin film structures such as cantilever beams. On the
other hand, this effect has to be reduced or prevented in
some applications, for instance to fabricate the proof
mass of the accelerometer [10, 11]. The DRIE is another
option for the anisotropic bulk silicon etching. The
sidewalls of MEMS structures will be protected by the
polymer deposition instead of {111} crystal planes
during DRIE [12, 13]. Various MEMS structures such as
circular and triangular mesas become available since
their shape is no longer limited to {111} crystal planes.
However, the DRIE process cannot fabricate free
suspended structures since the convex corner undercut
effect does not exist.
This study develops a novel bulk micromachining
platform on the (100) single crystal silicon substrate
using the integration of the wet anisotropic etching and
the DRIE. This platform employs the characteristics of
the DRIE process to prevent the convex corner
undercut and crystal plane dependent effects during bulk
etching. The protecting structures (or named vertical
convex corner compensation) [7] are then grown or
deposited to protect the convex corner and fast etching Fig. 1. Schematic drawings of the monolithically integration
of the micromachined structures on the substrate
crystal planes. The mesas with even circular and trian-
gular shapes become available after wet anisotropic
etching, as shown in Fig. 1. In addition, the characteris-
tics of the RIE lag during deep silicon etching and the DRIE was employed to etch the thin films and silicon
{111} crystal planes are used to fabricate the stepped substrate. Thus, the structure with arbitrary shape was
structures with various level depths. The undercut will defined. The height of these structures was determined
selectively occur at the convex corner without protecting by the etching depth of DRIE. In addition, the RIE-lag
structures, yet free suspend thin film structures remain was exploited to produce the stepped-structures. As
available through the bulk silicon etching. Thus, the shown in Fig. 2e, f, a second silicon dioxide was
characteristics of wet anisotropic etching are exploited to thermally grown on the substrate to form the
provide V-grooves, pyramid cavities, and various sus- protecting post, and then the silicon nitride was
pended structures in Fig. 1. Since all of the aforemen- removed. As shown in Fig. 2g, the wet anisotropic
tioned structures are fabricated using the presented etching was employed to fabricate inclined structures
fabrication platform, they can be integrated on the (100) and suspended structures. During the wet etching
silicon substrate as shown in Fig. 1. process, the non {111} crystal planes were protected
Figure 2 illustrates the typical process steps of this by the silicon dioxide post. It is possible to replace
study. A silicon dioxide film was thermally grown and the silicon dioxide film with the silicon nitride and
patterned on a (100) single crystal silicon wafer as heavily boron doped silicon films that have high
shown in Fig. 2a. As shown in Fig. 2b, a silicon nitride selectivity over silicon substrate during wet etching.
was deposited to act as the mask for the following Finally the vertical silicon dioxide protection was
thermal oxidation in Fig. 2e. As shown in Fig. 2c, d, removed to form circular mesas and stepped-structures,
after the photolithography of a thick photo resist, the as shown in Fig. 2h.
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Fig. 2. Flow chart of the fabrication
process
3 and all of its sidewall was covered with a 2 lm thick
Results SiO2 protecting structure. Hence, the non {111} crystal
To demonstrate the feasibility of the fabrication plat- planes of the circular mesa was not attacked by TMAH
form, the structures illustrated in Fig. 1 have been suc- during wet anisotropic etching in Fig. 2g. In addition,
cessfully fabricated using the process in Fig. 2. This this platform can also fabricate various thin suspended
study showed not only the fabrication results of each structures. A typical example is the 2 lm thick SiO2
structure but also the integration of these structures. cantilever shown in Fig. 3b. As shown in Fig. 2f, g, there
The protecting structure employed in this experiment was not any protecting structure on the convex corners
was a 2 lm thick SiO2 film, and the etching solution was and sidewall of the cantilever, so that it was released
TMAH. from the substrate using the undercut effect during wet
anisotropic etching.
3.1
Thick structures with arbitrary shapes 3.2
and thin suspended structures Stepped structures
This fabrication platform has successfully fabricated The RIE lag and {111} crystal planes were employed to
various thick structures such as mesa and post with fabricate stepped structures for this platform. Two
arbitrary shapes. A typical example is the 80 lm thick typical stepped structures fabricated using this
circular mesa shown in Fig. 3a. As shown in Fig. 2d, e, platform were demonstrated in Fig. 4. The cross section
the shape of the circular mesa was generated by DRIE, view of these two stepped structures illustrated in Fig. 4a
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Fig. 2. (Continued)
indicates that they had five different level depths. The
top level (level 1) was the original surface of the
substrate and did not experience any etching processes.
Hence, the depth of level 1 was regarded as 0 lm.
The second and third levels were only defined by the
wet etching shown in Fig. 2g. Since the second level
employed the {111} crystal planes to stop the anisotropic
etching, its depth was different from that of the third
level. The depth of level 2 and level 3 were 30
and 57 lm, respectively. As shown in Fig. 2c–h, the
depths of forth and the fifth levels were mainly
resulted form the DRIE etching. As indicated in Fig. 2c,
the line patterns on the etching mask are all 2 lm wide,
however, the spacing between these lines is different.
Therefore, various etching depths were produced as
shown in Fig. 2d after DRIE due to the RIE lag effect.
The 2 lm thick silicon line structures were fully
oxidized after the thermal oxidation shown in Fig. 2e. As
shown in Fig. 2h, the oxidized line structures were
etched away by BOE and formed the forth and the fifth
levels of stepped structures in Fig. 4a. The depth of
level 4 and level 5 were 91 and 108 lm, respectively. It
is possible to further increase the step number of the
structures after properly combining these two
characteristics.
Fig. 3a,b. Some typical structures fabricated using the presented
platform, a thick mesa, and b thin suspended beam
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Fig. 4a–c. Two typical stepped structures fabricated using this
platform, a the cross section view, b the circular stepped struc-
ture, and c the cross shape stepped structure
Fig. 5a–c. Typical pyramids fabricated using this platform, a the
3.3 four pyramids, b the eight pyramids, and c the complicated
Structures with both inclined and vertical sidewalls pyramid
The DRIE generally produces vertical sidewalls for
microstructures, whereas wet anisotropic etching
generally produces inclined sidewalls formed by {111}
crystal planes for microstructures. The presented microstructures with both inclined and vertical sidewalls
platform has combined these two different etching were available after the protecting structure was re-
mechanisms to fabricate microstructures with both moved, as shown in Fig. 2h. The typical fabrication re-
inclined and vertical sidewalls. As shown in Fig. 2d–f, sults shown in Fig. 5a are four pyramids, and each
the silicon substrate was removed using DRIE to form pyramid has two vertical sidewalls and one inclined
vertical sidewalls first, and then grown a protecting {111} sidewall. Apparently, these pyramids were fabri-
structure on them. As shown in Fig. 2g, the wet cated after the {111} planes divided by a cross shape
anisotropic etching was employed to generate the protecting structure. Similarly, the eight pyramids in
inclined {111} sidewalls. The non {111} vertical sidewalls Fig. 5b were fabricated after the {111} planes divided by
were covered with the protecting structures, and they an asteroid shape protecting structure. The SEM photos
were not attacked during wet anisotropic etching. The show that the pyramid in Fig. 5b is sharper than that in
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Fig. 6a–e. Integration of the suspended structure, V-groove, and mesa, d the suspended structure, cavity, and mesa, and e the
cavity, and mesa on the substrate, a the V-groove, cavity, and side view of the aforementioned structures
mesa, b the suspended structure, cavity, and mesa, c the cavity
Fig. 5a. Moreover, the complicated structure in Fig. 5c to demonstrate the potential combination of these struc-
was fabricated after the {111} planes divided by a cir- tures.
cular shape protecting structure. In summary, it is The suspended structure, V-groove, cavity, and mesa
possible to construct various three-dimensional micro- were integrated on the substrate based on the concept
structures by combining the inclined {111} sidewalls presented in Sects 3.1 and 3.3. As shown in Fig. 2d, e,
with protecting structures. the mesa with arbitrary shape was patterned by DRIE,
and all of its sidewall was covered with a 2 lm thick
SiO2 protecting structure. Before wet anisotropic
3.4 etching, various openings were defined on the etching
Structure integration mask either inside or outside the circular mesa. After
The structures presented in Figs. 3, 4, 5 were all fabricated that the wet anisotropic etching was employed to
using the fabrication platform illustrated in Fig. 2. It is remove the silicon substrate and generate the inclined
possible to integrate part or all of these structures for {111} sidewalls, as shown in Fig. 2g. As shown in Fig. 6a,
various applications. The following cases were employed b, the silicon substrate was removed by the wet
542/0441
Fig. 6d further employed the undercut effect to
fabricate the suspended structures. The schematic
illustration in Fig. 6e shows the side view of the
structures in Figs. 6a–d.
As a second example, the DRIE in Fig. 2d was used to
define a circular cavity first. In the mean time, posts with
arbitrary shapes were also defined by DRIE in the interior
of the cavity. As shown in Fig. 2e, the sidewalls of the
cavity and the post were covered with a 2 lm thick SiO2
protecting structure. After wet anisotropic etching, the
substrate inside the cavity yet outside the post was re-
7
moved, so as to produce the structures shown in Fig. 7a, b.
Moreover, the V-groove was also fabricated during wet
etching, as shown in Fig. 7c. Thus, these V-grooves can
perform as interconnections of the cavities. According to
the processes mentioned in Sect. 3.3, the pyramid structure
can be fabricated inside the cavity. A typical example is
shown in Fig. 8. In this case, the cross shape protecting
structure instead of the posts was placed inside the cavity
to divide the {111} planes, as shown in Fig. 8a. Thus, four
pyramids were fabricated inside the circular cavity after
the protecting structure was etched away, as shown in
Fig. 8b. The SEM photo in Fig. 8c is the close-up view of
these four pyramids.
The last example shows the integration of the cavity
and the stepped structure. As shown in Fig. 9, two
different stepped posts are located inside the cavity.
As mentioned in Sect. 3.2, the stepped structure due
to RIE lag was patterned, etched, and oxidized firstly,
as shown in Fig. 2c–f. In the mean time, the openings
were also defined on the etching mask for the
following wet anisotropic etching, as indicated in Fig. 9a.
Hence, the silicon substrate under these openings was
etched anisotropically, and the {111} planes formed the
etching stop layer. The depth d indicated in Fig. 9a was
mainly determined by the wet etching. Since a large
opening was placed outside the stepped posts, they
were surrounded by a cavity, as shown in Figs. 9b, c.
The cavity inside the stepped post was also obtained
in the same manner. Moreover, it is also obtained
from Fig. 9b, c that the wet anisotropic etching was able
Fig. 7a,b. Integration of the V-groove, cavity, and mesa on the to tune the depth of the cavity d. In other words, the
substrate, a the circular cavity and circular mesa, b the circular wet anisotropic etching can be exploited to tune the
cavity and rectangular mesa, and c the V-groove, circular cavity,
and mesa depths of level 3 and level 5 for the stepped structure in
Fig. 4a.
All of the aforementioned structures were fabricated
using the platform processes illustrated in Fig. 2. In
anisotropic etching to form a circular mesa. In addition, summary, the fabrication platform presented in this study
the V-grooves, cavities, and suspended structures in enables the monolithic integration of these structures on a
the interior of the mesa were also fabricated using the silicon substrate, as demonstrated by the SEM photos in
wet anisotropic etching. In Fig. 6c, d, the silicon Fig. 10.
substrate was also removed by the wet anisotropic
etching to produce nine circular posts and one square
post, respectively. These posts were located inside 4
a large cavity resulted from the wet anisotropic Conclusions
etching, and the {111} plane sidewalls can be clearly This study presents a bulk micromachining fabrication
observed from the SEM photos. Moreover, the design in platform on the (100) single crystal silicon substrate
542/0441
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Fig. 8a–c. The pyramid structure fabri-
cated inside the cavity, a the cross shape
protecting structure, b four pyramids, and
c the close-up of pyramids
using the integration of DRIE and the wet anisotropic ped-structures. The undercut will selectively occur at the
etching. This platform employs the characteristics of the convex corner without protecting structures, yet free
DRIE process to prevent the convex corner undercut suspend thin film structures remain available through
and crystal plane dependent effect during bulk etching. the bulk silicon etching. Thus, the characteristics of wet
The protecting structures are then grown or deposited to anisotropic etching are exploited to provide V-grooves,
protect the convex corner and fast etching crystal pyramid cavities, and various suspended structures.
planes. Thus, the mesas with even circular and trian- Since all of the aforementioned structures can be fab-
gular shapes become available after wet anisotropic ricated and integrated using the presented fabrication
etching. In addition, the RIE-lag in DRIE combines with platform, the applications of the bulk micromachining
the {111} crystal planes are used to fabricate the step- processes will significantly increase.
542/0441
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Fig. 9a,b. Integration of the cavity and the stepped structure a
the schematic drawings of the cross section view, b cavity with
d ¼ 80 lm, and c cavity with d ¼ 140 lm
Reference of 48th IEEE Electronic Components and Technology Con-
1. Helin P; Mita M; Fujita H (2000) Self aligned vertical ference, Seattle, May, 592–597
mirrors and V-grooves applied to a self-latching matrix 3. Fang W; Wickert JA (1996) Determining mean and gradient
switch for optical networks. IEEE MEMS’00, Miyazaki, Jan, residual stresses in thin film using micromachined cantilevers.
467–472 J Micromech Microeng, 6:301–309
2. Huang L-S; Lee S-S; Motamedi, E; Wu, MC; Kim, C-J (1998) 4. Gupta A; Denton JP; McNally H; Bashir R (2003) Novel fab-
MEMS packaging for micro mirror switches. In: Proceedings rication method for surface micromachined thin single-crystal
542/0441
6. Miyajima H; Asaoka N; Isokawa T; Ogata M; Aoki Y; Imai M;
Fujimori O; Katashiro M; Matsumoto K (2003) A MEMS
electromagnetic optical scanner for a commercial confocal
laser scanning microscope. J Microelectromech Systems
12:243–251
7 Chu H-Y; Fang W (2003) A novel vertical convex corner
compensation for wet anisotropic bulk micromachining.
J Micromech Microeng
8. Beam KE (1978) Anisotropic etching of silicon. IEEE Trans
Electron Devices 25:1185–1193
9. Tabata O; Asahi R; Funabashi H; Shimaoka K; Sugiyama S
(1991) Anisotropic etching of silicon in TMAH solutions. Int
10 Conf Solid-State Sensors Actuators (Transducer ’91), San
Francisco, June, 811–814
10.Rockstad HK; Reynolds JK; Tang TK; Kenny TW; Kaiser WJ;
Gabrielson TB (1995) A miniature, high-sensitivity, electron
tunneling accelerometer. Int Conf Solid-State Sensors Actua-
tors (Transducer ’95), Stockholm, June, 675–678
11.Liu C-H; Kenny TW (2001) A high-precision, wide-bandwidth
micromachined tunneling accelerometer. J Microelectromech
Systems 10:425–433
12.Bosch Gmbh RB (1994) U.S. Pat. 4855017, U.S. Pat. 4784720,
and Germany Pat. 4 241 045C1
13.Chen K-S; Ayon AA; Zhan X; Spearing SM (2002) Effect of
process parameters on the surface morphology and mechani-
cal performance of silicon structures after deep reactive ion
etching (DRIE). J Microelectromech Systems 11:264–275
Fig. 10. Monolithic integration of the aforementioned structures
on a silicon substrate
silicon cantilever beams. J Microelectromech Systems
12:185–192
5. Hsieh J; Fang W (2002) A boron etch-stop assisted lateral
silicon etching process for improved high-aspect-ratio silicon
micromachining and its applications. J Micromech Microeng
12:574–581


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