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Title
Thermal annealing in hydrogen for 3-D profile transformation on silicon-on-insulator and
sidewall roughness reduction
Permalink
https://escholarship.org/uc/item/6c41s9f0
Journal
Journal of Microelectromechanical Systems, 15(2)
ISSN
1057-7157
Authors
Lee, MCM
Wu, Ming C
Publication Date
2006-04-01
Peer reviewed
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University of California
338 JOURNAL OF MICROELECTROMECHANICAL SYSTEMS, VOL. 15, NO. 2, APRIL 2006
Thermal Annealing in Hydrogen for 3-D Profile
Transformation on Silicon-on-Insulator and
Sidewall Roughness Reduction
Ming-Chang M. Lee, Member, IEEE, and Ming C. Wu, Fellow, IEEE
Abstract—A fast, effective process using hydrogen annealing
is introduced to perform profile transformation on silicon-on-
insulator (SOI) and to reduce sidewall roughness on silicon
surfaces. By controlling the dimensions of as-etched structures,
microspheres with 1
m radii, submicron wires with 0.5 m radii,
and a microdisk toroid with 0.2
m toroidal radius have been suc-
cessfully demonstrated on SOI substrates. Utilizing this technique,
we also observe the root-mean-square (rms) sidewall roughness
dramatically reduced from 20 to 0.26 nm. A theoretical model is
presented to analyze the profile transformation, and experimental
results show this process can be engineered by several parameters
including temperature, pressure, and time. [1580]
Index Terms—Annealing, surface diffusion, surface roughness.
I. INTRODUCTION
S
IDEWALL roughness is an issue for many applications,
for example, scattering loss in optical waveguides [1]–[3]
and micromirrors [4]. For dry etch processes, roughness de-
pends on the quality of photomasks, photolithography, and
etching conditions. Though state-of-the-art complementary
metal-oxide-semiconductor (CMOS) technologies can achieve
sidewall roughness lessthan5 nm[5], in manyresearch labs, side-
wall roughness is about 10–20 nm. Anisotropic wet etching such
as potassium hydroxide (KOH) and tetramethylammonium hy-
droxide (TMAH) can form smooth sidewalls, but the geometry is
limited by crystal planes. Another solution is to develop post-dry
etching processes to reduce roughness. Thermal oxidation was
reported to smooth the sidewall for a silicon wire fabricated on
SOI [6], [7]. However, the process consumes silicon, more than
ten times the surface roughness, and it induces residual stress
in silicon. Therefore, it is not suitable for rough surfaces such
as the sidewall scalloping resulted from the Bosh process [8].
In contrast, hydrogen annealing was reported to remove surface
roughness effectively [9]–[12]. The surface mobility of silicon
atoms is enhanced by heated hydrogen [13], [14] at temperatures
Manuscript received April 20, 2005; revised July 1, 2005. This work was
supported in part by DARPA/CS-WDM programs (#MDA972-02-1-0019) and
by the University Photonics Research Program (HR0011-04-1-0040). Subject
Editor H. Zappe.
M.-C. M. Lee was with the Department of Electrical Engineering and
Computer Sciences, University of California, Berkeley, CA 94720 USA. He
is now with the Institute of Photonics Technologies and the Department of
Electrical Engineering, National Tsing Hua University, Taiwan, R.O.C. (e-mail:
mclee@ee.nthu.edu.tw).
M. C. Wu is with the Department of Electrical Engineering and Com-
puter Sciences, University of California, Berkeley, CA 94720 USA (e-mail:
wu@eecs.berkeley.edu).
Digital Object Identifier 10.1109/JMEMS.2005.859092
much lower than the melting point (1414
C). Based on this
phenomenon, migrating atoms smooth out the surface roughness
to minimize the total surface energy without losing volume.
Hydrogen-induced surface migration not only changes the
surface morphology but also affects the global profile if the sur-
face migration length is comparable to or larger than structural
dimensions. This effect is similar to the reflow process in glasses
[15] or polymers [16], but unlike the reflow process, this mech-
anism only depends on surface-atom movement and the crys-
talline structure is preserved. Thermal annealing in hydrogen
ambient has been reported to produce round corners [17] and
various voids [18], [19] in bulk silicon. However, previous ex-
periments were mostly performed on bulk Si. Microstructures
on silicon-on-insulator (SOI) behave differently in hydrogen
annealing because of the different boundary conditions. The
bottom oxide becomes a barrier for the mass transport. In our
studies, we have shown that it is possible to produce rounded
three-dimensional (3-D) microstructures such as microspheres,
submicron wires, and microtoroids on SOI [20], [21].
In this paper, we present a comprehensive study of the
hydrogen annealing process for SOI microstructures. Detailed
process characterization and modeling are described. In addi-
tion to producing various rounded 3-D structures, we will also
demonstrate reduction of sidewall scalloping created after deep-
reactive-ion-etch (DRIE). In Section II, we examine the process
parameters of hydrogen annealing and introduce a theoretical
model to predict the profile transformation. Several types of
profile transformation and their mechanisms are described
in Section III. In Section IV, we show the ability of sidewall
roughness reduction by hydrogen annealing.
II. P
ROCESS CHARACTERISTICS
As silicon is annealed in hydrogen ambient, the surface will
be terminated with hydrogen atoms [22]. Although the mass
transport of silicon atoms actually depends on the atomistic na-
ture of crystals [23] and is affected by the surface crystalline
structure [24], the global profile transformation can be approx-
imately modeled as atom motion on an isotropic continuum
surface. For isotropic materials, surface diffusion and evapora-
tion-condensation contribute to the fundamental surface mass
transport mechanisms [25]. However, for annealing tempera-
tures less than 1100
C, surface diffusion is dominant in pro-
file transformation [26]. In our applications, the temperature is
around 1050
C, which is compatible with typical microfabri-
cation, and only the surface diffusion mechanism is considered.
Fig. 1 shows a schematic of surface diffusion on a silicon profile.
1057-7157/$20.00 © 2006 IEEE
LEE AND WU: THERMAL ANNEALING IN HYDROGEN FOR 3-D PROFILE TRANSFORMATION 339
Fig. 1. Schematic illustrating atom migration on a silicon surface during
hydrogen annealing.
Fig. 2. The cross section of a line-and-space pattern etched on bulk silicon
(a) before and (b) after hydrogen annealing (1100
C and 10 torr for 5 min).
Surface atoms tend to leave from convex corners and accumu-
late at concave corners. Based on this mechanism, initial sharp
corners become rounded. Fig. 2 shows the cross-sectional scan-
ning electron micrographs (SEM) of silicon mesas before and
after hydrogen annealing (performed in ASM Episilon II single
wafer epitaxial reactor). The structure was initially etched on
bulk Si with 1
m width, 1 m spacing, and 2.5 m height.
After annealed in pure hydrogen at 10 torr, 1100
C for 5 min,
both the top and bottom corners became rounded.
As shown in Fig. 1, a surface proceeding or receding along
the normal direction depends on the surface atoms transferring
in the transverse direction. To gain more insight to the process,
we used Mullins’ model to simulate the 2-D profile evolution
due to surface diffusion [25]
(1)
where
is the speed of the profile developing along the normal
direction,
is the surface tension of solid substrate, is the
molecular volume,
is the number of atoms per unit area,
is the surface diffusion coefficient, T is the temperature, K is
the surface curvature, and s is the arc length along the profile.
From this equation, profile evolution dynamics are primarily
determined by the material properties, the surface diffusion coef-
ficient, and the second-order gradient of curvature. Among these
factors, the surface diffusion coefficient
can be controlled
by temperature and pressure. This process dependency was first
explored by Sato et al. on a trench evolution [12]. To gain more
quantitative information on this relationship, a parametric study
Fig. 3. Experimental surface diffusion coefficient as a function of (a) tempera-
ture and (b) pressure.
Fig. 4. The evolution of the radius of curvature on a rectangular corner at
1000
C, 1050 C, and 1100 C.
of the corner evolution was performed experimentally, and
was extracted for various temperatures and pressures. As indi-
cated in Fig. 3, the surface diffusion coefficient increases steadily
with temperature, and decreases with pressure. The relation be-
tween the diffusion coefficient and temperature actually follows
the Arrhenius equation. On the other hand, process pressure may
affect the activation energy of the diffusion coefficient. The acti-
vation energy is estimated to be about 2.26 eV under a hydrogen
pressure of 10 torr. As the pressure approaches to atmospheric
pressure, the diffusion coefficient decreases more rapidly.
In (1), the profile evolution is also driven by the second-order
gradient of curvature. To evaluate this characteristic, we ana-
lyzed the corner evolution on a silicon step with a rectangular
profile. Fig. 4 shows the radius of curvature of a corner versus
340 JOURNAL OF MICROELECTROMECHANICAL SYSTEMS, VOL. 15, NO. 2, APRIL 2006
TABLE I
E
XAMPLES OF
3-D SILICON
PROFILE TRANSFORMATION.T
HE FIGURES
SHOWN IN
THIS TABLE
REPRESENT THE CROSS-SECTIONAL
PROFILES OF
SEVERAL
STRUCTURES.THE
PROCESS
PARAMETERS ARE
SET TO BE
1100
C, 10
TORR,
AND 5
MIN TO
ACHIEVE A
LARGE ANNEALED
CORNER RADIUS
(ABOUT 1
m)
annealing time at 1000 C, 1050 C, and 1100 C with the pres-
sure held constant at 10 torr. As shown from these curves, the
radius of curvature initially increases very rapidly due to a large
gradient of curvature varying at the corner, but after a few min-
utes, the increase becomes much slower. The result shows the
radius of curvature approximately increases with the fourth root
of time. In addition, for a high annealing temperature, a large ra-
dius is easily achieved due to the large surface diffusion coeffi-
cient. This theoretical prediction provides important guidelines
to optimize the process for profile engineering.
III. 3-D S
ILICON PROFILE TRANSFORMATION
Micro-structures with round profiles are difficult to be fab-
ricated by conventional processing technologies. Although
such structures have been demonstrated by a reflow technique,
however, they are limited to noncrystalline materials such as
glass [15] or polymers [16]. Exploiting the hydrogen-enhanced
surface diffusion, we can make various round structures in
monocrystalline silicon by controlling the process parameters.
Profile transformation on arbitrary geometries with known
parameters can be analyzed by numerical methods [27]. A
simple profile evaluation can refer to the corner radius of a right
angle under the same annealing process conditions, as shown
in Fig. 4. If the dimensions of the structure are smaller than the
annealed corner radius (ACR), the global geometry will become
circular. Otherwise, only the edges will become rounded. If the
devices are fabricated on SOI, the profile transformation can
be performed on a released structure by removing the buried
oxide. Table I lists the cross sections of different released
profiles and shows the profiles are transformed into circular
beams, submicron wires, and toroidal structures after hydrogen
annealing. In this section, we introduce the fabrication process
for each structure and demonstrate the experimental results.
In addition to surface diffusion, hydrogen annealing also in-
duces chemical etching at the interface of silicon and silicon
dioxide [28]. Fig. 5 compares the cross sections of the annealed
rectangular structures on bulk silicon and SOI. The SOI wafer
was initially etched to expose the buried oxide, while the bulk
Si wafer was time-etched to the same depth. Then, both wafers
were annealed in pure hydrogen at 1100
C, 10 torr for 5 min.
In the bulk silicon sample, the annealed profile exhibits convex
Fig. 5. Cross-sectional views of the annealed profiles on (a) bulk Si and
(b) SOI (1100
C, 10 torr, 5 min) [21].
(top) and concave (bottom) corners. However, in the SOI wafer,
an undercut due to chemical etching is developed between the
etched Si and the SiO
substrate. It creates a slit at the interface
with 0.1
m thickness. This slit exposes the bottom corners to
hydrogen and makes the bottom corner convex.
A. Circular Cantilever Beams and Microspheres
As shown in Fig. 5, a feature profile with convex, rounded
corners can be fabricated on SOI even without the aid of a re-
leasing process. This phenomenon enables us to make circular
features if the dimensions of as-etched profiles are equivalent or
less than the ACR. Fig. 6 describes the transformation process.
First, a structure with a square profile is fabricated on a SOI
and the buried oxide is exposed. During hydrogen annealing, the
bottom interface is undercut and the four corners turn rounded.
If the sample is further annealed for a long time, the profile be-
come circular and the SOI structure could be completely sepa-
rated from the buried oxide, resulting in a self-released structure.
Based on this mechanism, we demonstrate circular cantilever
beams and microspheres. An array of microbeams with 1
m
LEE AND WU: THERMAL ANNEALING IN HYDROGEN FOR 3-D PROFILE TRANSFORMATION 341
Fig. 6. A schematic process flow for self-released circular cantilever beams.
Fig. 7. SEM images of (a) circular cantilever beams and (b) microshperes. The
radius of each microsphere is about 1
m. The radius of each circular micropillar
is 2
m [20].
width and 1 m spacing were patterned by lithography. The
structures were etched down to the buried oxide with vertical
sidewalls on a 1.5-
m-thick SOI film. Then, these microbeams
were annealed at 1100
C and 10 torr until they were com-
pletely released due to undercutting at the interface. Fig. 7(a)
presents the result. The rectangular beams became circular with
rounded tips. To avoid the structures falling on the buried oxide,
the microbeam array was anchored to a large-area silicon fea-
ture, which is still unreleased. Microspheres were created using
a similar process. An array of circular cylinders with 1
m radii
were patterned by lithography. After dry-etching, the sample
was annealed in pure hydrogen at 1100
C and 10 torr. The an-
nealing time is controlled to prevent complete release of the mi-
crospheres. Fig. 7(b) shows the SEM image of the microspheres
after hydrogen annealing. The radius of each microsphere ap-
proximately equals 1
m. For circular cylinders with radii larger
than the ACR, the annealed profiles are similar to micropillars.
B. Submicron Wires
For a very thin rectangular silicon structure, the profile varies
significantly. Due to a large gradient of curvature changes at the
edge, silicon atoms migrate rapidly from the edges to the top
and bottom surfaces. This migration makes the lateral dimension
Fig. 8. SEM image of top and cross-sectional views of the microbeam array
(a) before and (b) after annealing [20]. The as-etched line array is 1-
m wide
and 0.2-
m thick for each beam. After annealing, each beam turns circular with
0.25-
m radii. These circular beams are coated by spin-on-glass (SOG).
shrink and the vertical dimension expand. If both the thickness
and width are smaller than the ACR, the initial rectangular pro-
file eventually becomes circular after annealing. Based on this
mechanism, a submicron wire can be fabricated. To demonstrate
this effect, an array of 1
m wide strips with 1 m spacing was
first patterned on SOI substrate with 0.2-
m-thick Si film and
3-
m-thick buried oxide. The top and cross-sectional views of
the strips are shown in Fig. 8(a). To allow a vertical expansion
and to avoid the chemical etching at Si/SiO
interface during
annealing, the buried oxide underneath this array was etched
by BOE. Finally, the sample was annealed in hydrogen ambient
at 1100
C, 10 torr, for 5 min. As shown in Fig. 8(b), the cross
section of the Si beams was transformed from 0.2
1 m
rectangles to 0.25- m-radius circles. Note that the areas of the
cross sections are conserved during the annealing process. This
technique can be employed to design and fabricate submicron
features without advanced lithography.
C. Toroidal Structures
A similar migration mechanism can be applied on a thin SOI
structures with large area (lateral dimension
m). How-
