Encapsulation layers are explored for passivating the surfaces of silicon to
reduce optical absorption in the 1500-nm wavelength band. Surface-sensitive
test structures consisting of microdisk resonators are fabricated for this
purpose. Based on previous work in silicon photovoltaics, coatings of SiNx and
SiO2 are applied under varying deposition and annealing conditions. A short dry
thermal oxidation followed by a long high-temperature N2 anneal is found to be
most effective at long-term encapsulation and reduction of interface
absorption. Minimization of the optical loss is attributed to simultaneous
reduction in sub-bandgap silicon surface states and hydrogen in the capping
material.Comment: 4 pages, 3 figure

Borselli, M.

Henry, M. D.

Johnson, T. J.

Michael, C. P.

Painter, Oskar

English

arXiv

Encapsulation layers are explored for passivating the surfaces of silicon to reduce optical absorption in the 1500  nm wavelength band. Surface-sensitive test structures consisting of microdisk resonators are fabricated for this purpose. Based on previous work in silicon photovoltaics, coatings of SiNx and SiO2 are applied under varying deposition and annealing conditions. A short dry thermal oxidation followed by a long high-temperature N2 anneal is found to be most effective at long-term encapsulation and reduction of interface absorption. Minimization of the optical loss is attributed to simultaneous reduction in sub-band-gap silicon surface states and hydrogen in the capping material

Painter, O.

Caltech Authors - Main

Surface encapsulation for low-loss silicon photonics
M. Borselli, T. J. Johnson, C. P. Michael, M. D. Henry, and O. Paintera
Thomas J. Watson, Sr., Laboratory of Applied Physics, California Institute of Technology, Pasadena,
California 91125, USA
Received 2 July 2007; accepted 13 September 2007; published online 28 September 2007
Encapsulation layers are explored for passivating the surfaces of silicon to reduce optical absorption
in the 1500 nm wavelength band. Surface-sensitive test structures consisting of microdisk
resonators are fabricated for this purpose. Based on previous work in silicon photovoltaics, coatings
of SiNx and SiO2 are applied under varying deposition and annealing conditions. A short dry thermal
oxidation followed by a long high-temperature N2 anneal is found to be most effective at long-term
encapsulation and reduction of interface absorption. Minimization of the optical loss is attributed to
simultaneous reduction in sub-band-gap silicon surface states and hydrogen in the capping
material. © 2007 American Institute of Physics. DOI: 10.1063/1.2793820
The integration of high-speed photonics and electronics
on a silicon platform is currently being actively pursued due
to the soaring demand for bandwidth and the difficulties in
maintaining Moore’s Law scaling of microelectronics.1,2 Due
to the thermal and power limitations already faced by the
electronics industry,3,4 future systems incorporating on-chip
optical processing and communication will likely put a
heavy premium on low optical power levels and high optical
efficiency. With the many benefits of Si microphotonic cir-
cuits, such as the ability to create active and nonlinear optical
elements, there is also the adverse effect of increased optical
loss in comparison to conventional glass-based photonic cir-
cuits. In addition to the intrinsic nonlinear absorption present
in micron-scale Si waveguides,5 the high-index contrast of
such structures results in significant optical scattering loss
for surface roughness at the nanometer scale. Over the past
several years, there has been considerable effort put forth to
improve the quality of Si microphotonic device fabrication,
with optical scattering loss having been reduced to a level of
1 dB/cm in single-mode waveguides.6 This is still several
orders of magnitude above the bulk absorption limit of mod-
erately doped Si in the 1.3–1.5 m wavelength band,7 and it
is interesting to consider whether further reductions in opti-
cal loss can be made. One possible impediment is the absorp-
tion present at the Si surfaces of current high-index contrast
and, consequently, high surface-to-volume ratio Si micropho-
tonics.
In a previous work, we measured the effects of surface
chemistry on the optical losses in smoothly etched, high-Q Si
microdisk resonators8,9 and found that for losses below the
dB/cm level, surface absorption begins to play a role. In that
study, hydrogen-passivated surfaces showed marked im-
provement with optical losses measured as low as
0.15 dB/cm; however, the passivation was temporary and
easily spoiled by the atmosphere. In this letter, we investi-
gate, for photonics applications, several techniques originally
developed for electronic surface passivation of Si solar cells.
The reliability standards of Si photovoltaics fabrication10–13
routinely call for various passivation layers to be deposited
over the Si surfaces in order to preserve the lifetime of the
minority-carrier electrons and holes. Based upon our studies,
we report a simple recipe consisting of a short dry thermal
oxidation followed by a long high-temperature N2 anneal
that is effective at preserving the Si disks’ high quality fac-
tors in the 1.5 m wavelength band, indicating that the
Si/SiO2 interface provides adequate dangling bond passiva-
tion for photonic applications.
Schmidt et al.12 found that plasma-enhanced chemical
vapor deposition PECVD silicon nitride SiNx passivation
layers with or without an underlying thin thermal oxide layer
achieved effective free-carrier lifetimes of 1 ms, compa-
rable to the best passivation schemes to date. The optimized
PECVD recipe for SiNx layers was found to result in nearly
stoichiometric Si3N4 films and was generated through a di-
rect parallel-plate reactor system. Furthermore, they found
that the effective lifetime further increased for the first
50 min of a 400 °C anneal in forming gas 5% H2 and 95%
N2 before eventually decaying with increased forming gas
annealing FGA. As the effect of ambient hydrogen was
found to be negligible, they attributed the increase of lifetime
to the large reservoir of hydrogen in the SiNx
15–20 at. %  being released during the deposition and
anneal. The effective passivation was also found to benefit
from an initial thermal oxide, providing a higher-quality Si
interface, before the SiNx deposition and concomitant diffu-
sion of hydrogen through the SiO2 to the Si surface. In a
more recent study, McCann et al.13 found that a 25-nm-thick
thermal oxide grown at 900 °C followed by a 30 min FGA at
400 °C was sufficient to obtain equally high effective life-
times. In addition, they found that the lifetimes could be
spoiled by a 1 h high-temperature of 900–1000 °C N2 an-
neal and subsequently repaired by another FGA. They attrib-
uted this effect to reversibly adding or removing hydrogen
from the Si/SiO2 interface.
We hypothesized that the effective electronic lifetime
measurements from the previous references would be good
indicators of optical loss at the Si interface. Both PECVD
SiNx and dry thermal oxide TOX layers were applied as
encapsulation layers to Si microdisks Fig. 1. The samples
for these trials were fabricated from silicon-on-insulator
217 nm Si device layer, p type of 14–20  cm, 100 ori-
entation, and 2 m buried SiO2 layer. Microdisk resonators
with radii of 5, 7.5, and 10 m were patterned with
electron-beam lithography and a resist reflow process using
ZEP520A resist and were defined with a SF6–C4F8 induc-aElectronic mail: opainter@caltech.edu
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tively coupled-plasma reactive-ion etch.8 The devices were
tested using an evanescent coupling technique8 employing a
tunable laser source 1423–1496 nm and linewidth of
300 kHz and a local fiber-taper probe.14 A fiber-based
Mach-Zehnder interferometer was used to calibrate reso-
nance linewidth  measurements to ±0.01 pm— is re-
lated to the total cavity loss rate  by =0
2 /2c, where
0 is the free-space wavelength and c is the speed of light.
To ensure no ion contamination, all oxidations and an-
neals were done in a custom-built, HF-cleaned quartz-tube
oxidation furnace. Electronic grade II O2, N2, and forming
gases were plumbed into the 2 in. diameter furnace using
electropolished stainless steel tubing. Each gas’ flow rate was
independently controlled with a mass flow controller; typical
flow rates inside the approximately 1-m-long quartz furnace
were 0.3 standard liters per minute. Thermal shock to the
samples was avoided by using the unheated portion of the
furnace tube as a second and intermediate staging area where
the samples were allowed to slowly warm up and cool down
before transfer to the heated portion of the chamber or being
removed from the furnace. To assess the loss of each surface
treatment, the linewidth of a single surface-sensitive9 TM
mode is tracked throughout a given processing sequence.
This procedure gives a long effective optical path length and
ensures that the surface is sampled in the same way after
each step.
The first encapsulation trial was done on a sample that
had an initial 50 nm SiNx cap deposited on the surface prior
to lithography and etching. After removing the ZEP520A
resist with an hour-long piranha etch, Fig. 2a shows a bar
graph summary of the best linewidths at selected intermedi-
ate points during the fabrication. Having tested many devices
on the sample, the best linewidth after the initial piranha
clean was =4.5 pm. However, after an hour-long HF un-
dercut which also removed the SiNx cap, the linewidths re-
duced considerably to 0.8 pm. Further, piranha/HF
treatments9 had no discernible effect on the linewidths.
At this point, a nearly stoichiometric 30-nm-thick SiNx
encapsulation was deposited over the wafer. Immediately
prior to loading the wafer into the PECVD chamber, an ad-
ditional three piranha/HF treatments were done on the
sample to ensure a clean and well-passivated Si surface. The
processing conditions were adapted from Ref. 12, so gas
chemistries of 450 SCCM SCCM denotes cubic centimeter
per minute at STP of 5% SiH4/N2 and 50 SCCM of NH3
were applied to the chamber held at 200 mTorr and 400 °C.
The gas was cracked with 60 W from a 13.56 MHz radio-
frequency source; no low frequency source was used in an
attempt to suppress deposition damage by ion’s oscillating
below the 4 MHz plasma frequency.15 In contrast to the
photoconductance decay measurements of free-carrier life-
times on Si solar cells,12 the losses of the cavities signifi-
cantly increased to 4 pm, a difference of 3.2 pm. After test-
ing with the “as-deposited” SiNx cap, the sample underwent
a 40 min FGA at 400 °C. Subsequent testing revealed that
the FGA had further deleterious effects on the samples,
where the best linewidth was found to be 7 pm, as in Fig.
2a.
Assuming a SiNx index of refraction of 1.9, finite
element method simulations of the composite resonator
show that TM modes possess SiNx =11% of the optical en-
ergy inside the SiNx. Thus, if the sources of loss were
evenly distributed throughout the SiNx, the material quality
factor of the as-deposited material would be QSiNx,mat
=SiNx0 / after−before5.0	10
4
, corresponding to an
attenuation coefficient of 
SiNx,mat7.1 dB/cm. PECVD de-
posited material is known to have relatively high absorption
coefficients 1–10 dB/cm due to Si–H, O–H, and N–H
bond absorption overtones in the telecommunications wave-
length bands.16 Furthermore, as the FGA would not be ex-
pected to harm the Si surfaces, a consistent interpretation of
the results, shown in Fig. 2a, is that any benefits of a hy-
drogenated Si surface were overwhelmed by the increased
hydrogen content in the bulk SiNx layer. This is also consis-
tent with the fact that PECVD Si-rich nitride disks with no
high-temperature anneal were independently fabricated and
tested achieving quality factors of 6	104.
A second sample underwent identical processing as the
sample just described, including the deposition of a SiNx cap,
lithography, dry etching, ZEP removal, HF undercut, and
three piranha/HF treatments. As expected, the best measured
linewidth of 0.7 pm was very similar to the previous sample
and is shown in Fig. 2b. However, this time, a 10 nm TOX
layer was grown on the Si surface at 1000 °C for 3.1 min in
an attempt to form a good Si interface with a hydrogen free
material. After switching off the O2, the sample was allowed
to cool slowly under a N2 ambient for 5 min before retest-
ing. The best linewidth after thermal oxidation was 3.5 pm, a
result similar to the SiNx cap. However, the same 40 min
FGA had virtually no impact on the sample with the thermal
FIG. 1. Color online a An array of Si microdisk cavities with a radius of
5 m. b Finite element simulation of the normalized electric-field inten-
sity for the TM cavity mode studied in Fig. 3.
FIG. 2. Color online Summary of best linewidths after selected processing
steps for 5–10 m radii disks fabricated with a a stoichiometric SiNx
encapsulation layer and b a thermal oxide encapsulation layer along with
various annealing trials.
131117-2 Borselli et al. Appl. Phys. Lett. 91, 131117 2007
Downloaded 01 Oct 2007 to 131.215.225.176. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp
oxide cap. A 4.5 h high-temperature anneal in a N2 ambient
was found to significantly improve the losses, where the best
linewidth was measured to be 1.6 pm. The high-temperature
anneal consisted of holding the furnace at 1000 °C for 3 h
and then letting the temperature slowly ramp down to
400 °C over the course of the remaining 1.5 h. Assuming
that the high-temperature anneal healed the Si interface and
bulk TOX, a 40 min FGA was conducted on the sample. The
FGA was found to slightly reduce the quality factor of the
best resonance, where the linewidth was measured to be
1.8 pm. A second 4.5 h high-temperature anneal, aimed at
driving out any hydrogen added during the FGA step, suc-
cessfully reduced the optical losses back down to 
=1.2 pm on the best resonance. This set of four anneals
showed that the thermal oxide needed time at high tempera-
ture to remove material and surface defects. Also, the FGA
had a marginally degrading effect on the optical losses.
A third sample was similarly fabricated but did not have
an initial SiNx cap prior to lithography and dry etching. After
HF undercutting and three piranha/HF treatments, the best
linewidths were measured to be 1.0 and 0.6 pm, respectively
Fig. 3. The marginal improvement in this case was attrib-
uted to a simplified single material dry etch and a less dam-
aged top Si surface. The latter was confirmed after an iden-
tical 10 nm TOX cap with 5 min cool down showed a best
linewidth of 2.0 pm, much better than the second sample’s
3.5 pm linewidth after oxidation. Omitting any FGA step, a
final 4.5 h high-temperature anneal healed the Si interface
and bulk silica cap, showing identical linewidths prior to
oxidation. Similarly processed samples in which the Si mi-
crodisk was completely oxidized through and then annealed
had whispering gallery mode resonances with Q3	106,
indicating that the encapsulating oxide is of good optical
quality after high-temperature anneals.
Initial long-term data show that microdisks with a TOX
encapsulation layer and a high-temperature N2 anneal do not
degrade after 9 months stored in a clean-room environment
22 °C, 44% relative humidity. Going forward, formal-
ized reliability testing under increased temperature, humidity,
and pressure will still be necessary to prove true industrial
applicability. Furthermore, these tests could be coupled with
materials analysis such as secondary ion mass spectroscopy
SIMS or Auger profiling to yield even better insight into
the physical mechanisms at work. Follow-up work should
explore the sensitivity to furnace conditions including ramp
rate, gas chemistries, and anneal temperature. Low-pressure
chemical vapor deposition of SiNx could also be attempted to
provide a high quality Si photonic passivation layer.
The results shown in Fig. 3 represent the robust encap-
sulation of the once delicate Si-surfaces, as 10 nm of thermal
oxide will completely prevent native or chemical oxide
growth during any subsequent fabrication steps. While sili-
con nitride would have been slightly preferable in terms of
chemical resistance, the high-quality thermal oxide is a
simple and effective method of sealing the disks from envi-
ronmental contamination while still allowing optical access
to the mode’s near field. Having also demonstrated this tech-
nology on a planar ring resonator,14 this work represents a
significant step toward developing ultralow-loss silicon pho-
tonics technology.
This work was supported through the DARPA EPIC pro-
gram. For graduate fellowship support, we thank the Moore
Foundation M.B. and C.P.M., NPSC M.B., HRL Labora-
tories M.B., NSF C.P.M., and the Hertz Foundation
M.D.H..
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FIG. 3. Color online Summary of best linewidths after selected processing
steps for 5–10 m radii disks fabricated without an initial protective cap.
Inset: transmission spectrum of a TM-like WGM doublet resonance Ref. 8
o=1444.2 nm of a 7.5 m radius disk after the final high-temperature
anneal.
131117-3 Borselli et al. Appl. Phys. Lett. 91, 131117 2007
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Surface Encapsulation for Low-Loss Silicon Photonics

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