Multilayer 3-D Photonics in Silicon
Prakash Koonath and Bahram Jalali
University of California, Los Angeles, Department of Electrical Engineering, Los Angeles, CA 90095-1594
Abstract: Three-dimensionally integrated devices have been realized in Silicon using SIMOX 3D
sculpting. Devices are fabricated, for the first time, on three vertically-integrated silicon layers,
paving the way towards ultra-dense opto-electronic structures in Silicon.
©2005 Optical Society of America
OCIS Codes: (250.3140) Optoelectronics; (130.3120) Integrated Optics
1 . Introduction
The integration of optical and electronic devices on the same substrate is one of the most attractive features of
silicon. Recent advances in active and passive silicon photonic devices have brought the realization of optical
interconnects for intra-chip communications closer to reality [1-6]. However, it is desirable to realize opto-electronic
integration in silicon in such a way that the photonic devices do not consume the silicon real estate required for
CMOS electronics, to make the integration economically viable. This unequivocally point to the need for 3-D
integration of devices where devices can be realized on vertically stacked layers of silicon. From the perspective of
optical devices, apart from the fabrication of densely integrated 3-D structures, vertical integration offers the
prospect of precise control of coupling coefficient in vertically coupled devices. Thus, complex optical circuitry with
accurately controlled evanescent coupling between devices is possible by employing vertically integrated optical
devices. Innovative fabrication technologies need to be developed to realize monolithic 3-D integration, at the same
time being compatible with well established CMOS foundry processing techniques.
We have previously demonstrated monolithic 3-D integration of photonic devices and CMOS electronics as
well as a variety of vertically-coupled optical devices in Silicon utilizing the technique of SIMOX 3-D sculpting [7-
10]. Devices were integrated on two vertically stacked layers of silicon separated from each other by a layer of
silicon dioxide. In this paper, we demonstrate the capability of this technique to achieve ultra-dense opto-electronic
structures by integrating devices in multiple layers of Silicon. In a three layer structure, microresonators are realized
in two sub-surface layers of silicon that are coupled to each other and to bus waveguides in the 3rd (surface) silicon
layer. Microresonator based multistage filter structures implemented using this technique show extinction ratios of
2 . 3-D Sculpting of Multilayer Devices
The SIMOX process involves the implantation of Oxygen ions into a Silicon substrate, followed by a high
temperature (around 1300ºC) anneal of the substrate in order to cure the implantation damage. A method, utilizing
the implantation of Oxygen ions into a masked SOI substrate, is employed to realize buried waveguide devices.
Figure 1 depicts the process flow of the fabrication of vertically integrated multilayer structures using SIMOX 3-D
sculpting. Implantation of oxygen ions is performed on an SOI substrate that has been patterned with thermally
grown oxide. The thickness of the oxide mask is chosen suitably to decelerate the Oxygen ions that penetrate into
the area underneath the mask. High temperature anneal after the implantation results in the formation of buried
waveguide structures separated from a surface silicon layer by a silicon dioxide layer, as show in figure 1. This
surface silicon layer is used as the seed layer to grow silicon epitaxially on the substrate. After the epitaxial growth,
the substrate goes through another set of implantation and annealing steps, resulting in the formation of a second
layer of buried devices and a surface silicon layer. Photonic or electronic devices may be defined on the surface
silicon layer using conventional lithography and etching process, resulting in the realization of three layers of 3-D
3. Fabrication and Characterization of Devices
A SOI wafer (made by SOITEC Inc.) with 0.6 µm of Silicon on top of a buried oxide layer of 0.4 µm thickness
was oxidized and patterned using reactive ion etching process to form oxide patterns of thickness 0.16µm. The
wafer was then implanted with oxygen ions with a dose of 5×1017 ions per cm2, at energy of 160 KeV, and annealed
subsequently. This leads to the formation of a buried layer of devices, and a surface silicon layer of thickness
2ndanneal 2ndanneal 2ndanneal 2ndanneal
Silicon Dioxide Silicon DioxideSilicon Dioxide
1stOxygen Implant1stOxygen Implant1stOxygen Implant1stOxygen Implant
Substrate Substrate Substrate
1stanneal and epitaxy1stanneal and epitaxy1stanneal and epitaxy1stanneal and epitaxy
2ndOxygen Implant2ndOxygen Implant2ndOxygen Implant2ndOxygen Implant
Substrate Substrate Substrate
Surface layer definition Surface layer definition Surface layer definition Surface layer definition
Substrate Substrate Substrate
Fig. 1. The process flow of SIMOX 3-D Sculpting process flow
approximately 0.2 µm. An epitaxial layer of silicon, of thickness 0.4 µm is grown on this wafer after the first anneal,
and the wafer then goes through an identical series of processes to form a second layer of buried devices, separated
from the surface silicon layer by the oxide formed through oxygen implantation. Finally, lithography and etching is
performed to create devices in the surface silicon layer. Figure 2 shows the cross sectional SEM pictures of rib
waveguides realized in the three different layers of silicon. It is very clear that the process of SIMOX 3-D sculpting
has successfully been employed to realize multilayer 3-D photonic structures in silicon.
Fig. 2. SEM images of rib waveguides realized in 3 vertically integrated layers of silicon
It may be seen from figure 2b that the oxide layer that defines the second layer of buried devices is
discontinuous. This is due to the fact that the amount of oxygen ion dose that entered the wafer is less than the
optimum value of 5×1017 ions per cm2 required for the formation of a continuous oxide layer. This can be verified by
measuring the thickness of the second buried oxide layer that was formed, which is around 85 nm. For a dose of
5×1017 ions per cm2, the thickness of a stoichiometric oxide layer is expected to be around 115 nm, as is measured in
the case of the oxide layer formed in the first implantation step. We surmise that the difference in the dose that
penetrated the wafer during the second implantation step must arise from the process variations at the commercial
implantation facility where the implantation was performed. Thus by ensuring the presence of optimum dose inside
the substrate, a continuous layer of oxide can be realized. It needs to be mentioned here that, even though the oxide
layer is discontinuous, simulations based on the commercial available software FIMMWAVE show that the
structure supports guided optical modes.
Fig.3. 3-D schematic of the multilayer device structure Fig.4. Optical micrograph of the fabricated multilayer device
Si Si SiSi
240 nm240 nm240 nm240 nm
25 µ µm 25 µ µm
This approach has been utilized to fabricate a microresonator based multistage filter structure, the schematic of
which is shown in figure 3. Here, the microresonators are realized in the two buried layers of silicon that are coupled
to each other and to the bus waveguides fabricated on the surface silicon layer through the intervening oxide layer
(the intervening layer of oxide through which the coupling of light takes place is omitted in figure 3 for the
simplicity of illustration). Figure 4 shows the optical micrograph of the top view of the fabricated device with the
arrows indicating the direction of flow of optical energy through these devices. The microdisks have a radius of 20
µm, and the bus waveguides have a width of 2 µm. When optical energy is introduced to the input port of the device,
resonant wavelengths are transmitted to the drop port, after traversing through three layers of vertically coupled
The drop port responses of the filter were characterized using an Amplified Spontaneous Emission (ASE)
source, by launching optical power at the input port and collecting the optical spectra at the drop port using a
spectrum analyzer, the results of which are shown in figure 5. Fabricated disks show a free spectral range of around
5.6 nm, with maximum extinction ratios ~ 14 dB. These results clearly indicate the capability of the SIMOX 3-D
sculpting technique to integrate nanophotonic structures in a 3-D fashion on multiple layers of Silicon.
Fig.5.Drop port spectral characteristics of the 3 stage multilayer device
4 . Conclusion
In summary, 3-D integrated three layer devices have been realized for the first time in Silicon through a monolithic
process. Microresonator devices are realized in two subsurface layers of silicon coupled to each other and to bus
waveguides situated on the surface silicon layer. Nanophotonic devices are fabricated on three vertically integrated
layers of silicon, paving the way towards ultra-dense opto-electronic structures in Silicon.
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