Non-destructive
characterization of
membrane photonic
crystal devices
The study is based on the optical properties of QDs that are embedded in
PhC cavities (Figure 1a), leading to enhanced emission (the Purcell effect), orin WGs (Figure 1b), producing photon multiplexing devices. A typical device
is fabricated from a multi-layer GaAs/Al0.7Ga0.3As/GaAs epitaxially grownstack, where the top 250 nm thick GaAs layer contains the active part of thedevice, and the 1 µm Al0.7Ga0.3As sacrificial layer is eventually etched away toproduce a floating membrane device (Figure 1c). This membrane contains
one or several 20 nm In0.3Ga0.7As QDs in specific positions, and an etched
PhC structure (large array of 100 nm holes, with several holes missing in a
specific configuration), precisely aligned with the QDs. These devices require
accuracies of 1-20 nm in several processing steps, so they all involve highperformance electron-beam lithography and plasma (ICP) etching. The crucial
final step is the membrane release, involving wet etching of the sacrificiallayer. Since at this stage there is no possibility to look at a cross-section ofthe samples, top-view Optical Microscopy and Scanning Electron Microscopy(SEM) are the only possible tools to characterize the PhC structures.
CASE STUDY NON-DESTRUCTIVE CHARACTERIZATION OF MEMBRANE PHOTONIC CRYSTAL DEVICESEPFL The Laboratory for Physics ofNanostructures (LPN), led by Prof.
Eli Kapon, belongs to the IPYS atthe Ecole Polytechnique Fédérale
de Lausanne (EPFL), one of theleading Swiss technical universities.
Our main study is photonic nanodevices, based on the integrationof position- and energy-controlledsemiconductor quantum dots(QDs) with photonic crystal (PhC)
optical cavities and waveguides(WGs). Epitaxial growth of thenanostructures was performed byDr. Alok Rudra; Design, processingand characterization was performedby Dr. Benjamin Dwir.

1apyramidal
QDs

L3 PhCcavitiybc

1 μmFigure 1. a) SEM image of a network of QDs (darker spots) embedded in coupled L3PhC cavities.
b) SEM image of PhC waveguide with an out-coupler at the top, containing six QDs (indicatedby red triangles). c) SEM cross-section image of the suspended membrane device.

In this study, the S neox was used. The S neox provides up to 4 different lightsources (red, green, blue and white), which is particularly relevant in thisstudy as the use of 2D brightfield high-magnification imaging with a red
light source enables us to probe the features under the membrane that areotherwise invisible. The purpose of this study is the high-precision (sub-μm)
non-destructive characterization of photonic crystal devices fabricated onthin (250 nm) suspended GaAs membranes.

■ Measurements
We observed the floating membrane devices by brightfield microscopy,
using the 150X objective (sometimes combined with further image zoom),using the microscope’s four LED light sources. As a first approximation, wealways suppose that the observed image reflects the surface feature of the2CASE STUDY NON-DESTRUCTIVE CHARACTERIZATION OF MEMBRANE PHOTONIC CRYSTAL DEVICES

device. However, in some devices, e.g. those containing dielectric layers,
light can penetrate the top layer and provide information about the innerparts, which is extremely useful. Therefore, we tried to apply this techniqueto our floating membrane devices. As is well known, GaAs is opaque at thevisible spectral range, due to its gap energy of 1.52 eV (815 nm) at roomtemperature.
At all shorter wavelengths, the absorption coefficient of GaAs is large andstrongly wavelength-dependent:1 it varies by a factor of 5 between the red
LED (3.9µm-1 at 630 nm) and the blue LED (20.4µm-1 at 460 nm) used in
the Sensofar microscope. However, for the thin membranes (250-260 nm)
that we use, there is a good chance of recovering useful information fromthe bottom part of the device, at least with red light, which is less absorbedby the membrane.
In Figure2a-d we see brightfield images (taken with 150x objective plus x4zoom) of a PhC membrane device, depicted as a gray hexagon, as individual
100 nm holes cannot be imaged. We show images taken with the red (2a),green (2b), blue (2c), white (2d) LEDs. We can clearly observe the effectof the LED wavelength on the image: While the blue image (Figure2c)
shows only the top part of the device, the red image (Figure 2a) showsfeatures beyond the device edges, showing as a dark band. These features
correspond to the under-etched regions of the device, which are observed
through the membrane when using red light. The green (Figure 2b) andwhite (Figure 2d) images show very weak features of the underetched zone,
as we expect from the spectra in Figure 3. We also see that the central part(PhC zone) is brighter than the background in red light (2a) and darker inall other lights (2b-d). This could be an interference effect related to themembrane thickness. Another aspect is of course the higher resolution of
the blue images (Figure 2c,f ) compared to the red ones (Figure 2a,e), as canbe observed in the smaller images of the sub-micron (0.4x0.8 µm) cavity at
the center of the device. Finally, in Figure 2e,f we show the same devices indark-field, using red and blue LEDs, respectively. The edges of the undercut
zone (Figure 2e) and of the top of the device (Figure 2f ) are clearly seen. In
both images we also observe some granularity, which we can thus deduce
to be on the surface of the device and not underneath.

CASE STUDY NON-DESTRUCTIVE CHARACTERIZATION OF MEMBRANE PHOTONIC CRYSTAL DEVICES3abcd5 μmef

Figure 2. Brightfield images of a PhC cavity device, all taken using the x150 objective (Brightfield,zoom x4). Light source is Red LED (a). Green LED (b), Blue LED (c) and White LED (d). In (e) and (f)we see the dark-field images taken with Red and Blue LEDs, respectively (corresponding to theBF images in a and c)

Figure 3. a) Spectra of the four LEDs normalized to equal integrated intensities. b) The spectrumof attenuation (T2) of light passing twice (back and forth) through a 260 nm thick GaAs
membrane (Right axis), and the intensity of the reflected LED light from (a), after the double
passage (Left axis).

In Figure 4 we show blue (4a) and red (4b) images of a PhC WG device.
Similarly to Figure 2a and c, we observe the higher resolution of the blue
image, manifested in the sub-micron (0.4x0.8 µm) cavities. Otherwise, the4CASE STUDY NON-DESTRUCTIVE CHARACTERIZATION OF MEMBRANE PHOTONIC CRYSTAL DEVICES

surface is smooth-looking (except for some isolated defects). If we look at
the red-light image of the same area (Figure 4b), we again the device see
the whole array as the brightest part of the image, with a gray band aroundit. It is the image of the etched part under the membrane, visible only in redlight. We also observe some granularity of these “internal” surfaces (could beon the bottom part of the membrane or the top part of the substrate), again
visible only in this image.

a5 μmbFigure 4. Brightfield images of a PhC WG device, taken using the 150X objective. Light source isBlue LED (a) and Red LED (b).

■ Conclusions
When imaging thin membrane semiconductor devices, we are able to image
through the membrane by using red light, while blue light only shows thedevice’s surface. Using both blue and red brightfield imaging, we could
compare the “top” and “inside” images of our photonic devices. The S neox 3D
optical profiler in 2D brightfield imaging mode is very fast, non-destructive,and allows us to image device features that are otherwise not visible. It
would be very useful in the characterization of many types of MEMS devices,
as the optical absorption of Si shows a similar trend to that of GaAs.

■ Bibliography
1. J. O. Akinlami and A. O. Ashamu 2013 J. Semicond. 34 032002

CASE STUDY NON-DESTRUCTIVE CHARACTERIZATION OF MEMBRANE PHOTONIC CRYSTAL DEVICES

5SENSOFAR is a leading-edge technology
company that has the highest quality standards
within the field of surface metrology
Sensofar Metrology provides high-accuracy optical profilers
based on confocal, interferometry and focus variation
techniques, from standard setups for R&D and quality
inspection laboratories to complete non-contact metrology
solutions for in-line production processes. The Sensofar Grouphas its headquarters in Barcelona, also known as a technology
and innovation hub in Europe. The Group is represented in over
30 countries through a global network of partners and has its own
offices in Asia, Germany and the United States.
HEADQUARTERS

SENSOFAR METROLOGY | BARCELONA (Spain) | T. +34 93 700 14 92 | info@sensofar.com
SENSOFAR ASIA | SHANGHAI (China) | T. +86 021 51602735 | info.asia@sensofar.com
SENSOFAR GERMANY | MUNICH (Germany) | T. +49 151 14304168 | info.germany@sensofar.com
SENSOFAR USA | NEWINGTON (USA) | T. +1 617 678 4185 | info.usa@sensofar.com

sensofar.com

CS00-05A-EN-EPFL Membrane

SALES OFFICES