Supplementary Information
Optically monitoring and controlling nanoscale
topography during semiconductor etching
Chris Edwards, Amir Arbabi, Gabriel Popescu and Lynford L Goddard
IMAGING CAMERA
The CCD is a Hamamatsu model ORCA-ER C4742-80 with 12-bit dynamic range and an 8.67 mm x 6.60 mm
image sensor, 1344 x 1024 pixels, and a 6.45 μm x 6.45 μm pixel size. The focal lengths of lenses L3 and L4 are
7.5 cm and 40 cm, respectively, such that with a 10X objective, the overall transverse magnification is M = 53
and the field of view is approximately 160 m x 120 m.
ROTATING DIFFUSER
The surface of a circular piece of a polycarbonate was roughened by rubbing it against very fine grit
sandpaper by hand. A direct current (DC) motor rotated the polycarbonate piece, which operated as the
diffuser. Several pieces with different roughnesses were tested at various rotation speeds (on the order of
1000 rpm) and were characterized by measuring the spatial noise of the epi-DPM image with the diffuser in
the light path. The combination with the minimum spatial noise was used for future experiments.
FREQUENCY DOMAIN NOISE ANALYSIS
To further understand the nature of the spatial and temporal noise and the effect of the diffuser, we computed
the three-dimensional fast Fourier transform (FFT) of each series of 256 images of the flat sample and
recorded the power spectral density (PSD) of the noise as seen in the sample plane. To keep the memory
requirements to reasonable values, the images were cropped to retain the central 512 x 512 pixels. Figures
S1a-b show the spatially averaged noise PSD. With the diffuser, the noise appears to consist of 1/f noise
which dominates at low frequency and white noise which dominates at high frequency. Without the diffuser,
the low frequency noise increases faster than 1/f as f approaches zero. The width of the noise spectrum is
much narrower with the diffuser. This is because the diffuser randomizes the speckle pattern in time and
enables the camera to average out the slowly varying speckle patterns. The baseline noise level at high
frequency is slightly higher with the diffuser. This is probably due to the mechanical vibrations in the setup
caused by the rotating DC motor. The small side peaks in Figure S1b may also be due to vibrational
resonances in the setup. Figures S1c-d show the temporally averaged noise PSD. The noise is negligible
outside the diffraction limited circle |k| = 2πNA/λ = 2.9 rad/μm,1 where NA is the numerical aperture of the
objective. There is a very weak noise tail outside the diffraction limited circle along the kx = 0 and ky = 0 axes
approximately 65 dB lower than the peak near DC. This noise tail, which breaks the rotational symmetry of
the PSD, may be due to the grating alignment along the x direction and the nonlinear signal processing used
to convert from phase to corrected height, which adds higher order components.
Figure S1 Epi-DPM noise characterization on a flat unprocessed sample. 256 frames at 8.93 frames s-1 were collected and
cropped to retain the central 512 x 512 pixels. A three-dimensional fast Fourier transform was performed. (a) Spatially
averaged noise PSD without diffuser and (b) with diffuser. The diffuser narrows the noise spectrum as well as significantly
reduces the low frequency contribution. (c) Temporally averaged noise PSD without diffuser and (d) with diffuser. Almost all
of the noise is contained within the diffraction limited circle |k| < 2.9 rad/μm. epi-DPM, epi-diffraction phase microscopy.
PSD, power spectral density.
PROCESSING RECIPE FOR SPR 511A PHOTORESIST
Our recipe for SPR 511A photoresist (PR) begins with a standard degreasing followed by a dry bake at 125C
for 4 minutes to remove any remaining moisture on the samples. HMDS:Xylene (1:2) is spun at 2000 rpm for
30 s, followed by the resist at 5000 rpm for 30 s to obtain a thickness of about 1 μm. A soft bake is then done
at 90C for 90 s. The Karl Suss MJB-3 aligner C is used to do a 12 s exposure. Immediately after the exposure, a
post-bake is done for 1 minute at 110C. The sample is then developed using MF-CD-26 for 25 s. A hard bake at
110C for 90 s is done before etching.
MICROPILLAR FABRICATION
The micropillars were fabricated using n+ GaAs wafers. The photolithography and developing were done
using SPR 511A PR and the previously described recipe. A wet etch was then performed using a 1:1:50
solution of H3PO4:H2O2:H2O. Phosphoric acid is a standard wet etchant for GaAs. After the PR was removed,
the samples were observed under the surface profiler and under the scanning electron microscope (SEM) to
determine the height ranges for the given samples. Sample #1 was etched for 20 s and had heights of 60-70
nm. Sample #2 was etched for 40 s and had a height range of 120-130 nm. The heights measured by epi-DPM
for both samples were within their respective ranges listed above.
MICROPILLAR IMAGE WITHOUT DIFFUSER
Figure S2 shows a micropillar from sample #2 imaged without the diffuser. The pillar is at a different location
than the corresponding one of Figure 2c. Without the diffuser, the noise floor increased by 2.7 nm to 9.0 nm
for the pillar and by 0.3 nm to 5.1 nm for the etched region. The mean height of this sample was 122.9 nm.
Figure S2 Characterization of the accuracy of epi-DPM. (a) Height measurement of a micropillar using epi-DPM without diffuser
(field of view is 160 μm x 120 μm) (b) Histogram of a, showing the standard deviation for a single pixel on the pillar or etched
region. The noise is lower with the diffuser in both regions as shown by Figure 2d of the main text. The mean height was
determined from the histogram to be 122.9  0.1 nm. epi-DPM, epi-diffraction phase microscopy.
IN SITU MONITORING OF WET ETCHING
The sample containing the University of Illinois logo was also fabricated using n+ GaAs wafers. Plasma
enhanced chemical vapor deposition (PECVD) was used to deposit 50 nm of silicon dioxide on the front and
back sides of the GaAs sample. On the front side, SPR511A photoresist was spun and the logo was patterned
using photolithography. The resist pattern was then transferred to the oxide using reactive ion etching (RIE)
with Freon gases. The patterned oxide on the front side served as a hard mask for the wet etch process while
the oxide covering the backside prevented depletion of the etchant from unwanted backside etching.
After patterning, the sample was placed upside down in a small Petri dish filled with water and propped up
on two pieces of glass. The glass was used to allow the acid to diffuse under the inverted sample. The
microscope was then adjusted until the sample was in focus on the eyepiece. The sample was then carefully
adjusted until it was flat and then two drops of dilute acid (1:1:50 solution of H 3PO4:H2O2:H20) were added to
each side of the sample. The etch rate of PECVD silicon dioxide using such a dilute solution as 1:1:50 is less
than 1 nm min-1. Given the short etch times, etching of the oxide can be considered negligible2.
A 3D movie (Video S1) was recorded showing the dynamics of the wet etching process. A total of 400
frames were collected at 8.93 frames s-1 for a total etch time of 44.8 s. The movie is replayed at real time
speed for best representation of the etching dynamics. However, the slow changes can be more easily
perceived when replayed at 2x speed. The color scale used in the movie is the same as that used in Figure 3a.
At the beginning of the movie, before any etchant reaches the sample, the logo is slightly visible because there
is a patterned oxide mask. It is not very visible because there is only a small index difference between the
oxide and the water based ambient solution. At about 10 s, the etchant diffuses under the sample and begins
etching the top left corner. The diagonal line in the lower left corner may be a scratch in the sample. At about
18 s, the etch depth in the narrow gaps between the lines of the logo, especially near the curl, is comparable to
other unmasked portions; however near the end of the movie, one can see these gap regions do not etch as
deeply. There are some artifacts in the movie. The tiny fluctuations in the tilt of the sample are due to the
motion of the sample in the ambient etching solution. The quadratic fit process used to correct for sample tilt
and phase curvature of the beam did not perform as well as on the flat sample because of the lack of large
unetched areas in the image. The occasional sudden motions near the corners of the sample are attributed to
phase unwrapping errors. Despite these minor errors, the general trends and dynamics of wet etching have
been accurately captured in situ with epi-DPM.
Video S1 Dynamics of wet etching. 400 height images were collected at 8.93 frames s-1. The total etch time was 44.8 s. The
movie is replayed at real time speed. The field of view is 320 μm x 240 μm.
CROSS-SECTIONAL PROFILES OF ETCHED STRUCTURES
Figure S3a shows a cross-section of the micropillar imaged with the diffuser from Figure 2c. The image was
taken with a 10X objective (NA = 0.25) and has a field of view of 160 μm x 120 μm. The phosphoric acid wet
etch on GaAs is isotropic. Thus, the lateral undercutting is approximately equal to the etch depth. The inset is
taken from the edge of the pillar, represented by the dotted line, and shows that the sidewalls are vertical to
about 0.5 μm. Thus, the edge resolution of our system is better than its 1 μm lateral resolution.
Figure S3b shows a similar cross-section of the University of Illinois logo. The image was taken with a 5X
objective (NA = 0.1) and has a field of view of 320 μm x 240 μm.
Figure S3 Cross-sectional profiles of etched structures. (a) Height measurement (cross-section) of a micropillar using epi-DPM
with diffuser (field of view is 160 μm x 120 μm). The profile is taken along the dashed line across the pillar, as shown in the top
right inset. The vertical sidewalls in the middle inset show that the edge resolution is about 0.5 μm. (b) Etch depth measurement
(cross-section) of the University of Illinois logo using epi-DPM (field of view is 320 μm x 240 μm). The micropillars were etched
under more ideal conditions in a well-mixed acid solution and imaged with a higher numerical aperture objective, resulting in
images with more vertical sidewalls. epi-DPM, epi-diffraction phase microscopy.
CHARACTERIZATION OF MICROLENS ETCHING PROCESS
A spherical surface was fitted to the lens height data and the standard deviation of the residuals was
calculated to quantify the performance of the lens. Figure S4a shows a cross section of the data, fit, and
residuals. Figure S4b shows a top-down view of the residuals. The intended application is coupling arrays of
vertical cavity surface emitting lasers (VCSELs) emitting at λ = 980 nm into 62.5 μm diameter multi-mode
fibers. Thus, Figures S4b-c are limited to this diameter. Figure S4c shows a histogram of the residuals from
S4b. The standard deviation of the residuals was 11.4 nm.
Figure S4 Characterization of microlens etching process. (a) A spherical curve was fitted to the lens height data and the standard
deviation of the residuals was measured to quantify the lens performance. The images in b and c are limited to a diameter of
62.5 μm. (b) Top-down view of the residuals. Lens quality worsens near the edges because of the increased etch depth. (c)
Histogram of the residuals. The standard deviation was 11.4 nm.
SUPPLEMENTARY REFERENCES
1
2
Saleh, B. E. A. & Teich, M. C. Fundamental of Photonics. 2nd edition edn, (John Wiley and Sons, Inc., 2007).
Williams, K. R., Gupta, K. & Wasilik, M. Etch rates for micromachining processes- part II. Journal of
Microelectromechanical Systems 12, 761-778 (2003).