We describe a CMOS image sensor with column-parallel delta-sigma (ΔΣ) analog-to-digital converter (ADC). The
design employs three transistor pixels (3T<sup>1</sup>) where the unique configuration of the ΔΣ ADC reduces the noise
contribution of the readout transistor. A 128 x 128 pixel image sensor prototype is fabricated in 0.35μm TSMC
technology. The reset noise and the offset fixed pattern noise (FPN) are removed in the digital domain. The
measured readout noise is 37.8μV for an exposure time of 33ms. The low readout noise allows an improved low
light response in comparison to other state-of-art designs. The design is suitable for applications demanding
excellent low-light response such as astronomical imaging. The sensor has a measured intra-scene dynamic range
(DR) of 91 dB, and a peak signal-to-noise ratio (SNR) of 54 dB.
This paper is a progress report of the design and characterization of a monolithic CMOS detector with an on-chip ΣΔ
ADC. A brief description of the design and operation is given. Backside processing steps to allow for backside
illumination are summarized. Current characterization results are given for pre- and post-thinned detectors.
Characterization results include measurements of: gain photodiode capacitance, dark current, linearity, well depth,
relative quantum efficiency, and read noise. Lastly, a detector re-design is described; and initial measurements of its
photodiode capacitance and read noise are presented.
The Rochester Imaging Detector Laboratory, University of Rochester, Infotonics Technology Center, and Jet Process
Corporation developed a hybrid silicon detector with an on-chip sigma-delta (ΣΔ) ADC. This paper describes the process
and reports the results of developing a fabrication process to robustly produce high-quality bump bonds to hybridize a
back-illuminated detector with its ΣΔ ADC. The design utilizes aluminum pads on both the readout circuit and the
photodiode array with interconnecting indium bumps between them. The development of the bump bonding process is
discussed, including specific material choices, interim process structures, and final functionality. Results include
measurements of bond integrity, cross-wafer uniformity of indium bumps, and effects of process parameters on the final
product. Future plans for improving the bump bonding process are summarized.
The University of Rochester's Laboratory for Laser Energetics (LLE) has recently completed the construction of the
OMEGA EP short-pulse, petawatt laser system. A major structure for OMEGA EP is the grating compressor chamber
(GCC). This large (15,750-ft3) vacuum chamber contains numerous optics used in laser-pulse compression of two
40-cm-sq-aperture, IR (1054-nm) laser beams. Critical to this compression, within the GCC, are eight sets (four per
beamline) of tiled (e.g., three optical elements precisely held side by side to act as one element) multilayer-dielectric
(MLD)-diffraction-grating assemblies (three gratings per assembly) that provide the capability for producing 2.6-kJ
output IR energy per beam at 10 ps. The primary requirements for each of the 24 large-aperture (43-cm × 47-cm)
gratings are a high diffraction efficiency greater than 95%, a peak-to-valley wavefront quality of less than &lgr;/4 waves at
1054 nm, and a laser-induced-damage threshold greater than 2.7 J/cm2 at 10-ps pulse width (measured at normal beam
incidence). Degradation of grating laser-damage thresholds due to adsorption of contaminants must be prevented to
maintain system performance.
A critical component for the OMEGA EP short-pulse petawatt laser system is the grating compressor chamber (GCC).
This large (12,375 ft<sup>3</sup>) vacuum chamber contains critical optics where laser-pulse compression is performed at the output
of the system on two 40-cm-sq-aperture, IR (1054-nm) laser beams. Critical to this compression, within the GCC, are
four sets of tiled multilayer-dielectric- (MLD) diffraction gratings that provide the capability for producing 2.6-kJ output
IR energy per beam at 10 ps. The primary requirements for these large-aperture (43-cm × 47-cm) gratings are diffraction
efficiencies greater than 95%, peak-to-valley wavefront quality of less than λ/10 waves, and laser-induced-damage
thresholds greater than 2.7 J/cm<sup>2</sup> at 10-ps measured beam normal. Degradation of the grating laser-damage threshold due
to adsorption of contaminants from the manufacturing process must be prevented to maintain system performance.
In this paper we discuss an optimized cleaning process to achieve the OMEGA EP requirements. The fabrication of
MLD gratings involves processes that utilize a wide variety of both organic materials (photoresist processes) and
inorganic materials (metals and metal oxides) that can affect the final cleaning process. A number of these materials
have significant optical absorbance; therefore, incomplete cleaning of these residues may result in the MLD gratings
experiencing laser damage.
Multilayer-dielectric (MLD) diffraction gratings are used in high-power laser systems to compress laser-energy pulses.
The peak power deliverable on target for these short-pulse petawatt class systems is limited by the laser-damage
resistance of the optical components in the system, especially the MLD gratings. Recent experiments in our laboratory
have shown that vapor treatment of MLD gratings at room temperature with organosilanes such as hexamethyldisilazane
(HMDS) produces an <i>increase</i> in their damage threshold at 1054 nm (10-ps, 370- μm spot size) as compared to uncoated
MLD grating control samples. The 1-on-1 laser-damage threshold of an HMDS-treated grating increased by 4.5% as
compared to the uncoated control sample, while the <i>N</i>-on-1 damage threshold of an MLD grating treated with
tetramethyldisilazane increased by 16.5%. For an MLD grating treated with bis-(trifluoropropyl)tetramethyldisilazane,
the <i>N</i>-on-1 and 1-on-1 damage thresholds increased by 4.8% and 5.3%, respectively. Such increases in laser-damage
threshold are unprecedented and counterintuitive because it is widely believed that the presence of organic materials or
coatings on the surfaces of optical substrates will <i>inevitably</i> lead to reduced laser-damage resistance.
Multilayer dielectric (MLD) diffraction gratings are an essential component for the OMEGA EP short-pulse, highenergy
laser system. The MLD gratings must have both high-optical-diffraction efficiency and high laser-damage
threshold to be suitable for use within the OMEGA EP Laser System. Considerable effort has been directed toward
optimizing the process parameters required to fabricate gratings that can withstand the 2.6-kJ output energy delivered by
In this paper, we discuss a number of conventional semiconductor chemical cleaning processes that have been
investigated for grating cleaning, and present evidence of their effectiveness in the critical cleaning of MLD gratings
fabricated at LLE. Diffraction efficiency and damage-threshold data were correlated with both scanning electron
microscopy (SEM) and time-of-flight secondary ion-mass spectrometry (ToF-SIMS) to determine the best combination
of cleaning process and chemistry. We found that using these cleaning processes we were able to exceed both the LLE
diffraction efficiency (specification >97%) and laser-damage specifications (specification >2.7 J/cm<sup>2</sup>).