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Areas of research

My specialty is extreme light : the light you can't see, but that can see Nature in its finest details.

I'm lucky to work with extraordinary colleagues in the SHARP team at the Lawrence Berkeley National Lab, where we leverage the capabilities of the Advanced Light Source third generation synchrotron to advance microchip manufacturing technology.

the SHARP team
The SHARP team (photo credit : K. Goldberg)


The primary goal of our research is to provide companies (member companies of Sematech consortium, such as Intel or Samsung) tools to study masks for photo-lithography. This is done using Extreme ultraviolet light (having a wavelength 30 times smaller than visible light : 13.5nm), which is transitionning from the labs to the industry to become the next generation in microchip fabrication.
Typical sizes are in the order of tens of nanometers, therefore everything can be is a distubance : the slighthest imperfection of the optics (one atomic layer!) must be contained, vibrations need to be dampened thousand times to be, temperature variations must be kept below one tenth of a degree (C), while everything must work under a pressure a million times lower than atmospheric pressure. This is why we do this exciting work requires close collaboration with researchers and engineers from various backgrounds.
Since masks are the "gold master" for microchips, their study is of prime importance : a single defect can endanger a batch of a million chips, and developping new way to spot them then allows to alleviate them. This is why I am also interested in using new measurements techniques and computational power in tandem in order to get crucial information about the samples under study. This information can be phase (~thickness of the samples) or finer details; the goal always to reveal hidden contrast.

Before working in EUV lithography, I was interested in the properties of the far-infrared (terahertz radiation), at the exact opposite of the light spectrum. There, I studied the interactions of light and matter at extremely short time-scales (one millonth of a millionth of a second, or a picosecond), and application to microscopy and biological imaging.
I've also worked on optical communication systems, nanofabrication and optical characterization of physical properties.


SHARP rendering

EUV Microscopy

SHARP logo

SHARP is en EUV microscope designed to emulate state-of-the-art lithography printing tools. It is capable exploring minimal dimensions, and demonstrated resolution down to 25nm (6 nm on the wafer). Imaging at-wavelength is esential to factor out all the physical effects that arise during the formation of an aerial image, like the illumination conditions.
More info on SHARP's website

  • At-wavelength (13.5nm) large field microscope base on diffractive optical elements,
  • Emulation of industrial EUV photolithograhy tools (ASML NXE:3xxx),
  • Advanced illumination capability to control beam coherence.

SHARP image

EUV lithography

EUV lithography is the next generation in microchip fabrication.
I have worked on components of the 0.5NA Micro-Exposure tool, a lithography tool with printing capabilities down to 8 nm currently being commissionned.
It will be the single most precise optical nanofabrication device in the world.

Making microchips

Height sensor

photo credit : (left) Intel - (right) G. Gaines/CXRO

Sub-resolution imaging

We are developping new techniques increase resolution and extract more information, such as coded aperture imaging that allows to use large area detector to see fine features in a compressed sensing fashion, or Fourier Ptychography Microscopy, that allows to increase the resolution beyond the physical limit, while providing phase information about the object.

  • Measurement of an aerial image with a 40nm resolution without dedicated optics, using a standard EUV camera,
  • Study of the printability of sub-wavelength defects,
  • Quantitative analysis of phase defects on EUV masks.
Sub-resolution imaging

Optical design and metrology

We are developping metrology tools for extreme precision, like optical position sensor with 1 nm precision, or optical testing of mirrors with techniques such as ptychography (scanning coherent diffraction imaging).
I am using tools such as Zemax to design optical systems and perform tolerance analysis, and build prototypes in the visible range to validate concepts.

Optical design and metrology

  • Design of diffractive optics, for advanced imaging capabilities or programmed aberrations,
  • Tolerancing the components of a height sensor with 1 nm height resolution,
  • Large numerical aperture wavefront sensor.

Instrument control & simulations

Instrument control is key to develop high precision routines. I'm mainly using Matlab to provide interoperability within the scientific community, but I've also used C# to build performant graphical user interfaces. I also run various kind of custom numerical simulations (e.g. diffraction) and signal processing.

  • MET5 EUV printing tool operator interface (Matlab GUI)
  • Ptychography simulation & data acquisition (Matlab)
  • Terahertz Time-Domain Spectroscopy software (C# GUI)
Instrument control with C#, ptychography simulations with Matlab

Qualification of systems

Determining the performances of stage and sensors before they are being integrated is crucial to determine error budgets and system tolerances.
Making sure that all components are properly installed is also necessary for optimal operation.

Qualification of systems

  • Hysteresis of capacitive sensor with nitrogen venting cycles,
  • Correlation between thermal variations and stage drift,
  • Stage straightness for optical alignment (XM-2).

Terahertz and ultrafast optics

Terahertz radiations cover the part of the electro-magnetic spectrum located between radio-waves and visible light. They share desirable properties with these two domain in terms of interactions with material, but they also allow to perform very fine time analysis of light propagation, bringing new contrasts for imaging such as time of flight.

Terahertz ultrafast optics
  • Pico-second time scale dynamics and time-of-filght measurements,
  • Broad optical spectrum & optical dispersion,
  • Non-ionizing radiation, detection of concealed objects.

Guided optics & silicon

Semiconductors such as silicon are very interesting, since they can be used in various ways to shape the properties of light, such as its polarization, using interfacial effects.
Conversely, multiple interactions can be triggered at the same time to carry a certain kind of information.

  • Polarization shift upon reflection and broadband polarization effects.
  • Optical switching of the reflectance of a doped semi-conductor,
  • Frustrated total internal reflection and tunnel effects.
Guided optics silicon

Characterization of materials

I've performed many characterization of materials, either using spectroscopy or polarization techniques, sometimes using pump-probe experiment to determine the dynamics of the light-matter interactions.

Characterization
  • Time-domain spectroscopy of various polymers and semic-conductor,
  • Development of novel techniques based on total internal reflection effects to boost sensitivity,
  • Measure on carrier lifetime in solar cells to predict conversion efficiency.

Miscellaneous

  • Nano-imprinting,
  • Characterization of structures using AFM & SEM,
  • High-speed optical communication, in single-mode fibers or free-space.
Miscellaneous


You can find my PhD dissertation online - Unfortunately, due to France laws pertaining to education, it had to be written in French.
Lots of cool pictures and universal equations though !

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Title

Study of the polarization of terahertz waves in the subcycle-regime and application to imaging in biology

Summary

The terahertz domain is a vast yet largely unexplored part of the electromagnetic spectrum, in spite of the virtues conferred by its intermediary situation between radar waves and infrared. Recent technological breakthroughs now allow to generate very short terahertz pulses and to proceed to a time-resolved detection of the associated electric field. The joint determination of phase and amplitude empowers to build new measurement schemes and opens us new possibilities in the processing of collected data.
The phase plays a major role in the study on phenomena involving the polarization of electromagnetic waves, and assumes a peculiar meaning when dealing with ultra-sort pulses. The study of these particularities has motivated the design of achromatic polarizing elements fitted for the terahertz range using specific dielectric reflection conditions. These elements allow harnessing the differential phases and amplitudes between the two components of the electric field, so as to generate any kind of coherent polarization state.
Associated with terahertz pulses are with submilllimeter wavelengths, which provide a decent spatial resolution when it comes to image objects, while information in the time domain can still be exploited. We carried out some imaging procedures with a view to pick the ones who were the better adapted to biological imaging. In order to take advantage of the sensitivity of the waves to ionic content of solutions and to cope with the high absorption of water in this frequency range, we developed a new imaging technique based on total internal reflection phenomenon. This technique gains its sensitivity from the measurement of the phase, while having an excellent transverse resolution which is well suited when it comes to study thin biological objects such as neurons or cell layers.


You can read the thesis here (in French)

More information here