SPARC – High-Performance Cathodoluminescence

Imaging modes

Spectral mode

When the SPARC system is used in spectral mode, the light from the mirror is focused on a fiber, and the fiber is subsequently connected to a Czerny-Turner spectrograph. A silicon detector is connected to the spectrograph resulting in optimized detection over the range of 400-900 nm. By scanning the e-beam across the sample, a hyperspectral image is made.

Angular mode

The SPARC provides the unique option to acquire angle-resolved images. Instead of focusing the light signal on a fiber, an image of the mirror is projected onto an imaging camera. This allows for the detection of the directionality of the emitted light; also known as momentum spectroscopy. In this mode a filter wheel is used to spectrally distinguish the different emission wavelengths.

Polarisation mode

Using a polarizer in the angle resolved image allows for the detection of separate dipole orientations for nanophotonic applications, and the separation of polarization directions in a more general sense.

Operation and system

The system is closely integrated with the scanning electron microscope. This is seen in both the hardware and the software. The mounting of the hardware is done in such a way that it is minimally invasive for the SEM. It takes less than five minutes to bring the SEM back to its full original configuration. The software is straightforward to use and allows for easy alignment, acquisition and analysis of the results. The software can control the electron beam to allow for acquisition of electron images and to properly trigger the optical acquisition. For in-depth analysis, results can also easily be transferred to Matlab which are part of the software package.

In particular, the software has the following features for acquisition :

• Drift correction
• Simultaneous acquisition of the secondary electron and spectral or angle resolved images
• Easily obtain spectral images over an arbitrarily sized grid with an arbitrary number of pixels
• Easily obtain a large number of angle resolved images

In analysis :

• Easily evaluate spectra as a 2D map or pixel by pixel graph
• Immediate polar plotting of angle resolved images
• View overlaid spectral and electron microscope images
• Use correction files (such as the system response function) to obtain the final clean spectrum in one go
• For detailed analysis easily transport files to Matlab

Specifications & options

Mirror assembly

• Titanium precision translation/rotation stage with mirror mount
• Piezoelectric stepper motors with computer- controlled drivers
• x, y accuracy < 10 nm; θ, φ accuracy < 1 μrad
• Diamond-turned Al-coated half-parabolic precision mirror, collection angle 1.46 π sr., surface roughness < 20 nm

Optical analysis system

• Lightweight optical board in light-tight enclosure with SEM mounting assembly
• High-quality coated mirrors and achromat lens (400-1700 nm)
• Electrically controlled flip mount for remote switching between imaging and angular CL modes
• Filter holder with filters for angle-resolved measurements (50 nm band pass)
• Polarization analyzer
• Fiber coupling assembly

Spectrometer/detectors

• Computer-controlled fiber-coupled Czerny-Turner optical spectrometer with 3 custom interchangeable gratings
• Thermoelectrically cooled ultraviolet-visible Si CCD array detector (λ=400–900 nm) and/or thermoelectrically cooled infrared InGaAs CCD array detector (λ=900–1700 nm)
• Photomultiplier tube for ultra-fast alignment and video-rate CL mapping (λ=300–900 nm)
• 1024 x 1024-pixel thermoelectrically cooled CCD camera system for angle-resolved CL acquisition

Data acquisition

Spectroscopy mode

• 1D and 2D hyperspectral imaging
• Dwell time per e-beam position: 1 μs – 1 s
• Spectral resolution: < 1 nm

Angle-resolved mode

• Angular collection range: up to 1.46 π.sr
• Angular resolution : < 10 mrad
• Spectral resolution: depending on filter band width (typically: 50 nm)
• Simultaneous acquisition of secondary electron and cathodoluminescence signals
• Measurement of polarization-resolved spatial and angular distributions

Hardware

• Design of a dedicated flange assembly for integration with the SEM
• Computer for spectrometer grating control, read-out of CCD detectors and imaging CCD camera, interfacing with SEM x-y external input, controlling mirror stage, and read-out of PMT.
• Drivers and power supplies for piezoelectric stepper motors, spectrometer, CCD detectors, CCD imaging camera, PMT.

Software / data analysis

• Control and alignment of mirror stage
• Plotting 1D and 2D spectral images, crosscuts
  through spatial and angular data, comparison with SEM images, plotting angular radiation distributions