TPA-TCT: How to get 3D Imaging of CMOS Sensors

For the first time, the deep n-well (DNW) depletion space of a High Voltage CMOS sensor has been characterized using a Transient Current Technique based on the simultaneous absorption of two photons. This novel approach has allowed to resolve the DNW implant boundaries and therefore to accurately determine the real depleted volume and the effective doping concentration of the substrate. The unprecedented spatial resolution of this new method comes from the fact that measurable free carrier generation in two photon mode only occurs in a micrometric scale voxel around the focus of the beam. Real three-dimensional spatial resolution is achieved by scanning the beam focus within the sample.

The authors of this research presented the determination of the geometry of the space charge region of a depleted pixel cell using a novel Transient Current Technique (TCT) based on the Two Photon Absorption physical phenomena. TPA–TCT allows three dimensional mapping sensitivity even for detectors with a shallow depletion depth like CMOS pixel sensors.

In conventional laser TCT, silicon detectors are characterized by carrier generation using picosecond laser pulses. The laser wavelength for TCT is above the Si bandgap (λ ≤ 1150 nm) so Single Photon Absorption is dominant, inducing carrier generation along the beam path. The laser wavelength also determines spot size and beam divergence. Visible wavelengths (red, green) can be focused to small spots (≤ μ 1 m) but penetrate only few micrometers inside Si. Thus, good point spatial resolution is only possible at the surface. Very near infrared wavelengths (typically 1064 nm) can be collimated to ∼ 5 μm over several mm depth but carriers are generated along the whole beam path lacking point spatial resolution. In TPA–TCT, laser wavelength is below the Si bandgap ( λ ≥ 1150 nm), for example 1200–1500 nm. In this regime, only nonlinear absorption is relevant The laser has to generate femtosecond pulses because TPA absorption probability is significant only for very short pulses

“The advantage of TPA–TCT is to have both spatial resolution (carrier generation just concentrated around the focal point) and large penetration depth (because out-of-focus intensity does not lead to significant absorption)”

The advantage of TPA–TCT is to have both spatial resolution (carrier generation just concentrated around the focal point) and large penetration depth (because out-of-focus intensity does not lead to significant absorption). The approximately ellipsoidal carrier generation volume can be moved inside the sample in all three dimensions, adjusting the focus and displacing the sample. Looking at the detector response, we can establish a strong correlation between transient current and spatial focal point coordinates, being able to resolve detector internal structures and the depletion volume geometry.

After the experimental results described in the publication below ( See Figure below as an example ), for the first time, the dimension and geometry of the space charge region of a depleted CMOS pixel cell was accurately measured. This has been possible by measuring the collection time of the carriers, enabling the location of the boundaries of the DNW implant and the determination of the transition between drift and diffusion volumes. From the geometry of the space charge region we can compute the effective doping concentration of the silicon substrate, one of the main design parameters of the HVCMOS technology under optimization.

The enabling technology for this achievement is a novel transient current technique based on Two Photon Absorption (TPA–TCT), allowing a submicron spatial resolution in an edge-TCT configuration. TPA–TCT is the only transient current technique able to spatially resolve implants and to discriminate between drift and diffusion. This is because in TPA, focused light generates photocarriers only in a localized volume around the focus. The cross section of this volume is below 1 μm. However, in SPA-TCT, carriers are generated uniformly along the beam, therefore strong focusing only leads to a wide divergence out of the focus, and thus worse spatial resolution. The results presented by the authors  below prove the suitability of TPA–TCT as a high-resolution three dimensional probing tool for sensor characterization.

ABOVE – Left: HVCMOS sketch. Center: charge collection map in 10 ns. Right: Collection time map. Measurements at 20C, 80 V

This white paper is a summary extracted from the publication titled ” High-resolution three-dimensional imaging of a depleted CMOS sensor using an edge Transient Current Technique based on the Two Photon Absorption process (TPA-eTCT) “.

The authors of the previous contents and results are:

Marcos Fernández García (a), Javier González Sánchez (a), Richard Jaramillo Echeverría (a),  Michael Moll (b), Raúl Montero Santos (c), David Moya (a) , Rogelio Palomo Pinto (d) and Iván Vila (a).

(a) Instituto de Física de Cantabria (CSIC-UC), Avda. los Castros s/n, E-39005 Santander, Spain

(b) CERN, Organisation europénne pour la recherche nucléaire, CH-1211 Genéve 23, Switzerland

(c) SGIker Laser Facility, UPV/EHU, Sarriena, s/n – 48940 Leioa-Bizkaia, Spain

(d) Departamento de Ingeniería Electrónica, Escuela Superior de Ingenieros Universidad de Sevilla, Spain

 

Below the link to the complete paper:

http://inspirehep.net/record/1513604/files/10.1016_j.nima.2016.05.070.pdf

 

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