A Broad Photon Energy Range Multi-Strip Imaging Array Based upon Single Crystal Diamond Schottky Photodiode

At a Glance

Metadata	Details
Publication Date	2025-10-28
Journal	Instruments
Authors	C. Verona, M. Angelone, M. Marinelli, G. Verona‐Rinati
Institutions	University of Rome Tor Vergata
Analysis	Full AI Review Included

Executive Summary

Core Innovation: Development of a single crystal diamond (SCD) multi-strip photodetector designed for 1D imaging across the Extreme UV (EUV) to soft X-ray (SXR) spectrum.
SEE Mitigation: The device utilizes buried p-type diamond strips (non-metallic) instead of traditional metal electrodes to effectively suppress Secondary Electron Emission (SEE), ensuring stable and reproducible detection, particularly under EUV irradiation.
Device Structure: Transverse p-type/intrinsic/Schottky configuration fabricated via selective Chemical Vapour Deposition (CVD) on a High-Pressure High-Temperature (HPHT) SCD substrate.
Key Dimensions: The active area is 3.2 x 2.5 mm², patterned with 32 strips, each 100 µm wide with a 20 µm gap. The intrinsic diamond layer thickness was measured at 1.30 ± 0.05 µm.
Performance Metrics: Demonstrated high spatial resolution (SXR pixel resolution of 3.2 µm x 2.6 µm) and excellent response uniformity across the scanned surface. UV tests confirmed spatial resolution compatible with pinhole diameters (FWHM down to 0.289 mm).
Electrical Operation: The device operates efficiently at zero bias due to the Pt Schottky built-in potential (~2.1 V), exhibiting stable capacitance (~9.0 pF per strip) and highly linear response to photon flux (r² = 0.9972).
Future Development: The current 1D design is being adapted for 2D imaging capabilities by planning double-sided strip deposition on a thinned HPHT substrate (20-30 µm).

Technical Specifications

Parameter	Value	Unit	Context
Detector Material	Single Crystal Diamond (SCD)	N/A	P-type/Intrinsic/Schottky (PIM) configuration
Active Area	3.2 x 2.5	mm²	Total sensitive region
Number of Strips	32	N/A	Provides 1D imaging capability
Strip Width	100	µm	Length is 3 mm
Strip Gap	20	µm	Distance between adjacent strips
Intrinsic Layer Thickness (Nominal)	1	µm	CVD-grown layer
Intrinsic Layer Thickness (Measured)	1.30 ± 0.05	µm	Calculated from C-V measurements
Schottky Contact Material	Platinum (Pt)	N/A	Semitransparent, 10 nm thick
Ohmic Contact Material	Silver (Ag)	N/A	Deposited on p-type diamond strips
Schottky Built-in Potential	~2.1	V	For Pt contact, allows zero-bias operation
Operating Bias Range (Tested)	0 to 4	V	External bias increases current signal (up to double at 4 V)
Average Strip Capacitance (C_phd)	~9.0	pF	Stable across external bias range
SXR Test Energy	10	keV	Performed at DLS B16 beam line
SXR Scanning Pixel Resolution	3.2 x 2.6	µm	Horizontal (X) x Vertical (Y)
UV Test Wavelengths	220, 58, 30.4	nm	Tested using laser and He lamp/monochromator
Response Linearity (He Lamp)	r² = 0.9972	N/A	Measured versus plasma current
Spatial Resolution (UV Pinhole 0.25 mm)	0.289	mm	FWHM of Gaussian fit

Key Methodologies

The photodetector fabrication relies on a combination of standard photolithography and selective Chemical Vapour Deposition (CVD) growth on a commercial HPHT SCD substrate.

P-type Strip Patterning and Growth:
- A 200 nm thick Cr plasma-resistant coplanar mask was patterned on the HPHT substrate using photolithography.
- Boron-doped (p-type) diamond microstrip electrodes were selectively grown using Microwave Plasma Enhanced CVD (MWPECVD) in the exposed regions.
Intrinsic Layer Deposition:
- A second patterned Cr layer was deposited via the lift-off method to mask the strip contacts.
- An intrinsic homoepitaxial diamond layer (~1 µm thick) was selectively grown using MWPECVD, effectively burying the p-type diamond strips.
Surface Preparation and Contact Formation:
- The device was annealed in air to remove the surface conductive layer of the intrinsic diamond film.
- A semitransparent Platinum (Pt) electrode (10 nm thick) was thermally evaporated onto the top surface of the intrinsic diamond layer, forming the Schottky contact.
- Patterned Silver (Ag) pads were evaporated onto the exposed p-type diamond strips to form ohmic contacts.
Testing and Readout:
- Electrical characterization (I-V and C-V) was performed using a Keithley 6517A ammeter and an AGILENT 4263B LCR meter.
- Each of the 32 strips was connected to an independent channel of the XDAS (Sens-Tech Ltd.) data acquisition board via micro-wire bonding (25 µm diameter aluminum wire).
- SXR imaging tests utilized a micro-focused 10 keV beam at the Diamond Light Source (DLS) B16 beam line, with current read-out via Keythley 428 transimpedance amplifiers.

Commercial Applications

The unique properties of this SCD photodiode, particularly its visible-blindness, radiation hardness, and SEE suppression, make it suitable for demanding applications in high-energy physics and industrial monitoring.

Synchrotron and Free-Electron Laser Facilities: Used for high-resolution, position-sensitive detection and in situ beam monitoring of SXR and EUV radiation.
Nuclear Fusion Diagnostics: Deployment in magnetic-confinement reactors (e.g., JET) for VUV and soft X-ray measurements, leveraging diamond’s thermal and radiation robustness.
Advanced Semiconductor Manufacturing: High-sensitivity, visible-blind photodetectors required for quality control and monitoring in Extreme Ultraviolet (EUV) photolithographic processes.
Space and Astronomical Observation: Compact, radiation-hard detectors for imaging and spectroscopy of EUV and SXR sources in space environments.
Industrial and Medical Imaging: Potential use as a compact detector in tomography systems or as an active X-ray beam profiler, especially when high spatial resolution and stability are critical.

View Original Abstract

A multi-strip detector made of synthetic single crystal diamond (SCD), based on a p-type/intrinsic diamond/Schottky metal transverse configuration and operating at zero bias voltage, was developed for imaging from extreme UV (EUV) to soft X-rays. The photodetector was patterned with 32 strips made of boron-doped diamond directly deposited, by means of the CVD technique and the standard lithographic technique, on top of the HPHT diamond growth substrate. The width of each strip and the gap between two adjacent strips were 100 μm and 20 μm, respectively. The strips were embedded in intrinsic SCD of an active area of 3.2 × 2.5 mm2, also deposited using the CVD technique in a separate growing machine. In the present structure, the prototype photodetector is suitable for 1D imaging. However, all the dimensions above can be varied depending on the applications. The use of p-type diamond strips represents an attempt to mitigate the photoelectron emission from metal contacts, a non-negligible problem under EUV irradiation. The detector was tested with UV radiation and soft X-rays. To test the photodetector as an imaging device, a headboard (XDAS-DH) and a signal processing board (XDAS-SP) were used as front-end electronics. A standard XDAS software was used to acquire the experimental data. The results of the tests and the detector’s construction process are presented and discussed in the paper.

Tech Support

Original Source

DOI: https://doi.org/10.3390/instruments9040026

References

2013 - A Comprehensive Review of Semiconductor Ultraviolet Photodetectors: From Thin Film to One-Dimensional Nanostructures [Crossref]
2025 - Ultrafast Diamond Photodiodes for vacuum Ultraviolet Imaging in Space-Based Applications [Crossref]
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