A direct experimental comparison of single-crystal CVD diamond and silicon carbide X-ray beam position monitors

At a Glance

Metadata	Details
Publication Date	2023-07-13
Journal	Journal of Synchrotron Radiation
Authors	C. Houghton, Chris Bloomer, Lorraine Bobb
Institutions	Diamond Light Source
Citations	3
Analysis	Full AI Review Included

Executive Summary

This study provides a direct experimental comparison between single-crystal Chemical Vapour Deposition (CVD) diamond and 4H-Silicon Carbide (4H-SiC) X-ray Beam Position Monitors (XBPMs) for synchrotron diagnostics.

Core Value Proposition: 4H-SiC XBPMs offer performance comparable to sc-diamond while providing a larger transmissive aperture and eliminating the need for external bias voltage.
Bias Requirement: The 4H-SiC device, built as a p-n junction diode, operates effectively at 0 V bias, simplifying installation and operation compared to sc-diamond, which requires a minimum external bias (typically >1 V).
Aperture Advantage: 4H-SiC features a significantly larger position-sensitive area (9 mm x 3 mm) and a thin 2.3 µm central window, which aids in initial beam alignment and accommodates larger X-ray beams.
Position Resolution: Sc-diamond demonstrated superior position resolution (29 nm horizontal, 14 nm vertical at 5 V bias) compared to 4H-SiC (187 nm horizontal, 76 nm vertical at 5 V bias) at a 20 kHz sampling rate.
Uniformity Trade-off: Sc-diamond showed excellent spatial uniformity (0.47% RMS variation). 4H-SiC uniformity was slightly worse (1.5% RMS, ignoring inter-quadrant gaps), likely due to fabrication inhomogeneities in the thin 2.3 µm layer.
Temporal Response and Linearity: Both detectors showed excellent temporal response (sub-millisecond rise time) and maintained linearity across the tested flux range (10^-8 A to 10^-5 A).
Transmission: Both devices were selected for similar high transmission at 12.4 keV (sc-diamond: 99.2%; 4H-SiC window: 99.0%).

Technical Specifications

Parameter	Value	Unit	Context
Detector Material 1	Single-Crystal CVD Diamond	20 µm	Uniform thickness
Detector Material 2	4H-Silicon Carbide (4H-SiC)	362 µm frame / 2.3 µm window	Window diameter: 4.0 mm
Position-Sensitive Area (sc-diamond)	3 x 3	mm	Quadrant active region size
Position-Sensitive Area (4H-SiC)	9 x 3	mm	Quadrant active region size
X-ray Energy Tested	12.4	keV	Experimental condition (I22 beamline)
Transmission (sc-diamond)	99.2	%	At 12.4 keV
Transmission (4H-SiC, window)	99.0	%	At 12.4 keV
Transmission (4H-SiC, frame)	20	%	Thicker 362 µm region
Required External Bias (sc-diamond)	>1	V	Required for high Charge Collection Efficiency
Required External Bias (4H-SiC)	0	V	Operates effectively due to built-in p-n junction field
Sampling Rate	20	kHz	Maximum acquisition rate used
Spatial Uniformity (sc-diamond)	0.47	% RMS variation	Across position-sensitive face
Spatial Uniformity (4H-SiC)	1.5	% RMS variation	Ignoring inter-quadrant gaps
Horizontal Position Resolution (sc-diamond)	29	nm	5 V bias, 20 kHz sampling
Vertical Position Resolution (sc-diamond)	14	nm	5 V bias, 20 kHz sampling
Horizontal Position Resolution (4H-SiC)	187	nm	5 V bias, 20 kHz sampling
Vertical Position Resolution (4H-SiC)	76	nm	5 V bias, 20 kHz sampling
Flux Linearity Range	10^-8 to 10^-5	A	Linear response observed for both devices
Inter-Quadrant Gap (4H-SiC)	6	µm	Resulted in a 15% signal drop when beam centered on gap

Key Methodologies

Detector Configuration: A 20 µm-thick sc-diamond XBPM and a 362 µm-thick 4H-SiC XBPM (with a 2.3 µm central window) were mounted in-line on a motorized X-Y stage for synchronous measurement. The 4H-SiC device was placed upstream due to its larger transmissive aperture.
Beam Parameters: Experiments were conducted at the Diamond Light Source I22 beamline using a focused X-ray beam (12.4 keV) with a size of σ_x = 58 µm and σ_y = 30 µm.
Signal Acquisition: Quadrant currents from both devices were measured simultaneously using low-impedance electrometers (TetrAMM) at a maximum sampling rate of 20 kHz.
Bias Application: Tests were performed with 5 V external bias applied to both devices. Additional tests were conducted with 0 V bias applied to the 4H-SiC device to verify its p-n junction operation.
Spatial Uniformity Measurement: The detectors were raster scanned across the X-ray beam using the motorized stage (9 mm x 7 mm and fine 0.33 mm x 0.33 mm steps) to map the total signal current across the active area.
Flux Linearity Measurement: The incident X-ray flux was varied using different filter thicknesses, and the total current output of the 4H-SiC device was plotted against the sc-diamond device output.
Temporal Response Measurement: The response time was quantified by monitoring intensity changes during mechanical shutter opening/closing and during the 5 Hz electron injection pulses of the synchrotron top-up sequence.
Position Resolution Determination: Position resolution (noise floor) was determined by illuminating only one quadrant of the detector and measuring the standard deviation of the calculated position, isolating detector noise from actual beam motion.

Commercial Applications

The findings support the use of both sc-diamond and 4H-SiC in demanding radiation environments, with 4H-SiC offering specific advantages for scalability and ease of use.

Synchrotron and Light Source Diagnostics:
- Transmissive XBPMs: Real-time, non-intrusive monitoring of beam position and intensity at high flux rates (up to 20 kHz).
- Beamline Commissioning: The large aperture and semi-transmissive frame of 4H-SiC simplify initial coarse alignment and allow beam transmission to downstream diagnostics.
High-Power X-ray Optics:
- Both materials are suitable due to their exceptional radiation hardness and high thermal conductivity, minimizing thermal distortion under high photon flux.
Cost-Effective Large Area Detectors:
- 4H-SiC can be fabricated in large wafers (up to 150 mm diameter) at significantly lower costs than sc-diamond, making it ideal for applications requiring larger detector apertures.
Simplified Detector Systems:
- The ability of 4H-SiC to operate without an external bias voltage simplifies the electronic infrastructure required for detector installation on beamlines.
General Ionizing Radiation Detection:
- High-speed, linear detectors for measuring photon flux in environments where high charge collection efficiency is critical.

View Original Abstract

Single-crystal chemical vapour deposition (CVD) diamond detectors are an established transmissive synchrotron beamline diagnostic instrument used for beam position and beam intensity monitoring. A recently commercialized alternative is silicon carbide (4H-SiC) devices. These have the potential to provide the same diagnostic information as commercially available single-crystal CVD diamond X-ray beam position monitors, but with a much larger transmissive aperture. At Diamond Light Source an experimental comparison of the performance of single-crystal CVD diamond and 4H-SiC X-ray beam position monitors has been carried out. A quantitative comparison of their performance is presented in this paper. The single-crystal diamond and 4H-SiC beam position monitors were installed in-line along the synchrotron X-ray beam path enabling synchronous measurements at kilohertz rates of the beam motion from both devices. The results of several tests of the two position monitors’ performance are presented: comparing signal uniformity across the surface of the detectors, comparing kHz intensity measurements, and comparing kHz beam position measurements from the detectors. Each test is performed with a range of applied external bias voltages. A discussion of the benefits and limitations of 4H-SiC and single-crystal CVD diamond detectors is included.

Tech Support

Original Source

DOI: https://doi.org/10.1107/s1600577523005623