The eighteen H4RG-10 detectors used in the Roman WFI focal plane represent the most advanced IR detector technology at this time. This section of the Instrument Handbook describes the detectors, the programs aimed at characterizing their performance, and our current understanding of their anticipated performance for Roman science. 

The eighteen H4RG-10 detectors used in the Roman WFI focal plane represent the most advanced IR detector technology at this time. The science requirements for Roman demand that these detectors be calibrated to unprecedented levels, which requires that the detectors go through a rigorous characterization program on the ground and in orbit. This section of the Instrument Handbook describes the detectors, the programs aimed at characterizing their performance, and the best understanding of their performance at the current time.

This article provides insight into the characterization of the detector performance and an overview of the H4RG-10 technology with a focus on how the technology impacts scientific performance. Other articles in the handbook will provide overviews of the Detector Performance. Currently, there is a detailed description of WFI Ground Testing Campaigns and the availability of data and results and a brief overview of the types of calibrations in the current On-Orbit Calibration Plan. The detector performance will continue to be evaluated during commissioning and then monitored throughout operations. 

Overview

The Wide Field Instrument (WFI) focal plane is populated with 18 H4RG-10 detectors. The H4RG-10 detectors are HgCdTe 4096 pixel by 4096 pixel photodiode arrays with a 10 micron pixel pitch. Prototype detectors were extensively characterized at the Detector Characterization Laboratory (DCL) at Goddard Space Flight Center. More specifically, their performance was demonstrated in a relevant space-like environment (thermal vacuum, vibration, acoustic, and radiation testing). Detailed results were presented by Mosby et al. (2020).

Roman detector development was a decade long program with key early investments, dozens of detectors fabricated and screened, then characterized and tested at GSFC through the Detector Technology Advancement Program (DTAP). The testing undertaken by the Roman proejct advanced the H4RG-10’s technology readiness level (TRL) to TRL-6 (Mosby et al. 2020). The Table of Teledyne Infrared Detectors provides context on the evolution of these detectors into the Roman era (adapted from Schlieder 2022, presentation). 

Summary of H4RG Physical Characteristics

In this section, elements of the detector design are summarized with a focus on those elements that drive the detector properties and performance for Roman's science goals. Mosby et al. (2020) provides an extremely detailed theoretical and empirical description of the H4RG-10 detectors based on the testing campaign at the Detector Characterization Laboratory (DCL) in Goddard Space Flight Center.

H4RG Architecture

Infrared detectors are fundamentally different from silicon CCD detectors because it is not possible to have both photon detection and electronic readout on the same chip. The photon-sensitive mercury cadmium telluride (HgCdTe) layer is bonded to a silicon Read-out Integrated Circuit (ROIC) and the combination is referred to as a "hybrid". The hybrid is connected to a mechanical mount with electrical connections and the combination is referred to as a "Sensor Chip Assembly" or SCA. Throughout the WFI Imaging Mode User Guide, the terms SCA and detector will be used interchangeably. 

The precise mixture of HgCdTe, specifically the fraction of cadmium, can be varied to engineer a specific bandgap energy. For Roman's desired cutoff wavelength of approximately 2.5 microns, the fraction of cadmium was tuned to 0.445 (Mosby et al. 2020). Other characteristics in the HgCdTe layer induce an electric drift field within the layer that improve quantum efficiency (QE) and count rate non linearity (CRNL). 

Anti-Reflection Coating

An anti-reflection coating acts as an interference filter and ensures that in-band light is transmitted with high efficiency. This is the last optical surface in the WFI. The actual details of the anti-reflection coating are proprietary, but the impact of the coating on light transmission is not. The transmission as a function of wavelength is shown in the Figure of the Anti-reflection Coating Transmission, which is reproduced from Figure 3 of Mosby et al. (2020). Panel A shows the transmission, as a percentage of incident light, as a function of wavelength from roughly 400 nanometers to 2600 nanometers. Three angles of incidence are shown, 0 degrees, and 21 degrees with differing polarizations designated as “S” and “P”. S-polarized is transverse electric, with the electric field perpendicular to the plane of incidence and P-polarized is transverse magnetic with the electric field in the plane of incidence. Overall the variation in the transmission by the angle of incidence is small. Panel B shows the impact of differences in the anti-reflection coating thickness; these differences occur in the fabrication process, and a difference range of 20% of the optimal thickness is shown (e.g., 10% more and 10% less than optimal). These impacts produce significant changes in the throughput, particularly at blue wavelengths. Differences in anti-reflection coating thickness will be present across each detector and across the ensemble of the 18 detectors in Roman's focal plane array.  The result of this variation in anti-reflection coating thickness is variation in the effective blue edge at the pixel level. Significant effort in WFI Ground Testing Campaigns characterized the effective blue edge of the WFI Optical Elements and their impact on science return.

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References

Journal Papers or Reports:

• "Properties and characteristics of the WFIRST H4RG-10 detectors", Mosby et al. 2020

Instrument Update Presentations:

• "Wide Field Instrument Status" presentation at the Roman Community Forum on September 12, 2022 by J. Schlieder