Introduction to Infrared Sensors of Webb Telescope

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Introduction to Infrared Sensors of Webb Telescope

 The picture of the James Webb Telescope (JWST) has been released, which is really amazing. However, the domestic science and technology media have focused on the telescope mirror, optics, and photography technology. There are very few articles on infrared sensors. I checked on NASA’s official website. After a while, I feel that I have gained a lot of knowledge about infrared technology.

The following is the translation (personal correction) of NASA's official website, and I will share it with you.


Figure 1: The James Webb Space Telescope Near Infrared Camera (NIRCam) detector, with optical shields removed. Infrared light is collected by a thin film of violet cadmium telluride. Then pixelate it, each pixel is too small to see with the naked eye here. Credit: University of Arizona/NASA.

What are detectors and why are they important?

Figure 2: The above MIRI detector (green) is housed in a block-like unit called a focal plane module. It has a 1024x1024 pixel arsenic -doped silicon array. Image credit: NASA

Webb's mirrors collect light from the universe and direct it to scientific equipment. The light is filtered and split by a beam splitter before it is finally focused onto a sensor (Figure 1-2). Each device has its own detector. The detector is where photons are absorbed and eventually converted into a voltage signal that we measure. Webb needs extremely sensitive detectors to record the faint light from distant galaxies, stars and planets. It requires large-area detector arrays (Figure 3) to effectively observe space. Project Webb has expanded the advanced technology of infrared detectors by making arrays that are quieter, larger, and longer-lived than their predecessors.

Figure 3: This figure shows four 0.6 - 2.5 μm NIRCam H2RG sensors mounted in a focal plane module. Each of the four detectors is similar to the unit shown in Figure 1. The image shows the black optical baffle that allows light to enter the four detectors while preventing photons from hitting any potentially reflective surfaces, such as the edges of the detectors. Credit: University of Arizona/NASA

Different detectors for near-infrared and mid-infrared

Webb uses two different types of detectors: mercury cadmium telluride (abbreviated HgCdTe) "H2RG" detectors for 0.6-5 μm "near infrared" and arsenic -doped silicon (abbreviated Si :As) detectors "mid-infrared". Near-infrared detectors are manufactured by Teledyne Imaging Sensors, California. "H2RG" is the name of the Teledyne product line. Mid-infrared detectors are manufactured by Raytheon Vision Systems, also in California (Raytheon). Each Webb H2RG detector has approximately 4 million pixels. The mid-infrared detectors have approximately 1 million pixels each. 

HgCdTe is a very interesting material. By changing the ratio of mercury to cadmium, the material can be tuned to sense longer or shorter wavelengths of light. Weber achieved this by using a combination of two cadmium mercury tellurides: one with a lower proportion of mercury in the wavelength range of 0.6 - 2.5 microns, and one with a higher content of mercury in the wavelength range of 0.6 - 5 microns This technique has many advantages, including the ability to customize each NIRCam detector for peak performance at the specific wavelength needed. Chart 1 below shows the number of detectors of each type in each instrument. 


Webb detector architecture

All of Webb's detectors have the same basic sandwich architecture (Fig. 4). A sandwich consists of three parts:

(1) Thin semiconductor absorber layer,

(2) A layer of indium pillar interconnects connecting each pixel in the absorber layer to the readout,

(3) Silicon readout integrated circuits (ROICs) for readout of millions of pixels using a manageable number of outputs. 

The absorber layer and silicon ROIC are fabricated separately. This separation allows for the careful adjustment of each part of the process to the materials used. Indium is a soft metal that deforms under moderate pressure, creating a cold weld per pixel between the detector layer and the ROIC. To improve mechanical strength, detector suppliers flow low-viscosity epoxy between the indium bonds at a later stage of bonding.

Figure 4: The James Webb Space Telescope uses an infrared detector hybrid. A pixelated absorber layer (HgCdTe or Si:As) absorbs light and converts it into a voltage in a single pixel. Indium pillar interconnects connect the pixels in the absorber layer to the ROIC. The ROIC contains readout circuitry that crops signals in excess of 1 million pixels into a small number of readouts for further processing. Image Credit: Teledyne Imaging Sensors.

How does the Webb detector work?

The first step in the detection process is the same in the near-infrared HgCdTe detector and in the mid-infrared Si:As detector. Incident photons are absorbed by the semiconductor, generating mobile electron-hole pairs. They are accelerated under the influence of built-in and external electric fields until they are collected and processed by subsequent circuits

A pixel in a Weber detector can be read multiple times before being reset. This provides several benefits. The results of multiple reads can be stacked together to reduce read noise compared to taking only a single read. Another advantage is that by using multiple samples of the same pixel, "jumps" in signal levels can be seen, a sign that cosmic rays are interfering with the pixel. Once it is known that cosmic rays disturbed the pixels, corrections can be applied in ground-based processing to restore the affected pixels.

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